Clay mineralogy

@article{doi1010970001069419531000000009,
    author = "Grim, Ralph E.",
    title = "Clay Mineralogy",
    year = "1953",
    journal = "Soil Science",
    url = "https://doi.org/10.1097/00010694-195310000-00009",
    doi = "10.1097/00010694-195310000-00009",
    openalex = "W4247972398"
}

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@article{doi101126science1203109183,
    author = "Hunt, Charles B.",
    title = "Desert Varnish",
    year = "1954",
    journal = "Science",
    abstract = "71MetricsTotal Downloads7Last 6 Months2Last 12 Months2Total Citations1Last 6 Months0Last 12 Months0View all metrics",
    url = "https://doi.org/10.1126/science.120.3109.183",
    doi = "10.1126/science.120.3109.183",
    openalex = "W4240229988",
    references = "openalexw633827141"
}

Desert varnish forms a dark coating up to 0.10 mm thick on the exposed surfaces of many stones and outcrops in southern California deserts. Wet chemical analyses were made of varnish, the underlying weathered rind, and fresh rock for a rhyolite and two andesites. The principal elements in varnish are O, H, Si, Al, Fe, and Mn, and the last two give the deposit its distinctive physical characteristics. H_2O, Fe_2O_3, and especially MnO show the greatest enrichment. Field observations and a number of partial analyses indicate that the best varnishes are on fine-grained rocks relatively rich in Fe and Mn. \n \nSpectrographic analyses were made of 22 varnishes, 14 rocks, 8 soils, and 5 samples of air-borne material. In the varnishes Ti, Ba, and Sr are by far the most abundant trace elements, followed by Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb. Cd, W, Ag, Nb, Sn, Ga, Mo, Be, and Zn were recorded in some but not all varnishes. The trace-element content of all varnishes is similar, and the variations recorded are related to differences in the local geology. Most trace elements are considerably enriched in varnish—Cu and Co especially, and Ni, Pb, Ba, Cr, Yb, B, Y, Sr, and V. \n \nThe chemical data suggest that (1) varnish on stones seated in soil or colluvium is derived largely from that material, (2) varnish on large bedrock exposures come from weathered parts of the rock, (3) air-borne material is probably a minor contributor. \n \nThe formation of desert varnish is primarily a weathering process involving the solution, transportation, and deposition of Mn and Fe in particular and a host of trace elements. Most of these elements are derived from local sources, and the small amount of movement required can occur by transport in solution or possibly by ionic diffusion through moisture films. Dew may be as important a source of moisture as rain. Organic agents, such as bacteria, may cause deposition of varnish, but this has not yet been demonstrated. In the desert, evaporation and the catalytic action of MnO_2 should be capable of performing the task. \n \nThe rate of varnish formation varies widely with local conditions. Hundreds and thousands of years may be required to form a dark coating in some situations, but at one locality in the Mojave Desert a good varnish formed on the surface stones of an alluvial deposit in 25 years. Although the widespread evidence of varnish deterioration may be due to climatological change, conditions in some parts of this desert area are currently favorable to varnish formation.

@article{doi10113000167606195869487cdodv20co2,
    author = "Engel, Celeste G. and Sharp, Robert P.",
    title = "CHEMICAL DATA ON DESERT VARNISH",
    year = "1958",
    journal = "Geological Society of America Bulletin",
    abstract = "Desert varnish forms a dark coating up to 0.10 mm thick on the exposed surfaces of many stones and outcrops in southern California deserts. Wet chemical analyses were made of varnish, the underlying weathered rind, and fresh rock for a rhyolite and two andesites. The principal elements in varnish are O, H, Si, Al, Fe, and Mn, and the last two give the deposit its distinctive physical characteristics. H\_2O, Fe\_2O\_3, and especially MnO show the greatest enrichment. Field observations and a number of partial analyses indicate that the best varnishes are on fine-grained rocks relatively rich in Fe and Mn. \n \nSpectrographic analyses were made of 22 varnishes, 14 rocks, 8 soils, and 5 samples of air-borne material. In the varnishes Ti, Ba, and Sr are by far the most abundant trace elements, followed by Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb. Cd, W, Ag, Nb, Sn, Ga, Mo, Be, and Zn were recorded in some but not all varnishes. The trace-element content of all varnishes is similar, and the variations recorded are related to differences in the local geology. Most trace elements are considerably enriched in varnish—Cu and Co especially, and Ni, Pb, Ba, Cr, Yb, B, Y, Sr, and V. \n \nThe chemical data suggest that (1) varnish on stones seated in soil or colluvium is derived largely from that material, (2) varnish on large bedrock exposures come from weathered parts of the rock, (3) air-borne material is probably a minor contributor. \n \nThe formation of desert varnish is primarily a weathering process involving the solution, transportation, and deposition of Mn and Fe in particular and a host of trace elements. Most of these elements are derived from local sources, and the small amount of movement required can occur by transport in solution or possibly by ionic diffusion through moisture films. Dew may be as important a source of moisture as rain. Organic agents, such as bacteria, may cause deposition of varnish, but this has not yet been demonstrated. In the desert, evaporation and the catalytic action of MnO\_2 should be capable of performing the task. \n \nThe rate of varnish formation varies widely with local conditions. Hundreds and thousands of years may be required to form a dark coating in some situations, but at one locality in the Mojave Desert a good varnish formed on the surface stones of an alluvial deposit in 25 years. Although the widespread evidence of varnish deterioration may be due to climatological change, conditions in some parts of this desert area are currently favorable to varnish formation.",
    url = "https://doi.org/10.1130/0016-7606(1958)69[487:cdodv]2.0.co;2",
    doi = "10.1130/0016-7606(1958)69[487:cdodv]2.0.co;2",
    openalex = "W2016751313"
}

Abstract The oxidation potential of dithionite (Na 2 S 2 O 4) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na 2 S 2 O 4 -citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.

@article{doi101346ccmn19580070122,
    author = "Mehra, O. P. and Jackson, M. L.",
    title = "Iron Oxide Removal from Soils and Clays by a Dithionite-Citrate System Buffered with Sodium Bicarbonate",
    year = "1958",
    journal = "Clays and clay minerals (National Conference on Clays and Clay Minerals)",
    abstract = "Abstract The oxidation potential of dithionite (Na 2 S 2 O 4) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na 2 S 2 O 4 -citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.",
    url = "https://doi.org/10.1346/ccmn.1958.0070122",
    doi = "10.1346/ccmn.1958.0070122",
    openalex = "W3009397686",
    references = "doi1010970001069419441200000001, doi1010970001069419540300000001, doi101111j136523891950tb00733x, doi101111j136523891954tb02185x, doi102134agronj194200021962003400070003x, doi102136sssaj193703615995000100000013x, doi102136sssaj195303615995001700040015x, openalexw1512608287, openalexw2482216212"
}

Abstract Desert pavement and vesicular layer are described as distinct horizons of some Gray Desert soils. Distribution of > 2‐mm. and < 2‐mm. particles in the profile along with laboratory tests suggest that accumulation of gravel and stones at the surface to form a desert pavement is not due solely to removal of finer material by wind or water. There has also been some upward movement of coarse fragments from the nearly stone‐free layers below. Some properties of vesicular layers are pointed out by field and laboratory studies. Natural vesicular structure was destroyed by sieving and a new, but similar, structure formed by merely wetting and drying the soil. This led to a hypothesis for origin of the vesicular layer as a pedogenic horizon.

@article{doi102136sssaj195803615995002200010017x,
    author = "Springer, M. E.",
    title = "Desert Pavement and Vesicular Layer of Some Soils of the Desert of the Lahontan Basin, Nevada",
    year = "1958",
    journal = "Soil Science Society of America Journal",
    abstract = "Abstract Desert pavement and vesicular layer are described as distinct horizons of some Gray Desert soils. Distribution of > 2‐mm. and < 2‐mm. particles in the profile along with laboratory tests suggest that accumulation of gravel and stones at the surface to form a desert pavement is not due solely to removal of finer material by wind or water. There has also been some upward movement of coarse fragments from the nearly stone‐free layers below. Some properties of vesicular layers are pointed out by field and laboratory studies. Natural vesicular structure was destroyed by sieving and a new, but similar, structure formed by merely wetting and drying the soil. This led to a hypothesis for origin of the vesicular layer as a pedogenic horizon.",
    url = "https://doi.org/10.2136/sssaj1958.03615995002200010017x",
    doi = "10.2136/sssaj1958.03615995002200010017x",
    openalex = "W2162057244"
}

(1962). Applied Clay Mineralogy. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 84, No. 4, pp. 533-533.

@article{doi10108011035896209447314,
    author = "Grim, Ralph E.",
    title = "Applied Clay Mineralogy",
    year = "1962",
    journal = "Geologiska Föreningen i Stockholm Förhandlingar",
    abstract = "(1962). Applied Clay Mineralogy. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 84, No. 4, pp. 533-533.",
    url = "https://doi.org/10.1080/11035896209447314",
    doi = "10.1080/11035896209447314",
    openalex = "W2137505896"
}

@article{doi102136sssaj196303615995002700020003x,
    title = "Applied Clay Mineralogy",
    year = "1963",
    journal = "Soil Science Society of America Journal",
    url = "https://doi.org/10.2136/sssaj1963.03615995002700020003x",
    doi = "10.2136/sssaj1963.03615995002700020003x",
    openalex = "W2596921635"
}

@article{doi10113000167606196576803masord20co2,
    author = "Biscaye, Pierre E.",
    title = "Mineralogy and Sedimentation of Recent Deep-Sea Clay in the Atlantic Ocean and Adjacent Seas and Oceans",
    year = "1965",
    journal = "Geological Society of America Bulletin",
    url = "https://doi.org/10.1130/0016-7606(1965)76[803:masord]2.0.co;2",
    doi = "10.1130/0016-7606(1965)76[803:masord]2.0.co;2",
    openalex = "W1970108402",
    references = "doi101016002532276490012x, doi101021j150463a015, doi101086624619, doi10113000167606196172193adsc20co2, doi101306d42697b52b2611d78648000102c1865d, doi101306sv10340, doi101346ccmn19580070102, doi101346ccmn19580070104, doi101346ccmn19580070122, openalexw2580534269"
}

Desert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $$SiO_{2}$$ and usually $$Al_{2}O_{3}$$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $$Al_{2}O_{3}$$, and $$K_{2}O$$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $$Al_{2}O_{3}, K_{2}O$$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO: MnO ratio generally decreases outward through the varnish. This suggests that some of the Mn in varnish is dissolved whenever Fe-Mn-bearing solutions arrive on the rock and that fractionation occurs during subsequent precipitation such that Mn is precipitated last. This fractionation may be responsible for the existence of the orange bottom coating, the groundline band, and the darker varnish in hollows on the rock surface.

@article{doi101086627435,
    author = "Hooke, Roger LeB. and Yang, Houng‐Yi and Weiblen, P. W.",
    title = "Desert Varnish: An Electron Probe Study",
    year = "1969",
    journal = "The Journal of Geology",
    abstract = "Desert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $$SiO\_{2}$$ and usually $$Al\_{2}O\_{3}$$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $$Al\_{2}O\_{3}$$, and $$K\_{2}O$$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $$Al\_{2}O\_{3}, K\_{2}O$$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO: MnO ratio generally decreases outward through the varnish. This suggests that some of the Mn in varnish is dissolved whenever Fe-Mn-bearing solutions arrive on the rock and that fractionation occurs during subsequent precipitation such that Mn is precipitated last. This fractionation may be responsible for the existence of the orange bottom coating, the groundline band, and the darker varnish in hollows on the rock surface.",
    url = "https://doi.org/10.1086/627435",
    doi = "10.1086/627435",
    openalex = "W1964349919"
}

@article{doi101038259387a0,
    author = "Bauman, A. J.",
    title = "Desert varnish and marine ferromanganese oxide nodules: congeneric phenomena",
    year = "1976",
    journal = "Nature",
    url = "https://doi.org/10.1038/259387a0",
    doi = "10.1038/259387a0",
    openalex = "W1971904241"
}

Desert varnish has been characterized by infrared spectroscopy, x-ray diffraction, and electron microscopy. It is a distinct morphological entity having an abrupt boundary with the underlying rock. Clay minerals comprise more than 70 percent of the varnish. Iron and manganese oxides constitute the bulk of the remainder and are dispersed throughout the clay layer.

@article{potter1977desert,
    author = "Potter, Russell M. and Rossman, George R.",
    title = "Desert Varnish: The Importance of Clay Minerals",
    year = "1977",
    journal = "Science",
    abstract = "Desert varnish has been characterized by infrared spectroscopy, x-ray diffraction, and electron microscopy. It is a distinct morphological entity having an abrupt boundary with the underlying rock. Clay minerals comprise more than 70 percent of the varnish. Iron and manganese oxides constitute the bulk of the remainder and are dispersed throughout the clay layer.",
    url = "https://doi.org/10.1126/science.196.4297.1446",
    doi = "10.1126/science.196.4297.1446",
    number = "4297",
    openalex = "W2056380135",
    pages = "1446-1448",
    volume = "196",
    references = "doi101016037119516480165x, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi10113000167606197687725mobmoa20co2, doi101346ccmn19680160104, doi101346ccmn19680160305, doi101346ccmn19700180104, openalexw161080799, openalexw2240482887, openalexw2932319362"
}

@misc{potter1977desert1,
    author = "Potter, R. M. and Rossman, G. R",
    title = "Desert varnish",
    year = "1977",
    howpublished = "the importance of clay minerals: Science, v. 196, p. 1446-1448",
    note = "talkorigins\_source = {true}; raw\_reference = {Potter, R. M., and Rossman, G. R., 1977, Desert varnish: the importance of clay minerals: Science, v. 196, p. 1446-1448.}"
}

@article{doi101038276489a0,
    author = "Perry, Randall S. and Adams, J. B.",
    title = "Desert varnish: evidence for cyclic deposition of manganese",
    year = "1978",
    journal = "Nature",
    url = "https://doi.org/10.1038/276489a0",
    doi = "10.1038/276489a0",
    openalex = "W1997772382",
    references = "doi101007bf01820710, doi101016s0422989408x70485, doi101038259387a0, doi101086627435, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101130001676061972831493gofoca20co2, doi102307279516, openalexw653970988, potter1977desert"
}

@article{doi1010160009254179900858,
    author = "Potter, Russell M. and Rossman, George R.",
    title = "The manganese- and iron-oxide mineralogy of desert varnish",
    year = "1979",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/0009-2541(79)90085-8",
    doi = "10.1016/0009-2541(79)90085-8",
    openalex = "W2058306764",
    references = "doi101016b9780080092355500267, doi101016s0070457109x70019, doi101021ac60294a030, doi101086627339, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101346ccmn19580070122, doi102136sssaj197203615995003600050024x, openalexw2613288873, potter1977desert"
}

A number of structural problems in the mineralogy of the tetravalent manganese oxides have been addressed by infrared spectroscopy in conjunction with X-ray diffraction and chemical analysis. The first pyrolusite with proven orthorhombic symmetry is reported. Examination of pyrolusites with different degrees of orthorhombic distortion supports the view that this distortion is the result of micro-pores within the crystal. Pyrolusite infrared spectra exhibit variations which cannot be correlated to orthorhombic distortion, ramsdellite intergrowths, or other mineral impurity. The continuous structural variation of the nsutites from a rarnsdellite end-member to a pyrolusite end-member has been confirmed although much of the variation in synthetic nsutites appears to be related to crystalline order. Ramsdellite has a single, crystallographically-ordered type of water, which appears to be an integral part of the structure. The only hydrous component of romanechite is water, which is located in the channels in a specific crystallographic site. The only hydrous component of lithiophorite is hydroxide ion, which is oriented perpendicular to the cleavage planes. Birnessite appears to have a layer structure on the basis of its infrared spectrum. Its identity with proposed synthetic analogs is confirmed. Todorokite is a valid mineral species which is not analogous to any synthetic phases or to any alteration products of them. It appears to have a layered structure. Rancieite probably has a layer structure related to birnessite. It is possible to distinguish manganese oxides of different structural groups from one another by their spectra in the mid-infrared region, which is sensitive to the hydrous components and the manganese octahedral framework. Because of its sensitivity to short range order, infrared spectroscopy is often superior to X-ray diffraction for the determinative mineralogy of the manganese oxides, which often occur in a finely-particulate, poorly-crystalline state. Spectra in the region 4000 cm⁻¹ to 200 cm⁻¹ are presented for well-characterized manganese oxide samples in order to form a basis for identification of manganese oxide mineralogy. The following oxides are included: aurorite, birnessite, braunite, buserite, chalcophanite, coronadite, cryptomelane, groutite, hausmannite, hollandite, lithiophorite, manganite, manganosite, manjiroite, marokite, nsutite, partridgeite, pyrolusite, quenselite, ramsdellite, rancieite, romanechite (psilomelane), todorokite, and woodruffite. This data base has been applied to the study of the mineralogy of manganese oxide concentrations of the terrestrial weathering environment. Through the integrated application of a variety of infrared spectroscopic, X-ray diffraction, electron optic, and chemical techniques, the characteristic mineralogy of desert varnish has been identified as bimessite, hematite, and mixed-layer illite-montmorillonite clay minerals. Clay minerals comprise greater than 70 percent of the varnish, the oxides constitute t he bulk of the remainder and are in intimate physical association with the clays. An abrupt change in chemistry, mineralogy, and morphology exists at the varnish-rock interface. The origin of the material is external to the rock which it coats. The clays are most likely transported by wind or water. The oxides appear to be transported by water. Clay minerals are necessary for varnish formation, probably through their influence on oxide concentration or deposition. Morphological distinctions among desert varnish, manganese dendrites, river deposits and other manganese oxide concentrations of the terrestrial weathering environment have a sound basis in differences in their mineralogy. The manganese oxide in manganese dendrites is either romanechite or a hollandite group mineral. These are mixed with varying amounts of silicate minerals, which are a passive substrate for the oxide deposition. Manganese stream deposits are generally birnessite with minor amounts of silicate minerals; one nsutite stream deposit has been identified. Crack deposit mineralogy resembles that of manganese dendrites. Cave and subglacial deposit mineralogy resembles that of manganese stream deposits. No dendrite has been found to consist of pyrolusite. Hydropsilomelane concretions, which carry chemical remanent magnetization in sediments in Baja California, Mexico, consist of siltstone matrix minerals cemented by a manganese oxide having the chalcophanite structure. The high concentration of magnesium presumed to occur in the interlayer position of this mineral extends the known range of substitution in chalcophanite-structure minerals.

@misc{potter1979the,
    author = "Potter, Russell Marsh",
    title = "The Tetravalent Manganese Oxides : Clarification of their Structural Variations and Relationships and Characterization of their Occurrence in the Terrestrial Weathering Environment as Desert Varnish and other Manganese Oxide Concentrations",
    year = "1979",
    publisher = "California Institute of Technology",
    abstract = "A number of structural problems in the mineralogy of the tetravalent manganese oxides have been addressed by infrared spectroscopy in conjunction with X-ray diffraction and chemical analysis. The first pyrolusite with proven orthorhombic symmetry is reported. Examination of pyrolusites with different degrees of orthorhombic distortion supports the view that this distortion is the result of micro-pores within the crystal. Pyrolusite infrared spectra exhibit variations which cannot be correlated to orthorhombic distortion, ramsdellite intergrowths, or other mineral impurity. The continuous structural variation of the nsutites from a rarnsdellite end-member to a pyrolusite end-member has been confirmed although much of the variation in synthetic nsutites appears to be related to crystalline order. Ramsdellite has a single, crystallographically-ordered type of water, which appears to be an integral part of the structure. The only hydrous component of romanechite is water, which is located in the channels in a specific crystallographic site. The only hydrous component of lithiophorite is hydroxide ion, which is oriented perpendicular to the cleavage planes. Birnessite appears to have a layer structure on the basis of its infrared spectrum. Its identity with proposed synthetic analogs is confirmed. Todorokite is a valid mineral species which is not analogous to any synthetic phases or to any alteration products of them. It appears to have a layered structure. Rancieite probably has a layer structure related to birnessite. It is possible to distinguish manganese oxides of different structural groups from one another by their spectra in the mid-infrared region, which is sensitive to the hydrous components and the manganese octahedral framework. Because of its sensitivity to short range order, infrared spectroscopy is often superior to X-ray diffraction for the determinative mineralogy of the manganese oxides, which often occur in a finely-particulate, poorly-crystalline state. Spectra in the region 4000 cm⁻¹ to 200 cm⁻¹ are presented for well-characterized manganese oxide samples in order to form a basis for identification of manganese oxide mineralogy. The following oxides are included: aurorite, birnessite, braunite, buserite, chalcophanite, coronadite, cryptomelane, groutite, hausmannite, hollandite, lithiophorite, manganite, manganosite, manjiroite, marokite, nsutite, partridgeite, pyrolusite, quenselite, ramsdellite, rancieite, romanechite (psilomelane), todorokite, and woodruffite. This data base has been applied to the study of the mineralogy of manganese oxide concentrations of the terrestrial weathering environment. Through the integrated application of a variety of infrared spectroscopic, X-ray diffraction, electron optic, and chemical techniques, the characteristic mineralogy of desert varnish has been identified as bimessite, hematite, and mixed-layer illite-montmorillonite clay minerals. Clay minerals comprise greater than 70 percent of the varnish, the oxides constitute t he bulk of the remainder and are in intimate physical association with the clays. An abrupt change in chemistry, mineralogy, and morphology exists at the varnish-rock interface. The origin of the material is external to the rock which it coats. The clays are most likely transported by wind or water. The oxides appear to be transported by water. Clay minerals are necessary for varnish formation, probably through their influence on oxide concentration or deposition. Morphological distinctions among desert varnish, manganese dendrites, river deposits and other manganese oxide concentrations of the terrestrial weathering environment have a sound basis in differences in their mineralogy. The manganese oxide in manganese dendrites is either romanechite or a hollandite group mineral. These are mixed with varying amounts of silicate minerals, which are a passive substrate for the oxide deposition. Manganese stream deposits are generally birnessite with minor amounts of silicate minerals; one nsutite stream deposit has been identified. Crack deposit mineralogy resembles that of manganese dendrites. Cave and subglacial deposit mineralogy resembles that of manganese stream deposits. No dendrite has been found to consist of pyrolusite. Hydropsilomelane concretions, which carry chemical remanent magnetization in sediments in Baja California, Mexico, consist of siltstone matrix minerals cemented by a manganese oxide having the chalcophanite structure. The high concentration of magnesium presumed to occur in the interlayer position of this mineral extends the known range of substitution in chalcophanite-structure minerals.",
    url = "https://resolver.caltech.edu/CaltechTHESIS:09192022-193330791",
    doi = "10.7907/k2s0-9177"
}

Abstract In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.

@book{doi101180mono5,
    title = "Crystal Structures of Clay Minerals and their X-Ray Identification",
    year = "1980",
    booktitle = "Mineralogical Society of Great Britain and Ireland eBooks",
    abstract = "Abstract In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.",
    url = "https://doi.org/10.1180/mono-5",
    doi = "10.1180/mono-5",
    openalex = "W1821151640"
}

@article{doi101007bf00378791,
    author = "Krumbein, Wolfgang E. and Jens, K.",
    title = "Biogenic rock varnishes of the negev desert (Israel) an ecological study of iron and manganese transformation by cyanobacteria and fungi",
    year = "1981",
    journal = "Oecologia",
    url = "https://doi.org/10.1007/bf00378791",
    doi = "10.1007/bf00378791",
    openalex = "W2082823380",
    references = "doi101007bf01611203, doi1010160009254179900858, doi101016b978012135902750011x, doi101038276489a0, doi1010990022128711111, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101146annurevmi32100178003225, doi1023071797455, openalexw1526419501, openalexw368099268, potter1977desert"
}

Scanning electron microscopy and energy dispersive x-ray analyses of desert varnish reveal that microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish. Specific characteristics of desert varnish and of varnish bacteria support a microbial origin for manganese-rich films. Varnish microbes can be cultured and produce laboratory manganese films. Accordingly, natural desert varnish and also manganese-rich rock varnishes in nondesert environments appear to be a product of microbial activity.

@article{doi101126science21345131245,
    author = "Dorn, Ronald I. and Oberlander, Theodore M.",
    title = "Microbial Origin of Desert Varnish",
    year = "1981",
    journal = "Science",
    abstract = "Scanning electron microscopy and energy dispersive x-ray analyses of desert varnish reveal that microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish. Specific characteristics of desert varnish and of varnish bacteria support a microbial origin for manganese-rich films. Varnish microbes can be cultured and produce laboratory manganese films. Accordingly, natural desert varnish and also manganese-rich rock varnishes in nondesert environments appear to be a product of microbial activity.",
    url = "https://doi.org/10.1126/science.213.4513.1245",
    doi = "10.1126/science.213.4513.1245",
    openalex = "W1982129609"
}

This book is intended primarily for use in field-study tours of the Desert-Soil Geomorphology Project of the U.S. Soil Conservation Service (informally termed the Desert Project) in Dona Ana County, southern New Mexico. Main purposes are to illustrate major soils and landscapes of the project area, to illustrate principles of soil and landscape evolution in basin-and-range topography, to show the landscape positions in which the soils are most likely to occur, to describe soil development, and to illustrate the United States system of soil taxonomy as it applies to desert soils of the region. The Desert Project encompasses a 400-sq mi area near Las Cruces and includes a segment of the Rio Grande valley and parts of adjacent intermontane basins. Soil Survey Investigations field and laboratory studies were initiated in 1957; work was done in cooperation with the New Mexico State University College of Agriculture and the New Mexico Bureau of Mines and Mineral Resources. It represents one of the most comprehensive studies of soils and landscapes ever undertaken in an arid to semiarid region. The Desert Project is similar to many other areas in terms of terrain, soil parent materials, range in age of soils, and general climatic and geologic history. Thus principles of soil and landscape evolution worked out at the Desert Project will have widespread application. A number of formal field-study tours were held during progress of the research, and the Desert Project has proved to be of valuable study and training ground for a variety of professional workers and students. Field trip participants have included agronomists, anthropologists, biologists, foresters, geomorphologists, geologists, hydrologists, range scientists, and soil scientists. Numerous requests have been received for copies of previous field guides that are out of print. These were printed only in limited numbers and were much less comprehensive in scope than the present volume, which is intended to serve as permanent guide to many of the detailed study sites of the Desert Project. Field investigations included mapping the soils, geomorphic surfaces, and surficial deposits at a scale of 1:15,840. In addition, detailed studies at larger scales were conducted along selected transects. Joint laboratory and field investigations included studies of characteristics and genesis of a number of soils and soil horizons, radiocarbon dating of pedogenic carbonates and organic carbon, and studies of the effect of additions from dustfall to soil genesis and morphology. Since 1965, project investigations have been done in cooperations with the New Mexico Bureau of Mines and Mineral Resources; and since 1967, all major reports on geologic phases of the project have been published either by or with support of the Bureau. In 1977 the Desert Project became a formal part of the Bureau's environmental geology program, which includes sponsorship of this and future field-study tours. The Desert Project has been a good study and training ground for a wide variety of workers. The project area is similar to many arid and semiarid regions in terms of terrain, parent materials for soil, range in age of soils, and general climatic history. Thus principles of soil and landscape evolution worked out in the Desert Project also apply to many areas other than the southwest United States. A number of formal field-study tours were held during progress of the research. In addition to this memoir and several journal articles written during progress of the research, the Desert Project soil monograph has been written on the project as a whole, which may be attained from the National Technical Information Service. Complete laboratory analyses, details for methods of analyses reported in this book, many pedon descriptions, a list of all soils observed in the project area, and a detailed map of the entire project area may be found in this monograph. A final report on late Cenozoic geology of the project will be written after detailed mapping of the bedrock areas is complete. Companion to Bulletin 142.

@book{doi1058799m39,
    author = "Gile, Leland H. and Hawley, J. W. and Grossman, R. B.",
    title = "Soils and geomorphology in the Basin and Range area of southern New Mexico--Guidebook to the Desert Project",
    year = "1981",
    abstract = "This book is intended primarily for use in field-study tours of the Desert-Soil Geomorphology Project of the U.S. Soil Conservation Service (informally termed the Desert Project) in Dona Ana County, southern New Mexico. Main purposes are to illustrate major soils and landscapes of the project area, to illustrate principles of soil and landscape evolution in basin-and-range topography, to show the landscape positions in which the soils are most likely to occur, to describe soil development, and to illustrate the United States system of soil taxonomy as it applies to desert soils of the region. The Desert Project encompasses a 400-sq mi area near Las Cruces and includes a segment of the Rio Grande valley and parts of adjacent intermontane basins. Soil Survey Investigations field and laboratory studies were initiated in 1957; work was done in cooperation with the New Mexico State University College of Agriculture and the New Mexico Bureau of Mines and Mineral Resources. It represents one of the most comprehensive studies of soils and landscapes ever undertaken in an arid to semiarid region. The Desert Project is similar to many other areas in terms of terrain, soil parent materials, range in age of soils, and general climatic and geologic history. Thus principles of soil and landscape evolution worked out at the Desert Project will have widespread application. A number of formal field-study tours were held during progress of the research, and the Desert Project has proved to be of valuable study and training ground for a variety of professional workers and students. Field trip participants have included agronomists, anthropologists, biologists, foresters, geomorphologists, geologists, hydrologists, range scientists, and soil scientists. Numerous requests have been received for copies of previous field guides that are out of print. These were printed only in limited numbers and were much less comprehensive in scope than the present volume, which is intended to serve as permanent guide to many of the detailed study sites of the Desert Project. Field investigations included mapping the soils, geomorphic surfaces, and surficial deposits at a scale of 1:15,840. In addition, detailed studies at larger scales were conducted along selected transects. Joint laboratory and field investigations included studies of characteristics and genesis of a number of soils and soil horizons, radiocarbon dating of pedogenic carbonates and organic carbon, and studies of the effect of additions from dustfall to soil genesis and morphology. Since 1965, project investigations have been done in cooperations with the New Mexico Bureau of Mines and Mineral Resources; and since 1967, all major reports on geologic phases of the project have been published either by or with support of the Bureau. In 1977 the Desert Project became a formal part of the Bureau's environmental geology program, which includes sponsorship of this and future field-study tours. The Desert Project has been a good study and training ground for a wide variety of workers. The project area is similar to many arid and semiarid regions in terms of terrain, parent materials for soil, range in age of soils, and general climatic history. Thus principles of soil and landscape evolution worked out in the Desert Project also apply to many areas other than the southwest United States. A number of formal field-study tours were held during progress of the research. In addition to this memoir and several journal articles written during progress of the research, the Desert Project soil monograph has been written on the project as a whole, which may be attained from the National Technical Information Service. Complete laboratory analyses, details for methods of analyses reported in this book, many pedon descriptions, a list of all soils observed in the project area, and a detailed map of the entire project area may be found in this monograph. A final report on late Cenozoic geology of the project will be written after detailed mapping of the bedrock areas is complete. Companion to Bulletin 142.",
    url = "https://doi.org/10.58799/m-39",
    doi = "10.58799/m-39",
    openalex = "W4323765264",
    references = "doi101029jb081i005p00725, doi101086625662, doi101086625985, doi101086626313, doi101127zfg12196860, doi10130674d70c7c2b2111d78648000102c1865d, doi101306bc74368516be11d78645000102c1865d, doi102307634889, doi1056577ffc26297"
}

Microcolonial structures have been harvested from desert rock samples for cultivation and ultrastructural examination. The results indicate that these microcolonial structures are fungi previously unrecognized as inhabitants of desert rocks.

@article{doi101126science21545361093,
    author = "Staley, James T. and Palmer, F. and Adams, J. B.",
    title = "Microcolonial Fungi: Common Inhabitants on Desert Rocks?",
    year = "1982",
    journal = "Science",
    abstract = "Microcolonial structures have been harvested from desert rock samples for cultivation and ultrastructural examination. The results indicate that these microcolonial structures are fungi previously unrecognized as inhabitants of desert rocks.",
    url = "https://doi.org/10.1126/science.215.4536.1093",
    doi = "10.1126/science.215.4536.1093",
    openalex = "W2025020270"
}

@article{doi101007bf02097739,
    author = "Taylor‐George, Susan and Palmer, F. and Staley, James T. and Borns, David James and Curtiss, Brian and Adams, J. B.",
    title = "Fungi and bacteria involved in desert varnish formation",
    year = "1983",
    journal = "Microbial Ecology",
    url = "https://doi.org/10.1007/bf02097739",
    doi = "10.1007/bf02097739",
    openalex = "W2076852250"
}

Abstract Rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands. All varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing. They are valid only where manganese is not remobilized after deposition, for example, in most arid environments. The premise of a new age-determination method, cation-ratio dating, is that the ratio of the more mobile cations (e.g., K and Ca) to titanium in varnish decreases with time. Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California. Varnish cation ratios have been calibrated to independently dated surfaces in the Coso volcanic field and vicinity in California. Tentative absolute dates have been assigned to geomorphic surfaces in the Coso area. Cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern North America and to demonstrate that varnish at the South Stoddard locality, Mojave Desert, did not form in 25 yr.

@article{doi1010160033589483900650,
    author = "Dorn, Ronald I.",
    title = "Cation-Ratio Dating: A New Rock Varnish Age-Determination Technique",
    year = "1983",
    journal = "Quaternary Research",
    abstract = "Abstract Rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands. All varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing. They are valid only where manganese is not remobilized after deposition, for example, in most arid environments. The premise of a new age-determination method, cation-ratio dating, is that the ratio of the more mobile cations (e.g., K and Ca) to titanium in varnish decreases with time. Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California. Varnish cation ratios have been calibrated to independently dated surfaces in the Coso volcanic field and vicinity in California. Tentative absolute dates have been assigned to geomorphic surfaces in the Coso area. Cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern North America and to demonstrate that varnish at the South Stoddard locality, Mojave Desert, did not form in 25 yr.",
    url = "https://doi.org/10.1016/0033-5894(83)90065-0",
    doi = "10.1016/0033-5894(83)90065-0",
    openalex = "W2067887526",
    references = "doi101007bf00378791, doi1010160009254179900858, doi101016s0140196318317452, doi101021ba19680073ch021, doi101038276489a0, doi10108000401706196210490038, doi101086627271, doi101126science2044394701, doi10113000167606195869487cdodv20co2, doi101177030913338200600301, doi102136sssaj195803615995002200010017x, doi105962bhltitle132168, potter1977desert"
}

The relative importance of texture, structure, organic matter and clay mineralogy to the nature of the soil moisture characteristic is examined for an extensive group of Australian soils using numerical classification and diagnostic methods. The presence of pedality, particle size composition and grade of structure were the soil properties most consistently associated with differences between the groups of soils with similar moisture characteristics. By association, field texture was shown to be a useful property. Although the presence of pedality and grade of structure were important, the shape and size of ped had only weak associations with differences in the soil moisture retention. Montmorillonite, iron oxide, vermiculite and quartz were the minerals in the clay size fraction which appeared to be important if they were present. In contrast, the presence of illite did not show any strong associations with a particular position or form of the moisture characteristic. The soil moisture characteristic was successfully modelled as a power function. It appears that being able to group and classify the soil moisture characteristic and then to provide a description of these groups both in terms of soil properties and model parameters is a valuable means of developing simple predictive models for field soils. The error of our predictions for 44 horizons based on this simple approach appears to be only marginally larger than that encountered in conventional laboratory methods, and in view of soil heterogeneity it is argued that following further development these predictions may be adequate in many hydrological and agricultural applications.

@article{doi101071sr9830015,
    author = "WILLIAMS, J. B. and Prebble, RE and Williams, WT and Hignett, C.",
    title = "The influence of texture, structure and clay mineralogy on the soil moisture characteristic",
    year = "1983",
    journal = "Australian Journal of Soil Research",
    abstract = "The relative importance of texture, structure, organic matter and clay mineralogy to the nature of the soil moisture characteristic is examined for an extensive group of Australian soils using numerical classification and diagnostic methods. The presence of pedality, particle size composition and grade of structure were the soil properties most consistently associated with differences between the groups of soils with similar moisture characteristics. By association, field texture was shown to be a useful property. Although the presence of pedality and grade of structure were important, the shape and size of ped had only weak associations with differences in the soil moisture retention. Montmorillonite, iron oxide, vermiculite and quartz were the minerals in the clay size fraction which appeared to be important if they were present. In contrast, the presence of illite did not show any strong associations with a particular position or form of the moisture characteristic. The soil moisture characteristic was successfully modelled as a power function. It appears that being able to group and classify the soil moisture characteristic and then to provide a description of these groups both in terms of soil properties and model parameters is a valuable means of developing simple predictive models for field soils. The error of our predictions for 44 horizons based on this simple approach appears to be only marginally larger than that encountered in conventional laboratory methods, and in view of soil heterogeneity it is argued that following further development these predictions may be adequate in many hydrological and agricultural applications.",
    url = "https://doi.org/10.1071/sr9830015",
    doi = "10.1071/sr9830015",
    openalex = "W1990908911"
}

Abstract The physicochemical properties of smectite clay minerals that determine their industrial utilization are reviewed. Smectite is the name used for a group of phyllosilicate mineral species, the most important of which are montmorillonite, beidellite, nontronite, saponite and hectorite. These and several other less common species are differentiated by variations in chemical composition involving substitutions of Al for Si in tetrahedral cation sites and Al, Fe, Mg and Li in octahedral cation sites. Smectite clays have a variable net negative charge, which is balanced by Na, Ca, Mg and, or, H adsorbed externally on interlamellar surfaces. The structure, chemical composition, exchangeable ion type and small crystal size of smectite clays are responsible for several unique properties, including a large chemically active surface area, a high cation exchange capacity, interlamellar surfaces having unusual hydration characteristics, and sometimes the ability to modify strongly the flow behaviour of liquids. In terms of major industrial and chemical uses, natural smectite clays can be divided into three categories, Na smectites, Ca-Mg smectites and Fuller’s or acid earths. Large volumes of Na smectites and Na-exchanged Ca-M g smectites and Fuller’s earth are directly used in the foundry, oil well drilling, wine, and iron ore and feed pelletizing industries, and are also used in civil engineering to impede water movement. Significant volumes of Na smectites are used for various purposes in the manufacturing of many industrial, chemical and consumer products. Large quantities of Ca-M g smectites are used directly in iron foundries, in agricultural industries and for filtering and decolorizing various types of oils. A significant fraction of the Ca-M g smectites used for decolorizing has been acid treated. Large volumes of Fuller’s or acid earths are commercially used for preparing animal litter trays and oil and grease absorbents, as carriers for insecticides, and for decolorizing of oils and fats. Natural Na smectites occur in commercial quantities in only a few places, but Ca-M g smectite and Fuller’s earth deposits of considerable size occur on almost every continent.

@article{doi101098rsta19840036,
    author = "Odom, Ira Edgar",
    title = "Smectite clay minerals: properties and uses",
    year = "1984",
    journal = "Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences",
    abstract = "Abstract The physicochemical properties of smectite clay minerals that determine their industrial utilization are reviewed. Smectite is the name used for a group of phyllosilicate mineral species, the most important of which are montmorillonite, beidellite, nontronite, saponite and hectorite. These and several other less common species are differentiated by variations in chemical composition involving substitutions of Al for Si in tetrahedral cation sites and Al, Fe, Mg and Li in octahedral cation sites. Smectite clays have a variable net negative charge, which is balanced by Na, Ca, Mg and, or, H adsorbed externally on interlamellar surfaces. The structure, chemical composition, exchangeable ion type and small crystal size of smectite clays are responsible for several unique properties, including a large chemically active surface area, a high cation exchange capacity, interlamellar surfaces having unusual hydration characteristics, and sometimes the ability to modify strongly the flow behaviour of liquids. In terms of major industrial and chemical uses, natural smectite clays can be divided into three categories, Na smectites, Ca-Mg smectites and Fuller’s or acid earths. Large volumes of Na smectites and Na-exchanged Ca-M g smectites and Fuller’s earth are directly used in the foundry, oil well drilling, wine, and iron ore and feed pelletizing industries, and are also used in civil engineering to impede water movement. Significant volumes of Na smectites are used for various purposes in the manufacturing of many industrial, chemical and consumer products. Large quantities of Ca-M g smectites are used directly in iron foundries, in agricultural industries and for filtering and decolorizing various types of oils. A significant fraction of the Ca-M g smectites used for decolorizing has been acid treated. Large volumes of Fuller’s or acid earths are commercially used for preparing animal litter trays and oil and grease absorbents, as carriers for insecticides, and for decolorizing of oils and fats. Natural Na smectites occur in commercial quantities in only a few places, but Ca-M g smectite and Fuller’s earth deposits of considerable size occur on almost every continent.",
    url = "https://doi.org/10.1098/rsta.1984.0036",
    doi = "10.1098/rsta.1984.0036",
    openalex = "W2069694403",
    references = "doi101016s0070457109x70019"
}

In the Antarctic cold desert (Ross Desert), the survival of the cryptoendolithic microorganisms that colonize the near-surface layer of porous sandstone rocks depends on a precarious equilibrium of biological and geological factors. An unfavorable shift of this equilibrium results in death, and this may be followed by formation of trace fossils that preserve the characteristic iron-leaching pattern caused by microbial activity. Similar microbial trace fossil may exist in the geological record. If life ever arose on early Mars, similar processes may have occurred there and left recognizable traces.

@article{doi101126science11536571,
    author = "Friedmann, E. Imre and Weed, R.M.",
    title = "Microbial Trace-Fossil Formation, Biogenous, and Abiotic Weathering in the Antarctic Cold Desert",
    year = "1987",
    journal = "Science",
    abstract = "In the Antarctic cold desert (Ross Desert), the survival of the cryptoendolithic microorganisms that colonize the near-surface layer of porous sandstone rocks depends on a precarious equilibrium of biological and geological factors. An unfavorable shift of this equilibrium results in death, and this may be followed by formation of trace fossils that preserve the characteristic iron-leaching pattern caused by microbial activity. Similar microbial trace fossil may exist in the geological record. If life ever arose on early Mars, similar processes may have occurred there and left recognizable traces.",
    url = "https://doi.org/10.1126/science.11536571",
    doi = "10.1126/science.11536571",
    openalex = "W2035849767",
    references = "doi101177030913338200600301"
}

@article{openalexw1498145917,
    author = "Wilson, M. J.",
    title = "A Handbook of determinative methods in clay mineralogy",
    year = "1987",
    journal = "Medical Entomology and Zoology",
    url = "https://openalex.org/W1498145917",
    openalex = "W1498145917"
}

The field of pillared clays has recently expanded by discoveries of new pillaring species, and extensions of the pillaring concept to several new layered materials, including micas, silicic acids, zirconium phosphates and layered metal hydroxides. Pillaring in various sheet materials has been done with clusters of cations, anions and neutral molecules, resulting in a new compositional versatility. Pillared layered materials now represent a rapidly expanding category of catalytic microporous materials having a high degree of chemical and structural diversity.

@incollection{doi101021bk19880368ch019,
    author = "Vaughan, D. E. W.",
    title = "Recent Developments in Pillared Interlayered Clays",
    year = "1988",
    booktitle = "ACS symposium series",
    abstract = "The field of pillared clays has recently expanded by discoveries of new pillaring species, and extensions of the pillaring concept to several new layered materials, including micas, silicic acids, zirconium phosphates and layered metal hydroxides. Pillaring in various sheet materials has been done with clusters of cations, anions and neutral molecules, resulting in a new compositional versatility. Pillared layered materials now represent a rapidly expanding category of catalytic microporous materials having a high degree of chemical and structural diversity.",
    url = "https://doi.org/10.1021/bk-1988-0368.ch019",
    doi = "10.1021/bk-1988-0368.ch019",
    openalex = "W2505288476"
}

The nature and production of x-rays diffraction effects structures, composition, properties and occurences of clay minerals sample preparation techniques for clay minerals sample preparation techniques for clay minerals identification of individual clay minerals and associated minerals identification of mixed layered clay minerals quantitative analysis.

@book{openalexw2101874561,
    author = "Moore, D. M. and Reynolds, Robert C.",
    title = "X-Ray Diffraction and the Identification and Analysis of Clay Minerals",
    year = "1989",
    abstract = "The nature and production of x-rays diffraction effects structures, composition, properties and occurences of clay minerals sample preparation techniques for clay minerals sample preparation techniques for clay minerals identification of individual clay minerals and associated minerals identification of mixed layered clay minerals quantitative analysis.",
    openalex = "W2101874561"
}

Pecked figures – seeming to depict humans, animals, objects and ‘abstract’ shapes - are an important and recalcitrant aspect to the archaeology of the desert USA, in the Great Basin and the Southwest. Where they are covered by desert varnish, they provide an opportunity for an absolute dating by cation-ratio method. Here - as they did not for a similar study in South Australia reported in an earlier Antiquity - the cation-ratio dates do seem to run alongside the chronological pattern inferred by conventional means.

@article{doi101017s0003598x00079692,
    author = "Loendorf, Lawrence L.",
    title = "Cation-ratio varnish dating and petroglyph chronology in southeastern Colorado",
    year = "1991",
    journal = "Antiquity",
    abstract = "Pecked figures – seeming to depict humans, animals, objects and ‘abstract’ shapes - are an important and recalcitrant aspect to the archaeology of the desert USA, in the Great Basin and the Southwest. Where they are covered by desert varnish, they provide an opportunity for an absolute dating by cation-ratio method. Here - as they did not for a similar study in South Australia reported in an earlier Antiquity - the cation-ratio dates do seem to run alongside the chronological pattern inferred by conventional means.",
    url = "https://doi.org/10.1017/s0003598x00079692",
    doi = "10.1017/s0003598x00079692",
    openalex = "W2464647092",
    references = "doi1010160033589483900650"
}

@book{doi1010079789401107273,
    author = "Wilson, M. J.",
    title = "Clay Mineralogy: Spectroscopic and Chemical Determinative Methods",
    year = "1994",
    url = "https://doi.org/10.1007/978-94-011-0727-3",
    doi = "10.1007/978-94-011-0727-3",
    openalex = "W1601104640",
    references = "doi10100797894011072737"
}

Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene—aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene—aged surfaces containing soils with strongly developed argillic (clay—rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weathering—resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long—lived Larrea tridentata. Individuals of this shrub species can exhibit clone—like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long—lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata. Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay—rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought—deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal variability in water availability. Syntheses involving the study of various ecological processes (e.g., plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of arid land systems.

@article{doi1023072937038,
    author = "McAuliffe, Joseph R.",
    title = "Landscape Evolution, Soil Formation, and Ecological Patterns and Processes in Sonoran Desert Bajadas",
    year = "1994",
    journal = "Ecological Monographs",
    abstract = "Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene—aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene—aged surfaces containing soils with strongly developed argillic (clay—rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weathering—resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long—lived Larrea tridentata. Individuals of this shrub species can exhibit clone—like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long—lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata. Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay—rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought—deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal variability in water availability. Syntheses involving the study of various ecological processes (e.g., plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of arid land systems.",
    url = "https://doi.org/10.2307/2937038",
    doi = "10.2307/2937038",
    openalex = "W2099032229",
    references = "doi10113000917613198715504ioeapp20co2"
}

Manganiferous rock varnish collected from Death Valley and Antarctica contains the smallest known terrestrial sedimentary deposits, with some layers only a few nanometers thick. Irregularities in these nanometer-scale layers are consistent with shrinking, cracking, and weathering of clay minerals. In the Death Valley rock varnish, very different High Resolution Transmission Electron Microscope (HRTEM) textures coexist that may be related to climatic change. HRTEM observations contradict previous microbial models of Mn-Fe enhancement, requiring a new three-step model of biomineralization and diagenesis for varnish formation.

@article{doi101086629726,
    author = "Krinsley, David and Dorn, Ronald I. and Tovey, N. Keith",
    title = "Nanometer-Scale Layering in Rock Varnish: Implications for Genesis and Paleoenvironmental Interpretation",
    year = "1995",
    journal = "The Journal of Geology",
    abstract = "Manganiferous rock varnish collected from Death Valley and Antarctica contains the smallest known terrestrial sedimentary deposits, with some layers only a few nanometers thick. Irregularities in these nanometer-scale layers are consistent with shrinking, cracking, and weathering of clay minerals. In the Death Valley rock varnish, very different High Resolution Transmission Electron Microscope (HRTEM) textures coexist that may be related to climatic change. HRTEM observations contradict previous microbial models of Mn-Fe enhancement, requiring a new three-step model of biomineralization and diagenesis for varnish formation.",
    url = "https://doi.org/10.1086/629726",
    doi = "10.1086/629726",
    openalex = "W2089231110"
}

@article{doi102134jeq199500472425002400040041x,
    author = "Bigham, Jerry M.",
    title = "Clay Mineralogy: Spectroscopic and Chemical Determinative Methods",
    year = "1995",
    journal = "Journal of Environmental Quality",
    url = "https://doi.org/10.2134/jeq1995.00472425002400040041x",
    doi = "10.2134/jeq1995.00472425002400040041x",
    openalex = "W2079107066"
}

Abstract The palygorskite-sepiolite group of clay minerals has a wide range of industrial applications derived mainly from its sorptive, rheological and catalytic properties which are based on the fabric, surface area, porosity, crystal morphology, structure and composition of these minerals. For assessing potential industrial uses, the mineralogical and chemical composition of the clay and its basic physical and physico-chemical parameters must be determined. Then some particular properties of commercial interest can be modified and improved by appropriate thermal, mechanical and acid treatments, surface active agents, organo-mineral derivatives formation, etc. In this paper, a revision of the principal characteristics of commercial palygorskite-sepiolite clays is presented, and potential uses are suggested according to these data. New products and applications are being investigated and those concerning environmental protection in particular, are noted. Finally, possible health effects of these elongate clay minerals are discussed.

@article{doi101180claymin1996031401,
    author = "Galan, E.",
    title = "Properties and applications of palygorskite-sepiolite clays",
    year = "1996",
    journal = "Clay Minerals",
    abstract = "Abstract The palygorskite-sepiolite group of clay minerals has a wide range of industrial applications derived mainly from its sorptive, rheological and catalytic properties which are based on the fabric, surface area, porosity, crystal morphology, structure and composition of these minerals. For assessing potential industrial uses, the mineralogical and chemical composition of the clay and its basic physical and physico-chemical parameters must be determined. Then some particular properties of commercial interest can be modified and improved by appropriate thermal, mechanical and acid treatments, surface active agents, organo-mineral derivatives formation, etc. In this paper, a revision of the principal characteristics of commercial palygorskite-sepiolite clays is presented, and potential uses are suggested according to these data. New products and applications are being investigated and those concerning environmental protection in particular, are noted. Finally, possible health effects of these elongate clay minerals are discussed.",
    url = "https://doi.org/10.1180/claymin.1996.031.4.01",
    doi = "10.1180/claymin.1996.031.4.01",
    openalex = "W2154370331",
    references = "doi101016s0070457109x70019, doi1010970001069419810100000013"
}

Abstract The clay minerals kaolin, smectite and palygorskite-sepiolite are among the world's most important and useful industrial minerals. Clay minerals are important in a number of geological applications such as stratigraphic correlations, indicators of environments of deposition and temperature for generation of hydrocarbons. In agriculture, the clay minerals are a major component of soils and determinant of soil properties. The clay minerals are important in construction where they are a major constituent in brick and tile. The physical and chemical properties of the clay minerals determine their utilization in the process industries. What about tomorrow? Processing techniques will be improved and new equipment will be available so that improved clay mineral products will be available. Pillared clays and nanocomposites will become important. Further developments in organoclay technology and surface treatments will provide new usages for these special clays. Tomorrow will see further growth and utilization of the clay minerals.

@article{doi101180000985599546055,
    author = "Murray, Haydn H.",
    title = "Applied clay mineralogy today and tomorrow",
    year = "1999",
    journal = "Clay Minerals",
    abstract = "Abstract The clay minerals kaolin, smectite and palygorskite-sepiolite are among the world's most important and useful industrial minerals. Clay minerals are important in a number of geological applications such as stratigraphic correlations, indicators of environments of deposition and temperature for generation of hydrocarbons. In agriculture, the clay minerals are a major component of soils and determinant of soil properties. The clay minerals are important in construction where they are a major constituent in brick and tile. The physical and chemical properties of the clay minerals determine their utilization in the process industries. What about tomorrow? Processing techniques will be improved and new equipment will be available so that improved clay mineral products will be available. Pillared clays and nanocomposites will become important. Further developments in organoclay technology and surface treatments will provide new usages for these special clays. Tomorrow will see further growth and utilization of the clay minerals.",
    url = "https://doi.org/10.1180/000985599546055",
    doi = "10.1180/000985599546055",
    openalex = "W2102843004",
    references = "doi101016016913179190014z, doi1010160169131795000294, doi1010160920586188850028, doi101016s0070457108x70323, doi101016s0070457109062128, doi101021bk19880368ch019, doi10108011035896209447314, doi1010970001069419650700000019, doi101180claymin1996031401, doi101180mono5"
}

Rock varnish is a thin coating (<200 µm) of a cocktail rich in Mn, Fe, and clay minerals that is ubiquitous in desert regions. It has become the center of a contentious controversy revolving around its use to date geomorphic surfaces and/or to evaluate past climate conditions. We observe pronounced temporal variations in Mn and Ba concentration that are similar over large regions and that likely relate to variations in paleo-wetness. The mode of formation of varnish remains uncertain, but anthropogenic Pb concentrated in outermost varnish layers indicates its continued formation, and experiments using cosmogenic Be suggest that, while precipitation is a primary control, dust, dew, and aerosols may also be important in delivering the ingredients of varnish. We suggest several steps that may lead to rejuvenation and future breakthrough in varnish studies.

@article{doi1011301052517320010110004rvrodw20co2,
    author = "Broecker, Wallace S. and Liu, Tanzhuo",
    title = "Rock Varnish: Recorder of Desert Wetness?",
    year = "2001",
    journal = "GSA Today",
    abstract = "Rock varnish is a thin coating (<200 µm) of a cocktail rich in Mn, Fe, and clay minerals that is ubiquitous in desert regions. It has become the center of a contentious controversy revolving around its use to date geomorphic surfaces and/or to evaluate past climate conditions. We observe pronounced temporal variations in Mn and Ba concentration that are similar over large regions and that likely relate to variations in paleo-wetness. The mode of formation of varnish remains uncertain, but anthropogenic Pb concentrated in outermost varnish layers indicates its continued formation, and experiments using cosmogenic Be suggest that, while precipitation is a primary control, dust, dew, and aerosols may also be important in delivering the ingredients of varnish. We suggest several steps that may lead to rejuvenation and future breakthrough in varnish studies.",
    url = "https://doi.org/10.1130/1052-5173(2001)011<0004:rvrodw>2.0.co;2",
    doi = "10.1130/1052-5173(2001)011<0004:rvrodw>2.0.co;2",
    openalex = "W2081735009",
    references = "doi101006qres19931084, doi101016003101829090217u, doi1010160033589483900650, doi1010160033589487900469, doi101038276489a0, doi101111j136530911991tb00376x, doi101111j146783061996tb01750x, doi101126science2615118198, doi1011300091761319950230613chsedo23co2, doi10113000917613200028183hfdrvg20co2, openalexw1465883202, openalexw2912219260, potter1977desert"
}

Structure and surface acidity of clay minerals introduction to organo-clay complexes and interactions interactions of vermiculites with organic compounds organophilicity and hydrophobicity of organo-clays adsorption of organic cations on clays - experimental results and modelling nuclear magnetic resonance spectroscopy thermal analysis of organo-clay complexes IR-spectroscopy and thermo-IR-spectroscopy in the study of the fine structure of organo-clay complexes staining of clay minerals and visible-absorption spectroscopy of dye-dye complexes clay catalysis in reactions of organic matter organo-materials and organo-clay interactions and the origin of life on Earth.

@book{doi1012019781482270945,
    title = "Organo-Clay Complexes and Interactions",
    year = "2001",
    abstract = "Structure and surface acidity of clay minerals introduction to organo-clay complexes and interactions interactions of vermiculites with organic compounds organophilicity and hydrophobicity of organo-clays adsorption of organic cations on clays - experimental results and modelling nuclear magnetic resonance spectroscopy thermal analysis of organo-clay complexes IR-spectroscopy and thermo-IR-spectroscopy in the study of the fine structure of organo-clay complexes staining of clay minerals and visible-absorption spectroscopy of dye-dye complexes clay catalysis in reactions of organic matter organo-materials and organo-clay interactions and the origin of life on Earth.",
    url = "https://doi.org/10.1201/9781482270945",
    doi = "10.1201/9781482270945",
    openalex = "W1861064209"
}

X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.

@article{doi102138am20015611,
    author = "McKeown, David A. and Post, Jeffrey E.",
    title = "Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy",
    year = "2001",
    journal = "American Mineralogist",
    abstract = "X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.",
    url = "https://doi.org/10.2138/am-2001-5-611",
    doi = "10.2138/am-2001-5-611",
    openalex = "W2244524084",
    references = "doi101007bf00378791, doi1010160009254179900858, doi101016092145269400655f, doi101038276489a0, doi101107s0567740876007371, doi101107s0567740882004968, doi101111j136530911991tb00376x, doi101111j146783061996tb01750x, doi101346ccmn19860340503, openalexw1568216123, openalexw2146521523, openalexw2463361634, potter1977desert"
}

Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.

@article{doi101021es025955r,
    author = "Hofstetter, Thomas B. and Schwarzenbach, René P. and Haderlein, Stefan B.",
    title = "Reactivity of Fe(II) Species Associated with Clay Minerals",
    year = "2002",
    journal = "Environmental Science \& Technology",
    abstract = "Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10\% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.",
    url = "https://doi.org/10.1021/es025955r",
    doi = "10.1021/es025955r",
    openalex = "W2035758570",
    references = "doi102134jeq199500472425002400040041x"
}

Expansive clay soils—those that change significantly in volume with changes in water content—are the cause of distortions to structures that cost taxpayers several billion dollars annually in the United States. Much has been learned about their behavior over the past 60 years, and relatively successful methods have been developed to modify and stabilize them. This paper reviews some of the key advances developed over the past 60 years in improving our understanding of the nature and methods of modifying and stabilizing expansive clay soils. The state of the practice in stabilization is presented, and practical and research needs to help improve the state of the practice are discussed.

@article{doi101061asce089915612002146447,
    author = "Petry, Thomas M. and Little, Dallas N.",
    title = "Review of Stabilization of Clays and Expansive Soils in Pavements and Lightly Loaded Structures—History, Practice, and Future",
    year = "2002",
    journal = "Journal of Materials in Civil Engineering",
    abstract = "Expansive clay soils—those that change significantly in volume with changes in water content—are the cause of distortions to structures that cost taxpayers several billion dollars annually in the United States. Much has been learned about their behavior over the past 60 years, and relatively successful methods have been developed to modify and stabilize them. This paper reviews some of the key advances developed over the past 60 years in improving our understanding of the nature and methods of modifying and stabilizing expansive clay soils. The state of the practice in stabilization is presented, and practical and research needs to help improve the state of the practice are discussed.",
    url = "https://doi.org/10.1061/(asce)0899-1561(2002)14:6(447)",
    doi = "10.1061/(asce)0899-1561(2002)14:6(447)",
    openalex = "W1990083129",
    references = "doi1010970001069419531000000009"
}

Arsenic adsorption on amorphous Al and Fe oxides and the clay minerals, kaolinite, montmorillonite, and illite was investigated as a function of solution pH and As redox state, i.e., arsenite [As(III)] and arsenate [As(V)]. Arsenic adsorption experiments were carried out in batch systems to determine adsorption envelopes, amount of As(III), As(V), or both adsorbed as a function of solution pH per fixed total As concentration of 20 μ M As. Arsenate adsorption on oxides and clays was maximal at low pH and decreased with increasing pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. Arsenite adsorption exhibited parabolic behavior with an adsorption maximum around pH 8.5 for all materials. There was no competitive effect of the presence of equimolar arsenite on arsenate adsorption. The competitive effect of equimolar arsenate on arsenite adsorption was small and apparent only on kaolinite and illite in the pH range 6.5 to 9. The constant capacitance model was able to fit the arsenate and arsenite adsorption envelopes to obtain values of the intrinsic As surface complexation constants. These intrinsic surface complexation constants were then used in the model to predict competitive arsenate and arsenite adsorption from solutions containing equimolar As(III) and As(V) concentrations. The constant capacitance model was able to predict As adsorption from mixed As(III)‐As(V) solutions in systems where there was no competitive effect.

@article{doi102136sssaj20024130,
    author = "Goldberg, Sabine",
    title = "Competitive Adsorption of Arsenate and Arsenite on Oxides and Clay Minerals",
    year = "2002",
    journal = "Soil Science Society of America Journal",
    abstract = "Arsenic adsorption on amorphous Al and Fe oxides and the clay minerals, kaolinite, montmorillonite, and illite was investigated as a function of solution pH and As redox state, i.e., arsenite [As(III)] and arsenate [As(V)]. Arsenic adsorption experiments were carried out in batch systems to determine adsorption envelopes, amount of As(III), As(V), or both adsorbed as a function of solution pH per fixed total As concentration of 20 μ M As. Arsenate adsorption on oxides and clays was maximal at low pH and decreased with increasing pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. Arsenite adsorption exhibited parabolic behavior with an adsorption maximum around pH 8.5 for all materials. There was no competitive effect of the presence of equimolar arsenite on arsenate adsorption. The competitive effect of equimolar arsenate on arsenite adsorption was small and apparent only on kaolinite and illite in the pH range 6.5 to 9. The constant capacitance model was able to fit the arsenate and arsenite adsorption envelopes to obtain values of the intrinsic As surface complexation constants. These intrinsic surface complexation constants were then used in the model to predict competitive arsenate and arsenite adsorption from solutions containing equimolar As(III) and As(V) concentrations. The constant capacitance model was able to predict As adsorption from mixed As(III)‐As(V) solutions in systems where there was no competitive effect.",
    url = "https://doi.org/10.2136/sssaj2002.4130",
    doi = "10.2136/sssaj2002.4130",
    openalex = "W1981026196",
    references = "doi102136sssaj197203615995003600050024x"
}

An existing model of desert pavement formation suggests desert pavement clasts rise vertically on an accreting eolian mantle and the underlying vesicular horizon coevolves with pavement formation. Results presented here support this model and provide a mechanism whereby eolian material is transported from the ground surface to ped interiors, thereby increasing the thickness of the vesicular horizon underlying desert pavements. Basaltic desert pavements in the Cima Volcanic Field are underlain by a vesicular horizon with strong coarse columnar and strong medium platy soil structure. Peds collected from three sites along a topographic gradient were subsampled into seven ped domains to quantify physical, chemical, and micromorphological properties within peds and along the topographic transect. Ped interiors have up to 40% clay and 12% CaCO 3 whereas sediments adhering to ped sides have <7% clay and 2% CaCO 3 Argillans and siltans lining platy surfaces in ped centers and bottoms indicate desert dust is translocated vertically along the macropores between peds and horizontally along platy boundaries to ped interiors. Once soils develop a strong columnar and platy structure, translocation to ped interiors is enhanced. Calibrated radiocarbon ages between 5440 and 5045 BP for Av ped centers suggest enhanced dust flux and pedogenesis occurred during the drier middle Holocene, an inference supported by luminescence dating and correlations with regional eolian chronologies.

@article{doi102136sssaj20028780,
    author = "Anderson, K. C. and Wells, Stephen A. and Graham, Robert C.",
    title = "Pedogenesis of Vesicular Horizons, Cima Volcanic Field, Mojave Desert, California",
    year = "2002",
    journal = "Soil Science Society of America Journal",
    abstract = "An existing model of desert pavement formation suggests desert pavement clasts rise vertically on an accreting eolian mantle and the underlying vesicular horizon coevolves with pavement formation. Results presented here support this model and provide a mechanism whereby eolian material is transported from the ground surface to ped interiors, thereby increasing the thickness of the vesicular horizon underlying desert pavements. Basaltic desert pavements in the Cima Volcanic Field are underlain by a vesicular horizon with strong coarse columnar and strong medium platy soil structure. Peds collected from three sites along a topographic gradient were subsampled into seven ped domains to quantify physical, chemical, and micromorphological properties within peds and along the topographic transect. Ped interiors have up to 40\% clay and 12\% CaCO 3 whereas sediments adhering to ped sides have <7\% clay and 2\% CaCO 3 Argillans and siltans lining platy surfaces in ped centers and bottoms indicate desert dust is translocated vertically along the macropores between peds and horizontally along platy boundaries to ped interiors. Once soils develop a strong columnar and platy structure, translocation to ped interiors is enhanced. Calibrated radiocarbon ages between 5440 and 5045 BP for Av ped centers suggest enhanced dust flux and pedogenesis occurred during the drier middle Holocene, an inference supported by luminescence dating and correlations with regional eolian chronologies.",
    url = "https://doi.org/10.2136/sssaj2002.8780",
    doi = "10.2136/sssaj2002.8780",
    openalex = "W1996868670"
}

The mechanism of nanoclay exfoliation was investigated in epoxy−clay nanocomposites system. The elastic force exerted by cross-linked epoxy molecules inside the clay galleries was found responsible for exfoliation of clay layers from the intercalated tactoids. Complete exfoliation of clay galleries was observed under the conditions of slow increase of complex viscosity and fast rise of storage modulus. It was observed that faster intragallery polymerization, though expedited the exfoliation process, was not necessary for exfoliation. It was also observed that clays containing hydroxylated quaternary ammonium ions and quaternary ammonium ions with no polar functional groups produced exfoliated structures equally easily, provided the ratio of storage modulus to complex viscosity was maintained above 2−4 1/s. Both higher curing temperature and the presence of organically modified clay particles accelerated the formation of gels, and the gel time presented an upper bound of time available for exfoliation.

@article{doi101021ma021509c,
    author = "Park, Jong Hyun and Jana, Sadhan",
    title = "Mechanism of Exfoliation of Nanoclay Particles in Epoxy−Clay Nanocomposites",
    year = "2003",
    journal = "Macromolecules",
    abstract = "The mechanism of nanoclay exfoliation was investigated in epoxy−clay nanocomposites system. The elastic force exerted by cross-linked epoxy molecules inside the clay galleries was found responsible for exfoliation of clay layers from the intercalated tactoids. Complete exfoliation of clay galleries was observed under the conditions of slow increase of complex viscosity and fast rise of storage modulus. It was observed that faster intragallery polymerization, though expedited the exfoliation process, was not necessary for exfoliation. It was also observed that clays containing hydroxylated quaternary ammonium ions and quaternary ammonium ions with no polar functional groups produced exfoliated structures equally easily, provided the ratio of storage modulus to complex viscosity was maintained above 2−4 1/s. Both higher curing temperature and the presence of organically modified clay particles accelerated the formation of gels, and the gel time presented an upper bound of time available for exfoliation.",
    url = "https://doi.org/10.1021/ma021509c",
    doi = "10.1021/ma021509c",
    openalex = "W2137069897",
    references = "doi1010970001069419531000000009"
}

Identification of diagnostic soil organic matter (SOM) fractions and the mechanisms controlling their formation and turnover is critical for better understanding of C dynamics in soils. Enhanced microaggregate formation and stabilization of C due to reduced macroaggregate turnover has been proposed as a mechanism promoting C sequestration in no‐tillage (NT) compared with conventional tillage (CT) systems in temperate soils dominated by 2:1 clay mineralogy. We evaluated the contribution of macroaggregate‐protected microaggregates to total soil organic carbon (SOC) sequestration in NT relative to CT in three soils differing in clay mineralogy: a 2:1 clay‐dominated soil (2:1), a soil with mixed clay mineralogy [2:1 and 1:1] and oxides (mixed), and a soil dominated by (1:1) clay minerals and oxides (1:1). Microaggregates (mM) were isolated from macroaggregates from 0‐ to 5‐ and 5‐ to 20‐cm soil layers. Particulate organic matter (POM) located within the microaggregates (intra‐mM‐POM) was separated from POM outside of the microaggregates (inter‐mM‐POM) and the mineral fraction of the microaggregates (mineral‐mM). In all three soils, total SOC as well as microaggregate‐associated C (mM‐C) was greater with NT compared with CT. Although less than half of the total SOC under NT was associated with the microaggregate fraction, more than 90% of the total difference in SOC between NT and CT was explained by the difference in mM‐C in all three soils. Thus, we identified and isolated a fraction that explains almost the entire difference in total SOC between NT and CT across soils characterized by drastically different clay mineralogy.

@article{doi102136sssaj20041935,
    author = "Denef, Karolien and Six, Johan and Merckx, Roel and Paustian, Keith",
    title = "Carbon Sequestration in Microaggregates of No‐Tillage Soils with Different Clay Mineralogy",
    year = "2004",
    journal = "Soil Science Society of America Journal",
    abstract = "Identification of diagnostic soil organic matter (SOM) fractions and the mechanisms controlling their formation and turnover is critical for better understanding of C dynamics in soils. Enhanced microaggregate formation and stabilization of C due to reduced macroaggregate turnover has been proposed as a mechanism promoting C sequestration in no‐tillage (NT) compared with conventional tillage (CT) systems in temperate soils dominated by 2:1 clay mineralogy. We evaluated the contribution of macroaggregate‐protected microaggregates to total soil organic carbon (SOC) sequestration in NT relative to CT in three soils differing in clay mineralogy: a 2:1 clay‐dominated soil (2:1), a soil with mixed clay mineralogy [2:1 and 1:1] and oxides (mixed), and a soil dominated by (1:1) clay minerals and oxides (1:1). Microaggregates (mM) were isolated from macroaggregates from 0‐ to 5‐ and 5‐ to 20‐cm soil layers. Particulate organic matter (POM) located within the microaggregates (intra‐mM‐POM) was separated from POM outside of the microaggregates (inter‐mM‐POM) and the mineral fraction of the microaggregates (mineral‐mM). In all three soils, total SOC as well as microaggregate‐associated C (mM‐C) was greater with NT compared with CT. Although less than half of the total SOC under NT was associated with the microaggregate fraction, more than 90\% of the total difference in SOC between NT and CT was explained by the difference in mM‐C in all three soils. Thus, we identified and isolated a fraction that explains almost the entire difference in total SOC between NT and CT across soils characterized by drastically different clay mineralogy.",
    url = "https://doi.org/10.2136/sssaj2004.1935",
    doi = "10.2136/sssaj2004.1935",
    openalex = "W1965704018",
    references = "doi101016003807179500159x, doi101016s0038071700001796, doi101051agro2002043, doi101071sr9910815, doi101111j136523891982tb01755x, doi101111j147527431997tb00594x, doi102136sssaj198603615995005000030017x, doi102136sssaj199803615995006200050032x, doi102136sssaj19996351350x, doi105860choice355645"
}

Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

@article{doi1011800009855054040180,
    author = "Joussein, Emmanuel and Petit, Sabine and Churchman, G. Jock and Theng, Benny K.G. and Righi, D. and Delvaux, Bruno",
    title = "Halloysite clay minerals — a review",
    year = "2005",
    journal = "Clay Minerals",
    abstract = "Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.",
    url = "https://doi.org/10.1180/0009855054040180",
    doi = "10.1180/0009855054040180",
    openalex = "W2132656327",
    references = "doi101016s0070457109x70019, doi101017cbo9781139103848007, doi1010970001069419480200000020, doi101180mono4, doi101180mono5, doi102134jeq199500472425002400040041x, openalexw2267844404"
}

Rock varnish from Arizona's Whipple Mountains harbors a microbial community containing about 10(8) microorganisms g(-1) of varnish. Analyses of varnish phospholipid fatty acids and rRNA gene libraries reveal a community comprised of mostly Proteobacteria but also including Actinobacteria, eukaryota, and a few members of the Archaea. Rock varnish represents a significant niche for microbial colonization.

@article{doi101128aem722170817152006,
    author = "Kuhlman, K. R. and Fusco, William G. and Duc, Myron T. La and Allenbach, Lisa and Ball, Christopher L. and Kuhlman, G. M. and Anderson, Robert C. and Erickson, Issac K. and Stuecker, Tara and Benardini, James N. and Strap, Janice L. and Crawford, Ronald L.",
    title = "Diversity of Microorganisms within Rock Varnish in the Whipple Mountains, California",
    year = "2006",
    journal = "Applied and Environmental Microbiology",
    abstract = "Rock varnish from Arizona's Whipple Mountains harbors a microbial community containing about 10(8) microorganisms g(-1) of varnish. Analyses of varnish phospholipid fatty acids and rRNA gene libraries reveal a community comprised of mostly Proteobacteria but also including Actinobacteria, eukaryota, and a few members of the Archaea. Rock varnish represents a significant niche for microbial colonization.",
    url = "https://doi.org/10.1128/aem.72.2.1708-1715.2006",
    doi = "10.1128/aem.72.2.1708-1715.2006",
    openalex = "W2108211322",
    references = "doi101006qres19931084, doi101007bf00388810, doi101016016895259090186a, doi101016b9780123721808500421, doi101016jicarus200411022, doi101016s0022283605803602, doi101016s0169555x02003318, doi101093bioinformatics178754, doi101128aem5637827871990, doi101128aem713150115062005, doi101128mr5911431691995, doi1011301052517320010110004rvrodw20co2, openalexw1589603082, openalexw3217097258"
}

@article{doi101130g223521,
    author = "Perry, Randall S. and Lynne, Bridget Y. and Sephton, Mark A. and Kolb, Vera M. and Perry, Carole C. and Staley, James T.",
    title = "Baking black opal in the desert sun: The importance of silica in desert varnish",
    year = "2006",
    journal = "Geology",
    url = "https://doi.org/10.1130/g22352.1",
    doi = "10.1130/g22352.1",
    openalex = "W2036499309",
    references = "doi101007bf02097739, doi101016jsedgeo200505012, doi101016s0169555x02003318, doi101016s0927776501001436, doi101038276489a0, doi101111j136530911991tb00376x, doi101126science21345131245, doi101126science21545361093, doi101128aem722170817152006, doi101306011704740561, doi105962bhltitle132168, openalexw649466005, potter1977desert"
}

The composition and textural fabrics of clay materials control their applications for constructional and other purposes. Discussed below are the most commonly used techniques to characterize these aspects of clay materials. The analytical approaches, their advantages and their limitations are summarized without giving detailed discussions of each technique, but references to more detailed accounts are provided. The overall sampling and analytical approach, and how it will help define and/or solve potential problems, needs to be carefully planned. A preliminary reconnaissance sampling and analysis programme, taking into account desk studies (see Chapter 7) and any related available information needs to be made before embarking on full scale detailed and comprehensive analyses. Sampling and sample preservation methods are too often inadequately designed and this can limit or even negate the results from later sophisticated and costly analyses. Sampling needs to be planned to provide adequate representative samples taking into account the nature of the problem to be answered and the heterogeneity of the materials—clay materials are often very heterogeneous, even on a small scale. Once sampled, the material needs to be carefully preserved so as to minimize alteration of the sample from its original state prior to analysis. For example, drying of samples needs to be undertaken at temperatures which will not dehydrate minerals such as gypsum, i.e. at 40°C rather than the 105°C that is often required in standard procedures. Also the sampling and preservation procedures need to be designed in the context of the objectives of the investigation and

@article{doi101144gsleng20060210108,
    title = "8. Compositional and textural analysis of clay materials",
    year = "2006",
    journal = "Geological Society London Engineering Geology Special Publications",
    abstract = "The composition and textural fabrics of clay materials control their applications for constructional and other purposes. Discussed below are the most commonly used techniques to characterize these aspects of clay materials. The analytical approaches, their advantages and their limitations are summarized without giving detailed discussions of each technique, but references to more detailed accounts are provided. The overall sampling and analytical approach, and how it will help define and/or solve potential problems, needs to be carefully planned. A preliminary reconnaissance sampling and analysis programme, taking into account desk studies (see Chapter 7) and any related available information needs to be made before embarking on full scale detailed and comprehensive analyses. Sampling and sample preservation methods are too often inadequately designed and this can limit or even negate the results from later sophisticated and costly analyses. Sampling needs to be planned to provide adequate representative samples taking into account the nature of the problem to be answered and the heterogeneity of the materials—clay materials are often very heterogeneous, even on a small scale. Once sampled, the material needs to be carefully preserved so as to minimize alteration of the sample from its original state prior to analysis. For example, drying of samples needs to be undertaken at temperatures which will not dehydrate minerals such as gypsum, i.e. at 40°C rather than the 105°C that is often required in standard procedures. Also the sampling and preservation procedures need to be designed in the context of the objectives of the investigation and",
    url = "https://doi.org/10.1144/gsl.eng.2006.021.01.08",
    doi = "10.1144/gsl.eng.2006.021.01.08",
    openalex = "W4234508544",
    references = "doi10100797894011072737"
}

The surge of interest in and scientific publications on the structure and properties of nanocomposites has made it rather difficult for the novice to comprehend the physical structure of these new materials and the relationship between their properties and those of the conventional range of composite materials. Some of the questions that arise are: How should the reinforcement volume fraction be calculated? How can the clay gallery contents be assessed? How can the ratio of intercalate to exfoliate be found? Does polymerization occur in the clay galleries? How is the crystallinity of semi-crystalline polymers affected by intercalation? What role do the mobilities of adsorbed molecules and clay platelets have? How much information can conventional X-ray diffraction offer? What is the thermodynamic driving force for intercalation and exfoliation? What is the elastic modulus of clay platelets? The growth of computer simulation techniques applied to clay materials has been rapid, with insight gained into the structure, dynamics and reactivity of polymer-clay systems. However these techniques operate on the basis of approximations, which may not be clear to the non-specialist. This critical review attempts to assess these issues from the viewpoint of traditional composites thereby embedding these new materials in a wider context to which conventional composite theory can be applied. (210 references).

@article{doi101039b702653f,
    author = "Chen, Biqiong and Evans, Julian and Greenwell, H. Chris and Boulet, Pascal and Coveney, Peter V. and Bowden, A. and Whiting, Andrew",
    title = "A critical appraisal of polymer–clay nanocomposites",
    year = "2007",
    journal = "Chemical Society Reviews",
    abstract = "The surge of interest in and scientific publications on the structure and properties of nanocomposites has made it rather difficult for the novice to comprehend the physical structure of these new materials and the relationship between their properties and those of the conventional range of composite materials. Some of the questions that arise are: How should the reinforcement volume fraction be calculated? How can the clay gallery contents be assessed? How can the ratio of intercalate to exfoliate be found? Does polymerization occur in the clay galleries? How is the crystallinity of semi-crystalline polymers affected by intercalation? What role do the mobilities of adsorbed molecules and clay platelets have? How much information can conventional X-ray diffraction offer? What is the thermodynamic driving force for intercalation and exfoliation? What is the elastic modulus of clay platelets? The growth of computer simulation techniques applied to clay materials has been rapid, with insight gained into the structure, dynamics and reactivity of polymer-clay systems. However these techniques operate on the basis of approximations, which may not be clear to the non-specialist. This critical review attempts to assess these issues from the viewpoint of traditional composites thereby embedding these new materials in a wider context to which conventional composite theory can be applied. (210 references).",
    url = "https://doi.org/10.1039/b702653f",
    doi = "10.1039/b702653f",
    openalex = "W2024326866",
    references = "doi102136sssaj196303615995002700020003x"
}

Rock varnish is a very slow‐growing nanostratigraphic coating consisting of approximately 70% fine‐grained clay and 30% iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates. The microbial diversity associated with rock varnish collected from the hyperarid Yungay region of the Atacama Desert was investigated using culture‐independent biomolecular methods and an adenosine triphosphate (ATP) assay. The extraction of DNA from rock varnish collected at Yungay, a region in which little to no DNA has been extracted from the surface soil (<1 cm) to date, indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent. The clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms, ranging in diversity from Cyanobacterial to á‐proteobacteria lineages. The findings also show that only a few micrometers of varnish material are enough to shelter microbes like Chroococcidiopsis spp. from the intense ultraviolet radiation present in the Atacama Desert. Whether or not microorganisms are involved in its nucleation and/or growth, rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale.

@article{doi1010292007jg000677,
    author = "Kuhlman, K. R. and Venkat, Parth and Duc, Myron T. La and Kuhlman, G. M. and McKay, Christopher P.",
    title = "Evidence of a microbial community associated with rock varnish at Yungay, Atacama Desert, Chile",
    year = "2008",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Rock varnish is a very slow‐growing nanostratigraphic coating consisting of approximately 70\% fine‐grained clay and 30\% iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates. The microbial diversity associated with rock varnish collected from the hyperarid Yungay region of the Atacama Desert was investigated using culture‐independent biomolecular methods and an adenosine triphosphate (ATP) assay. The extraction of DNA from rock varnish collected at Yungay, a region in which little to no DNA has been extracted from the surface soil (<1 cm) to date, indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent. The clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms, ranging in diversity from Cyanobacterial to á‐proteobacteria lineages. The findings also show that only a few micrometers of varnish material are enough to shelter microbes like Chroococcidiopsis spp. from the intense ultraviolet radiation present in the Atacama Desert. Whether or not microorganisms are involved in its nucleation and/or growth, rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale.",
    url = "https://doi.org/10.1029/2007jg000677",
    doi = "10.1029/2007jg000677",
    openalex = "W2027173010",
    references = "doi101038345063a0, doi101093nar22224673, doi101093nargkg039, doi101093nargki038, doi10109900207713444846, doi101111j157469761997tb00351x, doi101128aem5637827871990, doi101128aem713150115062005, doi101128mmbr5911431691995, doi101128mr5911431691995"
}

The study of terrestrial geomicrobiology and its relationship to rock weathering processes is an essential tool in developing analogues for similar processes that may have occurred on Mars. Most studies of manganese-enhanced rock varnish have focused on samples taken from warm arid desert regions. Here, we examine samples obtained from eolian-abraded lava flows of the 4700-4800 m high Ashikule Basin in Tibet. Because it receives approximately 300 mm of precipitation annually, this site is nowhere near as dry as Atacama Desert locales. However, the dusty, sulfate-rich, high-altitude and high-UV flux environment of the Tibetan locale offers new insight into rock varnish formation processes in a terrestrial environment that displays some attributes similar to those expected on early Mars. Microprobe measurements reveal that Mn enhancements in varnish are two orders of magnitude above the dust source, but Fe is only enhanced by a factor of three. Manganese-enhancing bacterial forms are not abundant but are still approximately 3 times more common than in Mojave and Sonoran Desert varnishes. In addition to its occurrence in subaerial positions, Tibetan varnish also occurs in micron-scale "pods" enveloped by silica glaze and as remobilized constituents that have migrated into the underlying weathering rind. A lack of surficial Mn-rich varnish, therefore, might not imply the absence of varnish. In contrast to suggestions that silica glaze might be a good source of microbial fossils and a key to varnish formation, we did not observe any clear microfossil forms entombed in silica glaze; further, there is no gradation between varnish and silica glaze but only distinct contacts.

@article{doi101089ast20080238,
    author = "Krinsley, David and Dorn, Ronald I. and DiGregorio, Barry E.",
    title = "Astrobiological Implications of Rock Varnish in Tibet",
    year = "2009",
    journal = "Astrobiology",
    abstract = {The study of terrestrial geomicrobiology and its relationship to rock weathering processes is an essential tool in developing analogues for similar processes that may have occurred on Mars. Most studies of manganese-enhanced rock varnish have focused on samples taken from warm arid desert regions. Here, we examine samples obtained from eolian-abraded lava flows of the 4700-4800 m high Ashikule Basin in Tibet. Because it receives approximately 300 mm of precipitation annually, this site is nowhere near as dry as Atacama Desert locales. However, the dusty, sulfate-rich, high-altitude and high-UV flux environment of the Tibetan locale offers new insight into rock varnish formation processes in a terrestrial environment that displays some attributes similar to those expected on early Mars. Microprobe measurements reveal that Mn enhancements in varnish are two orders of magnitude above the dust source, but Fe is only enhanced by a factor of three. Manganese-enhancing bacterial forms are not abundant but are still approximately 3 times more common than in Mojave and Sonoran Desert varnishes. In addition to its occurrence in subaerial positions, Tibetan varnish also occurs in micron-scale "pods" enveloped by silica glaze and as remobilized constituents that have migrated into the underlying weathering rind. A lack of surficial Mn-rich varnish, therefore, might not imply the absence of varnish. In contrast to suggestions that silica glaze might be a good source of microbial fossils and a key to varnish formation, we did not observe any clear microfossil forms entombed in silica glaze; further, there is no gradation between varnish and silica glaze but only distinct contacts.},
    url = "https://doi.org/10.1089/ast.2008.0238",
    doi = "10.1089/ast.2008.0238",
    openalex = "W2051921900",
    references = "doi101016jicarus200411022"
}

@misc{crossref2010desert,
    title = "Desert Varnish",
    year = "2010",
    booktitle = "Encyclopedia of Geography",
    url = "https://doi.org/10.4135/9781412939591.n276",
    doi = "10.4135/9781412939591.n276",
    openalex = "W4232645615"
}

Scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms. To investigate this enigma, we utilized a three‐pronged approach that combined (1) culture‐independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of Black Canyon, New Mexico, and rocks with no visible varnish; (2) culturing of varnish in media supplemented with reduced forms of manganese and/or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and/or manganese oxides; and (3) scanning electron microscopy (SEM) of varnish and nearby rock that lacks macroscopically visible varnish. Our culture‐independent studies revealed significant differences between varnish and nonvarnish communities. Chloroflexi and Ktedobacteria dominated one varnish site, while the other varnish site was dominated by Cyanobacteria. The nonvarnish sites were dominated by Actinobacteria and, to a lesser extent, Cyanobacteria and were the only samples to contain Deinococcus ‐ Thermus sequences. Approximately 65% of varnish cultures produced visible manganese precipitates. Most culture isolates were not closely related to known manganese oxidizers, with the exception of Bacillus spp. SEM revealed microbial morphologies and two types of varnish morphologies: (1) relatively smooth layers and (2) patches of botryoidal pinnacles, which were often associated with increased manganese concentrations. “Bare” rock showed evidence of incipient varnish. These results have important implications for the detection of life on extraterrestrial planets such as Mars, where putative varnish coatings have been observed, and represent some of the first culture‐independent characterizations of varnish communities.

@article{doi1010292009jg001107,
    author = "Northup, Diana E. and Snider, Jessica R. and Spilde, M. and Porter, Megan L. and van de Kamp, Jodie and Boston, Penelope J. and Nyberg, A. and Bargar, John",
    title = "Diversity of rock varnish bacterial communities from Black Canyon, New Mexico",
    year = "2010",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms. To investigate this enigma, we utilized a three‐pronged approach that combined (1) culture‐independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of Black Canyon, New Mexico, and rocks with no visible varnish; (2) culturing of varnish in media supplemented with reduced forms of manganese and/or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and/or manganese oxides; and (3) scanning electron microscopy (SEM) of varnish and nearby rock that lacks macroscopically visible varnish. Our culture‐independent studies revealed significant differences between varnish and nonvarnish communities. Chloroflexi and Ktedobacteria dominated one varnish site, while the other varnish site was dominated by Cyanobacteria. The nonvarnish sites were dominated by Actinobacteria and, to a lesser extent, Cyanobacteria and were the only samples to contain Deinococcus ‐ Thermus sequences. Approximately 65\% of varnish cultures produced visible manganese precipitates. Most culture isolates were not closely related to known manganese oxidizers, with the exception of Bacillus spp. SEM revealed microbial morphologies and two types of varnish morphologies: (1) relatively smooth layers and (2) patches of botryoidal pinnacles, which were often associated with increased manganese concentrations. “Bare” rock showed evidence of incipient varnish. These results have important implications for the detection of life on extraterrestrial planets such as Mars, where putative varnish coatings have been observed, and represent some of the first culture‐independent characterizations of varnish communities.",
    url = "https://doi.org/10.1029/2009jg001107",
    doi = "10.1029/2009jg001107",
    openalex = "W2100072849",
    references = "doi101016jicarus200411022, doi101016s0169555x02003318, doi1011301052517320010110004rvrodw20co2"
}

Understanding completely the interaction of clay mineral and anionic dye is very important for the applications of clay minerals. In this paper, the adsorption behaviors of heat-treated palygorskite clays for methyl orange (MO) from an aqueous medium were studied using equilibrium batch and theoretic calculation techniques. The crystal structure and surface appearance of the heat-treated samples were studied using Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The adsorption results reveal that a higher calcining temperature helps in improving removal performance of palygorskite clay for methyl orange. The isothermal adsorption experiments show that there is almost no adsorption at low dye concentrations but a sudden linear increase occurs when the dye concentration exceeds to a certain value. A lower change point is observed for 700 °C treated sample compared with that of the natural sample. The leaching of exchangeable Mg2+ ions located in palygorskite should be a key factor affecting the adsorption capacity. Both the experimental and theoretical studies suggest that the adsorption of MO onto heat-treated palygorskite clay is controlled by the two mechanisms: for samples treated at lower temperatures (400 °C), electrostatic interaction between the dye–Mg2+ complexes and negatively charged surface of heat-treated palygorskite clay becomes dominant. In addition, the latter becomes stronger with the increasing the calcination temperature, which can be ascribed to the formation of a higher ratio of complexes and stronger negatively charged surface of the adsorbent caused by leaching of more metal ions. This work provides a deep insight into the interaction of heat-treated palygorskite clay and anionic dye, which paves the way for their practical applications in anionic dye adsorption.

@article{doi101021ie300702j,
    author = "Chen, Hao and Zhong, Aiguo and Wu, Junyong and Zhao, Jie and Yan, Hua",
    title = "Adsorption Behaviors and Mechanisms of Methyl Orange on Heat-Treated Palygorskite Clays",
    year = "2012",
    journal = "Industrial \& Engineering Chemistry Research",
    abstract = "Understanding completely the interaction of clay mineral and anionic dye is very important for the applications of clay minerals. In this paper, the adsorption behaviors of heat-treated palygorskite clays for methyl orange (MO) from an aqueous medium were studied using equilibrium batch and theoretic calculation techniques. The crystal structure and surface appearance of the heat-treated samples were studied using Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The adsorption results reveal that a higher calcining temperature helps in improving removal performance of palygorskite clay for methyl orange. The isothermal adsorption experiments show that there is almost no adsorption at low dye concentrations but a sudden linear increase occurs when the dye concentration exceeds to a certain value. A lower change point is observed for 700 °C treated sample compared with that of the natural sample. The leaching of exchangeable Mg2+ ions located in palygorskite should be a key factor affecting the adsorption capacity. Both the experimental and theoretical studies suggest that the adsorption of MO onto heat-treated palygorskite clay is controlled by the two mechanisms: for samples treated at lower temperatures (400 °C), electrostatic interaction between the dye–Mg2+ complexes and negatively charged surface of heat-treated palygorskite clay becomes dominant. In addition, the latter becomes stronger with the increasing the calcination temperature, which can be ascribed to the formation of a higher ratio of complexes and stronger negatively charged surface of the adsorbent caused by leaching of more metal ions. This work provides a deep insight into the interaction of heat-treated palygorskite clay and anionic dye, which paves the way for their practical applications in anionic dye adsorption.",
    url = "https://doi.org/10.1021/ie300702j",
    doi = "10.1021/ie300702j",
    openalex = "W2327323237",
    references = "doi101016jsaa201207085"
}

Desert varnish and endolithic organisms are two widespread phenomena that have been studied in detail separately; their interaction and their genetic relationships have virtually escaped the attention of researchers. Both phenomena are of indubitable interest for pedology: endolithic organisms as an agent of soil formation and rock varnish as a probable product of pedogenesis. It is argued that the system of endolithic organisms, their functioning products, and the rock has all the features inherent to soils: the rock layer subjected to the influence of external abiogenic factors and living organisms dwelling in the rock and synthesizing and decomposing organic substances. The action of biogenic and abiogenic agents leads to the in situ transformation of the rock with the accumulation and removal of the products of this transformation and with the development of vertical heterogeneity in the form of microhorizons composing the soil microprofile. Instrumental measurements indicate that the carbon content in the endolithic horizons developed by biota in granitoid rocks of the Larsemann Hills oasis varies from 0.2 to 3.3%, the nitrogen content in these horizons varies from 0.02 to 0.47%, and the radiocarbon age of their organic matter reaches 480 ± 25 yrs. The products of the pedogenesis are represented by fine earth materials and by abundant and often multilayered films and coatings on the rock surface and on the lower sides of the desquamation (spalling) plates. Scanning electron microscopy with X-ray microprobe analysis indicates that the major elements composing these films are O, C, Si, Al, Fe, Ca, Mg, and S. It is shown that the films of the rock varnish and the organomineral films in the fissured zone of the rock under the plate with endolithic communities have certain similarity in their morphology and composition: the films of the rock varnish also contain biota (dead cells or cells in the dormant state), and their botryoidal structure is similar to the structure of the biofilms inside the endolithic system. In both types of films, amorphous aluminum and silicon compounds are present, and the accumulation of Fe, Ca, Mg, S, Cl, and some other elements takes place. It is argued that some varieties of rock varnish are the products of endolithic pedogenesis; in essence, they represent the horizons of micropaleosols exposed to the surface in the course of spalling and then transformed by the external environmental agents.

@article{doi101134s1064229312100067,
    author = "Mergelov, N. S. and Goryachkin, S. V. and Shorkunov, I. G. and Zazovskaya, E. P. and Cherkinsky, Alexander",
    title = "Endolithic pedogenesis and rock varnish on massive crystalline rocks in East Antarctica",
    year = "2012",
    journal = "Eurasian Soil Science",
    abstract = "Desert varnish and endolithic organisms are two widespread phenomena that have been studied in detail separately; their interaction and their genetic relationships have virtually escaped the attention of researchers. Both phenomena are of indubitable interest for pedology: endolithic organisms as an agent of soil formation and rock varnish as a probable product of pedogenesis. It is argued that the system of endolithic organisms, their functioning products, and the rock has all the features inherent to soils: the rock layer subjected to the influence of external abiogenic factors and living organisms dwelling in the rock and synthesizing and decomposing organic substances. The action of biogenic and abiogenic agents leads to the in situ transformation of the rock with the accumulation and removal of the products of this transformation and with the development of vertical heterogeneity in the form of microhorizons composing the soil microprofile. Instrumental measurements indicate that the carbon content in the endolithic horizons developed by biota in granitoid rocks of the Larsemann Hills oasis varies from 0.2 to 3.3\%, the nitrogen content in these horizons varies from 0.02 to 0.47\%, and the radiocarbon age of their organic matter reaches 480 ± 25 yrs. The products of the pedogenesis are represented by fine earth materials and by abundant and often multilayered films and coatings on the rock surface and on the lower sides of the desquamation (spalling) plates. Scanning electron microscopy with X-ray microprobe analysis indicates that the major elements composing these films are O, C, Si, Al, Fe, Ca, Mg, and S. It is shown that the films of the rock varnish and the organomineral films in the fissured zone of the rock under the plate with endolithic communities have certain similarity in their morphology and composition: the films of the rock varnish also contain biota (dead cells or cells in the dormant state), and their botryoidal structure is similar to the structure of the biofilms inside the endolithic system. In both types of films, amorphous aluminum and silicon compounds are present, and the accumulation of Fe, Ca, Mg, S, Cl, and some other elements takes place. It is argued that some varieties of rock varnish are the products of endolithic pedogenesis; in essence, they represent the horizons of micropaleosols exposed to the surface in the course of spalling and then transformed by the external environmental agents.",
    url = "https://doi.org/10.1134/s1064229312100067",
    doi = "10.1134/s1064229312100067",
    openalex = "W2063517698"
}

Biological soil crusts (BSCs) are bio-sedimentary complexes that play critical ecological roles in arid landscapes; however, the interactions between component biota and sediments are poorly understood. A detailed micromorphological investigation of BSC development and crust microstructure in the Muddy Mountains Wilderness Area, Nevada, examined features in thin section using petrographic microscopy, light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic history and diverse formative processes, including: (i) stabilization and authigenic mineral precipitation; (ii) wetting–drying and expansion–contraction; (iii) dust capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon formation. A new conceptual model for hot deserts illustrates how these processes co-develop with BSC succession, during countless wet–dry cycles, to build up pinnacle microtopography while simultaneously forming Av horizons in the bio-rich and bio-poor zones. Complex surficial and internal bio-sedimentary structures, which vary as a function of crust morphology, trap surface water for uptake by crust organisms, while dust influx provides a source of nutrients. These phenomena influence landscape-scale water dynamics and biogeochemical cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, and ecosystem function of dust and should, therefore, be considered critical agents in arid pedogenesis and landscape development.

@article{doi102136sssaj20120021,
    author = "Williams, Amanda J. and Buck, Brenda J. and Beyene, Mengesha A.",
    title = "Biological Soil Crusts in the Mojave Desert, USA: Micromorphology and Pedogenesis",
    year = "2012",
    journal = "Soil Science Society of America Journal",
    abstract = "Biological soil crusts (BSCs) are bio-sedimentary complexes that play critical ecological roles in arid landscapes; however, the interactions between component biota and sediments are poorly understood. A detailed micromorphological investigation of BSC development and crust microstructure in the Muddy Mountains Wilderness Area, Nevada, examined features in thin section using petrographic microscopy, light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic history and diverse formative processes, including: (i) stabilization and authigenic mineral precipitation; (ii) wetting–drying and expansion–contraction; (iii) dust capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon formation. A new conceptual model for hot deserts illustrates how these processes co-develop with BSC succession, during countless wet–dry cycles, to build up pinnacle microtopography while simultaneously forming Av horizons in the bio-rich and bio-poor zones. Complex surficial and internal bio-sedimentary structures, which vary as a function of crust morphology, trap surface water for uptake by crust organisms, while dust influx provides a source of nutrients. These phenomena influence landscape-scale water dynamics and biogeochemical cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, and ecosystem function of dust and should, therefore, be considered critical agents in arid pedogenesis and landscape development.",
    url = "https://doi.org/10.2136/sssaj2012.0021",
    doi = "10.2136/sssaj2012.0021",
    openalex = "W2051130578",
    references = "doi101016s0169555x97000950, doi102136sssaj195803615995002200010017x"
}

@article{doi101016jgca201310040,
    author = "Goldsmith, Yonaton and Stein, Mordechai and Enzel, Yehouda",
    title = "From dust to varnish: Geochemical constraints on rock varnish formation in the Negev Desert, Israel",
    year = "2013",
    journal = "Geochimica et Cosmochimica Acta",
    url = "https://doi.org/10.1016/j.gca.2013.10.040",
    doi = "10.1016/j.gca.2013.10.040",
    openalex = "W1986052302",
    references = "doi1010079783642730931, doi101007bf00322470, doi101016001670379290334f, doi1010160021979772901749, doi101016jgca200406013, doi101016s0016703700005780, doi101023a1022973114090, doi101073pnas9673447, openalexw1624806571"
}

@incollection{crossref2014desert,
    title = "desert varnish",
    year = "2014",
    booktitle = "Dictionary Geotechnical Engineering/Wörterbuch GeoTechnik",
    url = "https://doi.org/10.1007/978-3-642-41714-6\_41392",
    doi = "10.1007/978-3-642-41714-6\_41392",
    openalex = "W4243791910",
    pages = "362-362"
}

Increasing amount of dyes in the ecosystem particularly in wastewater has propelled the search for more efficient low-cost adsorbents. The effective use of the sorption properties (high surface area and surface chemistry, lack of toxicity and potential for ion exchange) of different clays as adsorbents for the removal of different type of dyes (basic, acidic, reactive) from water and wastewater as potential alternatives to activated carbons has recently received widespread attention because of the environmental-friendly nature of clay materials. Insights into the efficiencies of raw and modified/activated clay adsorbents and ways of improving their efficiencies to obtain better results are discussed. Acid-modified clay resulted in higher rate of dye adsorption and an increased surface area and porosity (49.05 mm2 and 53.4 %). Base-modified clay has lower adsorption capacities, while ZnCl2-modified clay had the least rate of adsorption with a surface area of 44.3 mm2 and porosity of 43.4 %. This review also explores the grey areas of the adsorption properties of the raw clays and the improved performance of activated/modified clay materials with particular reference to the effects of pH, temperature, initial dye concentration and adsorbent dosage on the adsorption capacities of the clays. Various challenges encountered in using clay materials are highlighted and a number of future prospects for the adsorbents are proposed.

@article{doi101007s132010150322y,
    author = "Adeyemo, Aderonke Ajibola and Adeoye, Idowu Olatunbosun and Bello, Olugbenga Solomon",
    title = "Adsorption of dyes using different types of clay: a review",
    year = "2015",
    journal = "Applied Water Science",
    abstract = "Increasing amount of dyes in the ecosystem particularly in wastewater has propelled the search for more efficient low-cost adsorbents. The effective use of the sorption properties (high surface area and surface chemistry, lack of toxicity and potential for ion exchange) of different clays as adsorbents for the removal of different type of dyes (basic, acidic, reactive) from water and wastewater as potential alternatives to activated carbons has recently received widespread attention because of the environmental-friendly nature of clay materials. Insights into the efficiencies of raw and modified/activated clay adsorbents and ways of improving their efficiencies to obtain better results are discussed. Acid-modified clay resulted in higher rate of dye adsorption and an increased surface area and porosity (49.05 mm2 and 53.4 \%). Base-modified clay has lower adsorption capacities, while ZnCl2-modified clay had the least rate of adsorption with a surface area of 44.3 mm2 and porosity of 43.4 \%. This review also explores the grey areas of the adsorption properties of the raw clays and the improved performance of activated/modified clay materials with particular reference to the effects of pH, temperature, initial dye concentration and adsorbent dosage on the adsorption capacities of the clays. Various challenges encountered in using clay materials are highlighted and a number of future prospects for the adsorbents are proposed.",
    url = "https://doi.org/10.1007/s13201-015-0322-y",
    doi = "10.1007/s13201-015-0322-y",
    openalex = "W1777998439",
    references = "doi10108011035896209447314"
}

A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.

@article{doi101093femsrefuu011,
    author = "Makhalanyane, Thulani P. and Valverde, Ángel and Gunnigle, Eoin and Frossard, Aline and Ramond, Jean‐Baptiste and Cowan, Don A.",
    title = "Microbial ecology of hot desert edaphic systems",
    year = "2015",
    journal = "FEMS Microbiology Reviews",
    abstract = "A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.",
    url = "https://doi.org/10.1093/femsre/fuu011",
    doi = "10.1093/femsre/fuu011",
    openalex = "W2156850034",
    references = "doi101007bf00378791"
}

In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.

@incollection{doi101180mono55,
    author = "Brown, G. and Brindley, G. W.",
    title = "X-ray Diffraction Procedures for Clay Mineral Identification",
    year = "2015",
    booktitle = "Mineralogical Society of Great Britain and Ireland eBooks",
    abstract = "In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.",
    url = "https://doi.org/10.1180/mono-5.5",
    doi = "10.1180/mono-5.5",
    openalex = "W2498112629",
    references = "doi101016b9780080092355500267"
}

The mineralogical composition of coarse fraction and characteristic features of the micro- and submicrofabrics and chemical composition of desert varnish on gravels of desert pavements and the underlying vesicular crust soil horizons were studied in the extremely arid soils of the Mojave (USA) and Trans-Altai Gobi (Mongolia) deserts. A set of common diagnostic features of elementary pedogenetic processes was identified in the automorphic desert soils developed on ancient (70–90 ka) piedmont plains composed of alluvial deposits with the high content of red-earth clay. The results of this study attest to the long and complicated history of the extremely arid soils with alternation of the humid and arid phases of pedogenesis reflected in a specific combination of textural (clay-illuvial) and carbonate pedofeatures and in the distribution patterns of iron, manganese, titanium, and barium in different layers of the desert varnish. The chemical composition of the latter did not depend on the mineralogical composition of the underlying substrates and was formed with active participation of soil microorganisms. This allowed us to conclude about the polygenetic (accretionary–microbiological) nature of desert varnish.

@article{doi101134s106422931512011x,
    author = "Лебедева, М. П. and Шишков, В. А.",
    title = "A Comparative Analysis of the Microfabrics of Surface Horizons and Desert Varnish in Extremely Arid Soils of the Mojave (USA) and Trans-Altai Gobi (Mongolia) Deserts",
    year = "2016",
    journal = "Eurasian Soil Science",
    abstract = "The mineralogical composition of coarse fraction and characteristic features of the micro- and submicrofabrics and chemical composition of desert varnish on gravels of desert pavements and the underlying vesicular crust soil horizons were studied in the extremely arid soils of the Mojave (USA) and Trans-Altai Gobi (Mongolia) deserts. A set of common diagnostic features of elementary pedogenetic processes was identified in the automorphic desert soils developed on ancient (70–90 ka) piedmont plains composed of alluvial deposits with the high content of red-earth clay. The results of this study attest to the long and complicated history of the extremely arid soils with alternation of the humid and arid phases of pedogenesis reflected in a specific combination of textural (clay-illuvial) and carbonate pedofeatures and in the distribution patterns of iron, manganese, titanium, and barium in different layers of the desert varnish. The chemical composition of the latter did not depend on the mineralogical composition of the underlying substrates and was formed with active participation of soil microorganisms. This allowed us to conclude about the polygenetic (accretionary–microbiological) nature of desert varnish.",
    url = "https://doi.org/10.1134/s106422931512011x",
    doi = "10.1134/s106422931512011x",
    openalex = "W2379527323",
    references = "doi101007bf00378791, doi101016b9780444531568000064, doi101016s0169555x97000950, doi101038276489a0, doi10113000917613198715504ioeapp20co2, doi101130g223521, doi1021362003guidelinesforanalysis, doi102136sssaj195803615995002200010017x, doi102138am20015611, potter1977desert"
}

@incollection{doi101016bsagron201711001,
    author = "Singh, Mandeep and Sarkar, Binoy and Sarkar, Subhas and Churchman, G. Jock and Bolan, Nanthi and Mandal, Sanchita and Menon, Manoj and Purakayastha, Tapan Jyoti and Beerling, David J.",
    title = "Stabilization of Soil Organic Carbon as Influenced by Clay Mineralogy",
    year = "2017",
    booktitle = "Advances in agronomy",
    url = "https://doi.org/10.1016/bs.agron.2017.11.001",
    doi = "10.1016/bs.agron.2017.11.001",
    openalex = "W2775180186",
    references = "doi101016jgeoderma200403005, doi101016s0038071700001796, doi101023a1016125726789, doi101038nature10386, doi1010970001069419830200000014, doi101111j136523891982tb01755x, doi101126science1097396, doi1023071941811, openalexw1566774663, openalexw2133119982"
}

Clay-size minerals play important roles in terrestrial biogeochemistry and atmospheric physics, but their data have been only partially compiled at global scale. We present a global dataset of clay-size minerals in the topsoil and subsoil at different spatial resolutions. The data of soil clay and its mineralogical composition were gathered through a literature survey and aggregated by soil orders of the Soil Taxonomy for each of the ten groups: gibbsite, kaolinite, illite/mica, smectite, vermiculite, chlorite, iron oxide, quartz, non-crystalline, and others. Using a global soil map, a global dataset of soil clay-size mineral distribution was developed at resolutions of 2' to 2° grid cells. The data uncertainty associated with data variability and assumption was evaluated using a Monte Carlo method, and validity of the clay-size mineral distribution obtained in this study was examined by comparing with other datasets. The global soil clay data offer spatially explicit studies on terrestrial biogeochemical cycles, dust emission to the atmosphere, and other interdisciplinary earth sciences.

@article{doi101038sdata2017103,
    author = "Ito, Akihiko and Wagai, Rota",
    title = "Global distribution of clay-size minerals on land surface for biogeochemical and climatological studies",
    year = "2017",
    journal = "Scientific Data",
    abstract = "Clay-size minerals play important roles in terrestrial biogeochemistry and atmospheric physics, but their data have been only partially compiled at global scale. We present a global dataset of clay-size minerals in the topsoil and subsoil at different spatial resolutions. The data of soil clay and its mineralogical composition were gathered through a literature survey and aggregated by soil orders of the Soil Taxonomy for each of the ten groups: gibbsite, kaolinite, illite/mica, smectite, vermiculite, chlorite, iron oxide, quartz, non-crystalline, and others. Using a global soil map, a global dataset of soil clay-size mineral distribution was developed at resolutions of 2' to 2° grid cells. The data uncertainty associated with data variability and assumption was evaluated using a Monte Carlo method, and validity of the clay-size mineral distribution obtained in this study was examined by comparing with other datasets. The global soil clay data offer spatially explicit studies on terrestrial biogeochemical cycles, dust emission to the atmosphere, and other interdisciplinary earth sciences.",
    url = "https://doi.org/10.1038/sdata.2017.103",
    doi = "10.1038/sdata.2017.103",
    openalex = "W2748099678",
    references = "doi101180000985599546055"
}

The present review covers the regeneration capacity and adsorption efficiency of different adsorbents for the treatment of industrial dyes to control water pollution. Various techniques and materials have been employed to remove organic pollutants from water; however, adsorption techniques using cost-effective, ecofriendly, clay-supported adsorbents are widely used owing to their simplicity and good efficiency. Among all the natural adsorbents, activated carbon has been found to be the most effective for dye adsorption; however, its use is restricted due to its high regeneration cost. Clays and modified clay-based adsorbents are the most efficient clarifying agents for organic pollutants as compared to activated carbon, organic/inorganic, and composite materials. Regeneration is an important aspect to stimulate the adsorption efficiency of the exhausted/spent adsorbent for water treatment. A number of techniques, including chemical treatment, supercritical extraction, thermal, and photocatalytic and biological degradation, have been developed to regenerate spent or dye-adsorbed clays. This review discusses how these techniques enhance the adsorption and retention potential of spent low-cost adsorbents and reflects on the future perspectives for their use in wastewater treatment.

@article{doi101039c8ra04290j,
    author = "Momina, Momina and Shahadat, Mohammad and Suzylawati, Isamil",
    title = "Regeneration performance of clay-based adsorbents for the removal of industrial dyes: a review",
    year = "2018",
    journal = "RSC Advances",
    abstract = "The present review covers the regeneration capacity and adsorption efficiency of different adsorbents for the treatment of industrial dyes to control water pollution. Various techniques and materials have been employed to remove organic pollutants from water; however, adsorption techniques using cost-effective, ecofriendly, clay-supported adsorbents are widely used owing to their simplicity and good efficiency. Among all the natural adsorbents, activated carbon has been found to be the most effective for dye adsorption; however, its use is restricted due to its high regeneration cost. Clays and modified clay-based adsorbents are the most efficient clarifying agents for organic pollutants as compared to activated carbon, organic/inorganic, and composite materials. Regeneration is an important aspect to stimulate the adsorption efficiency of the exhausted/spent adsorbent for water treatment. A number of techniques, including chemical treatment, supercritical extraction, thermal, and photocatalytic and biological degradation, have been developed to regenerate spent or dye-adsorbed clays. This review discusses how these techniques enhance the adsorption and retention potential of spent low-cost adsorbents and reflects on the future perspectives for their use in wastewater treatment.",
    url = "https://doi.org/10.1039/c8ra04290j",
    doi = "10.1039/c8ra04290j",
    openalex = "W2826216626",
    references = "doi101016jclay200902002"
}

Desert varnishes are dark rock coatings observed in arid environments and might resemble Mn-rich coatings found on Martian rocks. Their formation mechanism is not fully understood and the possible microbial involvement is under debate. In this study, we applied DNA metagenomic Shotgun sequencing of varnish and surrounding soil to evaluate the composition of the microbial community and its potential metabolic function. We found that the α diversity was lower in varnish compared to soil samples (p value < 0.05), suggesting distinct populations with significantly higher abundance of Actinobacteria, Proteobacteria and Cyanobacteria within the varnish. Additionally, we observed increased levels of transition metal metabolic processes in varnish compared to soil samples. Nevertheless, potentially relevant enzymes for varnish formation were detected at low to insignificant levels in both niches, indicating no current direct microbial involvement in Mn oxidation. This finding is supported by quantitative genomic analysis, elemental analysis, fluorescence imaging and scanning transmission X-ray microscopy. We thus conclude that the distinct microbial communities detected in desert varnish originate from settled Aeolian microbes, which colonized this nutrient-enriched niche, and discuss possible indirect contributions of microorganisms to the formation of desert varnish.

@article{doi1011111758222912634,
    author = "Lang‐Yona, Naama and Maier, Stefanie and Macholdt, Dorothea S. and Müller-Germann, I. and Yordanova, Petya and Rodríguez‐Caballero, Emilio and Jochum, Klaus Peter and Alamri, Abdullah and Andreae, Meinrat O. and Fröhlich‐Nowoisky, Janine and Weber, Bettina",
    title = "Insights into microbial involvement in desert varnish formation retrieved from metagenomic analysis",
    year = "2018",
    journal = "Environmental Microbiology Reports",
    abstract = "Desert varnishes are dark rock coatings observed in arid environments and might resemble Mn-rich coatings found on Martian rocks. Their formation mechanism is not fully understood and the possible microbial involvement is under debate. In this study, we applied DNA metagenomic Shotgun sequencing of varnish and surrounding soil to evaluate the composition of the microbial community and its potential metabolic function. We found that the α diversity was lower in varnish compared to soil samples (p value < 0.05), suggesting distinct populations with significantly higher abundance of Actinobacteria, Proteobacteria and Cyanobacteria within the varnish. Additionally, we observed increased levels of transition metal metabolic processes in varnish compared to soil samples. Nevertheless, potentially relevant enzymes for varnish formation were detected at low to insignificant levels in both niches, indicating no current direct microbial involvement in Mn oxidation. This finding is supported by quantitative genomic analysis, elemental analysis, fluorescence imaging and scanning transmission X-ray microscopy. We thus conclude that the distinct microbial communities detected in desert varnish originate from settled Aeolian microbes, which colonized this nutrient-enriched niche, and discuss possible indirect contributions of microorganisms to the formation of desert varnish.",
    url = "https://doi.org/10.1111/1758-2229.12634",
    doi = "10.1111/1758-2229.12634",
    openalex = "W2793400010",
    references = "doi101016jchemgeo201704009, doi101080014904512011619636, doi101126science1203109183"
}

Transmission electron microscopy linked with energy-dispersive X-ray spectroscopy (TEM-EDX) was applied to characterize mineralogical signals of weathering processes in the Di Linh bentonite deposit (Vietnam) and to visualize the effects of Na activation on the smectitic phases. Modelling of X-ray diffraction patterns (oriented mount) was applied in order to refine the computed structural formula. X-ray diffraction, X-ray fluorescence and Fourier-transform infrared spectroscopy (FT-IR) methods were also applied to verify the TEM-EDX results. An Excel -based routine has been developed in this research to allow fast computation of structural formulae and classification of the investigated clay particles. This routine supports the acquirement of 100–300 TEM-EDX analyses as a representative set of individual particles for each sample. The Excel -based routine involves end members of different clay-mineral groups and interstratifications with two or three members (e.g. illite–smectite interstratifications – IS-ml; dioctahedral vermiculite–smectite interstratifications – diVS-ml; and kaolinite–montmorillonite–dioctahedral vermiculite interstratifications – KSV-ml). The routine is now freely available. According to the identification procedure, the <2 µm fraction of the Di Linh bentonite (Vietnam) is composed mainly of K- and charge-deficient illite–smectite interstratifications (or diVS-ml): montmorillonite-rich randomly ordered (R0) type and illite-rich regularly ordered (R1) type. Additionally, Fe-poor KSV-ml was identified. Industrial Na activation of the Di Linh bentonite resulted in an increase of the R1 diVS-ml portion and dissolution of a large part of the smectite-rich phases. The TEM-EDX approach also gave analytical proof of a sedimentary process for Di Linh smectite. The parent muscovite was altered in two different environments: (i) K-leaching and layer-wise alteration into kaolinite (weathering), and (ii) further edge-controlled alteration of mica into lath-like montmorillonite particles associated with a dissolution of kaolinite layers from the former kaolinite–mica intergrowths by heat impact (basalt flow).

@article{doi101107s1600576718018162,
    author = "Hoang‐Minh, Thao and Kasbohm, Jörn and Nguyen-Thanh, Lan and Nga, Pham Thi and Lai, Le Thi and Duong, Nguyen Thuy and Thành, Nguyễn Đức and Thuyet, Nguyen Thi Minh and Anh, Dao Duy and Pusch, Roland and Knutsson, Sven and Mählmann, Rafael Ferreiro",
    title = "Use of TEM-EDX for structural formula identification of clay minerals: a case study of Di Linh bentonite, Vietnam",
    year = "2019",
    journal = "Journal of Applied Crystallography",
    abstract = "Transmission electron microscopy linked with energy-dispersive X-ray spectroscopy (TEM-EDX) was applied to characterize mineralogical signals of weathering processes in the Di Linh bentonite deposit (Vietnam) and to visualize the effects of Na activation on the smectitic phases. Modelling of X-ray diffraction patterns (oriented mount) was applied in order to refine the computed structural formula. X-ray diffraction, X-ray fluorescence and Fourier-transform infrared spectroscopy (FT-IR) methods were also applied to verify the TEM-EDX results. An Excel -based routine has been developed in this research to allow fast computation of structural formulae and classification of the investigated clay particles. This routine supports the acquirement of 100–300 TEM-EDX analyses as a representative set of individual particles for each sample. The Excel -based routine involves end members of different clay-mineral groups and interstratifications with two or three members (e.g. illite–smectite interstratifications – IS-ml; dioctahedral vermiculite–smectite interstratifications – diVS-ml; and kaolinite–montmorillonite–dioctahedral vermiculite interstratifications – KSV-ml). The routine is now freely available. According to the identification procedure, the <2 µm fraction of the Di Linh bentonite (Vietnam) is composed mainly of K- and charge-deficient illite–smectite interstratifications (or diVS-ml): montmorillonite-rich randomly ordered (R0) type and illite-rich regularly ordered (R1) type. Additionally, Fe-poor KSV-ml was identified. Industrial Na activation of the Di Linh bentonite resulted in an increase of the R1 diVS-ml portion and dissolution of a large part of the smectite-rich phases. The TEM-EDX approach also gave analytical proof of a sedimentary process for Di Linh smectite. The parent muscovite was altered in two different environments: (i) K-leaching and layer-wise alteration into kaolinite (weathering), and (ii) further edge-controlled alteration of mica into lath-like montmorillonite particles associated with a dissolution of kaolinite layers from the former kaolinite–mica intergrowths by heat impact (basalt flow).",
    url = "https://doi.org/10.1107/s1600576718018162",
    doi = "10.1107/s1600576718018162",
    openalex = "W2911856978",
    references = "doi10100797894011072737"
}

"The composition and microfabric of the Av diagnostic surface horizon (crust) and desert varnish of soils formed on various parent materials covering ancient alluvial fans in extremely arid deserts of Central Asia (Trans-Altai Gobi, Mongolia) and North America (Mojave, USA) are analysed. Application of a wide range of methods—morphological and micromorphological descriptions, scanning electron microscopy, electron microprobe analysis and microtomography, together with traditional physicochemical methods—has shown that desert varnish retains the same composition and relative contents of elements (Fe/Ti and Mn/Ba) independently from the initial composition of parent materials and geographical location of the studied regions. The absolute dominance of isolated closed (vesicular) pores, identified by micromorphology and confirmed by microtomography, can account for specific hydrophysical properties of the crust horizon; that is, low water-holding capacity and permeability. We explain both phenomena of desert pedogenesis—light-coloured crust horizon and dark-coloured desert varnish—the burst of microbiological activity and physicochemical processes after rare but heavy showers during the hot season. The development of iron-depleted zones around pores is considered the evidence of that phenomenon. Consequently, we propose that Mn2+ undergoes mobilization within the crust horizon, followed by its upward migration with capillary and film moisture flows, against the gravity gradient to the surface of desert pavement with its subsequent fixation at the oxygen barrier. The processes of sulphate reduction are indirectly confirmed by the mobilization of Ba2+, which is immobile in the presence of sulphates. These elements (Mn and Ba) accumulate within the lower part of two-layer films. These films are located in microcavities on the upper sides of rock fragments of desert pavements in the studied regions. Films on lower sides of rock fragments and the upper part of those two-layered films are enriched in Fe and Ti, and therefore acquire reddish-brownish colours. Despite its transitory character, the process, which we suggest to call ‘cryptosolodization’, includes five stages: (1) Fe, Mn and sulphate reduction with mobilization of Ba2+ from BaSO4, (2) precipitation of FeS and migration of Ba2+ and Mn2+ to the stone surface, (3) coprecipitation of Ba and Mn at a combined oxygen-sorption barrier, (4) oxidation of S2- to SO4 2- with mobilization of Ti2+ from TiO2 and (5) oxidation of Ti2+ and Fe2+ and their coprecipitation in the form of oxides/ hydroxides TiO2 and Fe(OH)3."

@article{doi1018268bsgm2019v71n1a3,
    author = "Лебедева, М. П. and Golovanov, Dmitriy L. and Shishkov, V. and Абросимов, К. Н.",
    title = "Microscopic and tomographic studies for interpreting the genesis of desert varnish and the vesicular horizon of desert soils in Mongolia and the USA",
    year = "2019",
    journal = "Boletín de la Sociedad Geológica Mexicana",
    abstract = {"The composition and microfabric of the Av diagnostic surface horizon (crust) and desert varnish of soils formed on various parent materials covering ancient alluvial fans in extremely arid deserts of Central Asia (Trans-Altai Gobi, Mongolia) and North America (Mojave, USA) are analysed. Application of a wide range of methods—morphological and micromorphological descriptions, scanning electron microscopy, electron microprobe analysis and microtomography, together with traditional physicochemical methods—has shown that desert varnish retains the same composition and relative contents of elements (Fe/Ti and Mn/Ba) independently from the initial composition of parent materials and geographical location of the studied regions. The absolute dominance of isolated closed (vesicular) pores, identified by micromorphology and confirmed by microtomography, can account for specific hydrophysical properties of the crust horizon; that is, low water-holding capacity and permeability. We explain both phenomena of desert pedogenesis—light-coloured crust horizon and dark-coloured desert varnish—the burst of microbiological activity and physicochemical processes after rare but heavy showers during the hot season. The development of iron-depleted zones around pores is considered the evidence of that phenomenon. Consequently, we propose that Mn2+ undergoes mobilization within the crust horizon, followed by its upward migration with capillary and film moisture flows, against the gravity gradient to the surface of desert pavement with its subsequent fixation at the oxygen barrier. The processes of sulphate reduction are indirectly confirmed by the mobilization of Ba2+, which is immobile in the presence of sulphates. These elements (Mn and Ba) accumulate within the lower part of two-layer films. These films are located in microcavities on the upper sides of rock fragments of desert pavements in the studied regions. Films on lower sides of rock fragments and the upper part of those two-layered films are enriched in Fe and Ti, and therefore acquire reddish-brownish colours. Despite its transitory character, the process, which we suggest to call ‘cryptosolodization’, includes five stages: (1) Fe, Mn and sulphate reduction with mobilization of Ba2+ from BaSO4, (2) precipitation of FeS and migration of Ba2+ and Mn2+ to the stone surface, (3) coprecipitation of Ba and Mn at a combined oxygen-sorption barrier, (4) oxidation of S2- to SO4 2- with mobilization of Ti2+ from TiO2 and (5) oxidation of Ti2+ and Fe2+ and their coprecipitation in the form of oxides/ hydroxides TiO2 and Fe(OH)3."},
    url = "https://doi.org/10.18268/bsgm2019v71n1a3",
    doi = "10.18268/bsgm2019v71n1a3",
    openalex = "W2969758197",
    references = "doi1010160009254179900858, doi101016jgeomorph200606008, doi101016jgeomorph200703015, doi101016s0169555x97000950, doi101029wr011i006p00929, doi101086629726, doi101126science11536571, doi101134s1064229312100067, doi101134s106422931512011x, doi102138am20015611, potter1977desert"
}

To future-proof alkali-activation technology, there is a need to look beyond well-established precursors such as fly ash and blast furnace slag, due to resource competition, geographical distribution and technical limitations. Clay minerals are abundant and diverse aluminosilicate resources available around the world. However, due to the mineralogical complexity amongst the most common 1:1 (kaolinite, halloysite) and 2:1 (montmorillonite, illite) clay minerals, and practical issues such as workability, their use has been more limited. Recent advances have improved understanding both of pre-activation treatments (thermal, mechanical, chemical), and of the factors influencing clay reactivity, phase assemblages and properties of final products. This opens new opportunities for the exploitation of these resources to produce sustainable cements. A one-size-fits-all approach for processing and activating clay minerals is not viable. Instead, activation routes need to be tailored according to the clay mineralogy to achieve the binder properties required for key applications.

@article{doi101016jcemconres2020106050,
    author = "Khalifa, Ahmed and Çizer, Özlem and Pontikes, Yiannis and Heath, Andrew and Patureau, Pascaline and Bernal, Susan A. and Marsh, Alastair",
    title = "Advances in alkali-activation of clay minerals",
    year = "2020",
    journal = "Cement and Concrete Research",
    abstract = "To future-proof alkali-activation technology, there is a need to look beyond well-established precursors such as fly ash and blast furnace slag, due to resource competition, geographical distribution and technical limitations. Clay minerals are abundant and diverse aluminosilicate resources available around the world. However, due to the mineralogical complexity amongst the most common 1:1 (kaolinite, halloysite) and 2:1 (montmorillonite, illite) clay minerals, and practical issues such as workability, their use has been more limited. Recent advances have improved understanding both of pre-activation treatments (thermal, mechanical, chemical), and of the factors influencing clay reactivity, phase assemblages and properties of final products. This opens new opportunities for the exploitation of these resources to produce sustainable cements. A one-size-fits-all approach for processing and activating clay minerals is not viable. Instead, activation routes need to be tailored according to the clay mineralogy to achieve the binder properties required for key applications.",
    url = "https://doi.org/10.1016/j.cemconres.2020.106050",
    doi = "10.1016/j.cemconres.2020.106050",
    openalex = "W3015248526",
    references = "doi101016jclay200902002, doi101016jfuel201203024, doi101016s1386142598000407, doi101180000985599546055"
}

Abstract We conducted rock varnish measurements at four rock art sites in north‐western Saudi Arabia, including Kilwa and the Camel Site near Sakaka. We determined the areal densities of Mn and Fe in rock varnish that had accumulated on petroglyph surfaces since their creation, complemented by a detailed analysis of varnish samples. We inferred varnish accumulation rates by relating the Mn areal density on inscriptions to their ages estimated based on the type of script used. Applying these rates to the varnish densities on the rock art indicated that the art was produced during two distinct periods, corresponding to the Pre‐Pottery/Late Neolithic and the Bronze/Iron Age, respectively, with different artistic traditions, reflecting distinct socioeconomic and ecological conditions. Our dating approach, while admittedly burdened with substantial uncertainties, yields ages consistent with archaeological and historical evidence and it provides a unique quantitative tool to obtain at least rough ages for otherwise undatable rock art.

@article{doi101111aae12167,
    author = "Andreae, Meinrat O. and Alamri, Abdullah and Andreae, Claire M. and Guagnin, Maria and Jochum, Klaus Peter and Stoll, Brigitte and Weis, Ulrike",
    title = "Archaeometric studies on the petroglyphs and rock varnish at Kilwa and Sakaka, northern Saudi Arabia",
    year = "2020",
    journal = "Arabian archaeology and epigraphy",
    abstract = "Abstract We conducted rock varnish measurements at four rock art sites in north‐western Saudi Arabia, including Kilwa and the Camel Site near Sakaka. We determined the areal densities of Mn and Fe in rock varnish that had accumulated on petroglyph surfaces since their creation, complemented by a detailed analysis of varnish samples. We inferred varnish accumulation rates by relating the Mn areal density on inscriptions to their ages estimated based on the type of script used. Applying these rates to the varnish densities on the rock art indicated that the art was produced during two distinct periods, corresponding to the Pre‐Pottery/Late Neolithic and the Bronze/Iron Age, respectively, with different artistic traditions, reflecting distinct socioeconomic and ecological conditions. Our dating approach, while admittedly burdened with substantial uncertainties, yields ages consistent with archaeological and historical evidence and it provides a unique quantitative tool to obtain at least rough ages for otherwise undatable rock art.",
    url = "https://doi.org/10.1111/aae.12167",
    doi = "10.1111/aae.12167",
    openalex = "W3092893687",
    references = "doi101016jatmosenv201710022, doi1018268bsgm2019v71n1a3"
}

We investigated rock varnish, a thin, manganese- and iron-rich, dark surface crust, on basaltic lava flows and petroglyphs in the Owens and Rose Valleys (California) by portable X-ray fluorescence (pXRF) and femtosecond laser-ablation inductively-coupled-plasma mass spectrometry (fs-LA-ICPMS). The major element composition of the varnish was consistent with a mixture of Mn-Fe oxyhydroxides and clay minerals. As expected, it contained elevated concentrations of elements that are typically enriched in rock varnish, e.g., Mn, Pb, Ba, Ce, and Co, but also showed unusually high enrichments in U, Cu, and Th. The rare earth and yttrium (REY) enrichment pattern revealed a very strong positive cerium (Ce) anomaly and distinct negative europium (Eu) and Y anomalies. The light rare earth elements (REE) were much more strongly enriched than the heavy REY. These enrichment patterns are consistent with a formation mechanism by leaching of Mn and trace elements from aeolian dust, reprecipitation of Mn and Fe as oxyhydroxides, and scavenging of trace elements by these oxyhydroxides. We inferred accumulation rates of Mn and Fe in the varnish from their areal densities measured by pXRF and the known ages of some of the lava flow surfaces. The areal densities of Mn and Fe, as well as their accumulation rates, were comparable to our previous results from the desert of Saudi Arabia. There was a moderate dependence of the Mn areal density on the inclination of the rock surfaces, but no relationship to its cardinal orientation. We attempted to use the degree of varnish regrowth on the rock art surfaces as an estimate of their age. While an absolute dating of the petroglyphs was not possible because of the lack of suitable calibration surfaces and a considerable amount of variability, the measured degree of varnish regrowth on the various petroglyphs was consistent with chronologies based on archeological and other archaeometric techniques. In particular, our results suggest that rock art creation in the study area continued over an extended period of time, possibly starting around the Pleistocene/Holocene transition and extending into the last few centuries.

@article{doi101371journalpone0235421,
    author = "Andreae, Meinrat O. and Alamri, Abdullah and Andreae, T. W. and Garfinkel, Alan and Haug, Gerald H. and Jochum, Klaus Peter and Stoll, Brigitte and Weis, Ulrike",
    title = "Geochemical studies on rock varnish and petroglyphs in the Owens and Rose Valleys, California",
    year = "2020",
    journal = "PLoS ONE",
    abstract = "We investigated rock varnish, a thin, manganese- and iron-rich, dark surface crust, on basaltic lava flows and petroglyphs in the Owens and Rose Valleys (California) by portable X-ray fluorescence (pXRF) and femtosecond laser-ablation inductively-coupled-plasma mass spectrometry (fs-LA-ICPMS). The major element composition of the varnish was consistent with a mixture of Mn-Fe oxyhydroxides and clay minerals. As expected, it contained elevated concentrations of elements that are typically enriched in rock varnish, e.g., Mn, Pb, Ba, Ce, and Co, but also showed unusually high enrichments in U, Cu, and Th. The rare earth and yttrium (REY) enrichment pattern revealed a very strong positive cerium (Ce) anomaly and distinct negative europium (Eu) and Y anomalies. The light rare earth elements (REE) were much more strongly enriched than the heavy REY. These enrichment patterns are consistent with a formation mechanism by leaching of Mn and trace elements from aeolian dust, reprecipitation of Mn and Fe as oxyhydroxides, and scavenging of trace elements by these oxyhydroxides. We inferred accumulation rates of Mn and Fe in the varnish from their areal densities measured by pXRF and the known ages of some of the lava flow surfaces. The areal densities of Mn and Fe, as well as their accumulation rates, were comparable to our previous results from the desert of Saudi Arabia. There was a moderate dependence of the Mn areal density on the inclination of the rock surfaces, but no relationship to its cardinal orientation. We attempted to use the degree of varnish regrowth on the rock art surfaces as an estimate of their age. While an absolute dating of the petroglyphs was not possible because of the lack of suitable calibration surfaces and a considerable amount of variability, the measured degree of varnish regrowth on the various petroglyphs was consistent with chronologies based on archeological and other archaeometric techniques. In particular, our results suggest that rock art creation in the study area continued over an extended period of time, possibly starting around the Pleistocene/Holocene transition and extending into the last few centuries.",
    url = "https://doi.org/10.1371/journal.pone.0235421",
    doi = "10.1371/journal.pone.0235421",
    openalex = "W3047255755",
    references = "doi101016jatmosenv201710022, doi101016jchemgeo201704009, doi101016jchemgeo201905016, doi101086691147, doi1018268bsgm2019v71n1a3"
}

Rock varnishes are known to be fine, dark, glossy submicron films found in deserts bare rock surfaces. The oxides and hydroxides of manganese and iron bind together the clay minerals present in the varnish layer. The processes of oxide-hydroxide accumulation at varnish sites are due to iron and manganese oxidizing bacteria which may require clay minerals for additional nutrition. Quantification and identification of clay minerals in this biofilm is needed to understand its formation. Past attempts to analyze the mineralogical composition of rock varnish have led to inconclusive results as varnish is a submicron thin layer composed of a complex mineral matrix. The elimination of non-crystalline cementing groups comprising of free iron oxides is a key step in the identification of many types of clay minerals, particularly in soil/sediment mineral studies.•The Fe-Mn oxide-hydroxide coatings, acting as cementing materials, can be easily removed using a one-step reduction method employing Na2S2O4 at 70 °C, leading to separation of clay minerals.•We have taken the lead from earlier reported Jackson (1958) method, wherein a combination of reagents was used such as sodium acetate, sodium citrate, hydrogen peroxide, sodium bicarbonate, and sodium dithionite for removing carbonate, organic carbon and Fe-Mn oxy-hydroxide coatings respectively from sediment grains to segregate individual grains from each other.•Our modification helps in the unveiling of clay minerals from a solid substrate and reports the X-ray diffraction peaks, which are elsewise hard to detect and therefore earlier studies are inconclusive.

@article{doi101016jmex2021101511,
    author = "Chaddha, Amritpal Singh and Sharma, Anupam and Singh, Narendra Kumar",
    title = "Clay minerals identification in rock varnish by XRD: A one-step reduction approach.",
    year = "2021",
    journal = "MethodsX",
    abstract = "Rock varnishes are known to be fine, dark, glossy submicron films found in deserts bare rock surfaces. The oxides and hydroxides of manganese and iron bind together the clay minerals present in the varnish layer. The processes of oxide-hydroxide accumulation at varnish sites are due to iron and manganese oxidizing bacteria which may require clay minerals for additional nutrition. Quantification and identification of clay minerals in this biofilm is needed to understand its formation. Past attempts to analyze the mineralogical composition of rock varnish have led to inconclusive results as varnish is a submicron thin layer composed of a complex mineral matrix. The elimination of non-crystalline cementing groups comprising of free iron oxides is a key step in the identification of many types of clay minerals, particularly in soil/sediment mineral studies.•The Fe-Mn oxide-hydroxide coatings, acting as cementing materials, can be easily removed using a one-step reduction method employing Na2S2O4 at 70 °C, leading to separation of clay minerals.•We have taken the lead from earlier reported Jackson (1958) method, wherein a combination of reagents was used such as sodium acetate, sodium citrate, hydrogen peroxide, sodium bicarbonate, and sodium dithionite for removing carbonate, organic carbon and Fe-Mn oxy-hydroxide coatings respectively from sediment grains to segregate individual grains from each other.•Our modification helps in the unveiling of clay minerals from a solid substrate and reports the X-ray diffraction peaks, which are elsewise hard to detect and therefore earlier studies are inconclusive.",
    url = "https://pmc.ncbi.nlm.nih.gov/articles/PMC8563653/",
    doi = "10.1016/j.mex.2021.101511",
    openalex = "W3196649250",
    pmcid = "PMC8563653",
    pmid = "34754782",
    references = "doi101002jpln19590850311, doi101016b9780080092355500267, doi101017s0885715614000840, doi101107s0021889869006558, doi101111j152513141991tb00556x, doi101180mono5, openalexw1512608287, openalexw2101874561"
}

Desert varnish is a dark rock coating that forms in arid environments worldwide. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese-over two orders of magnitude higher manganese content than other cells. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant-a valuable adaptation for coping with the substantial oxidative stress present in this environment. Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities.

@article{doi101073pnas2025188118,
    author = "Lingappa, Usha F and Yeager, Chris M and Sharma, Ajay and Lanza, Nina L and Morales, Demosthenes P and Xie, Gary and Atencio, Ashley D and Chadwick, Grayson L and Monteverde, Danielle R and Magyar, John S and Webb, Samuel M and Valentine, Joan Selverstone and Hoffman, Brian M and Fischer, Woodward W",
    title = "An ecophysiological explanation for manganese enrichment in rock varnish.",
    year = "2021",
    journal = "Proceedings of the National Academy of Sciences of the United States of America",
    abstract = "Desert varnish is a dark rock coating that forms in arid environments worldwide. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese-over two orders of magnitude higher manganese content than other cells. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant-a valuable adaptation for coping with the substantial oxidative stress present in this environment. Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities.",
    url = "https://pmc.ncbi.nlm.nih.gov/articles/PMC8237629/",
    doi = "10.1073/pnas.2025188118",
    openalex = "W3172329579",
    pmcid = "PMC8237629",
    pmid = "34161271",
    references = "doi1010160033589483900650, doi101016jchemgeo201704009, doi101016jchemgeo201905016, doi101016jgca201310040, doi101016jicarus201405038, doi101038nmeth3869, doi101038s4158701902099, doi101080014904512011619636, doi101093bioinformaticsbts174, doi101093bioinformaticsbts252, doi101093bioinformaticsbtu170, doi101093bioinformaticsbtv033, doi101093bioinformaticsbtz848, doi101093sysbiosyq010, doi101128aem0300605, doi10113000167606195869487cdodv20co2, doi101186147121059386"
}

Laser-induced breakdown spectroscopy (LIBS) is a form of optical emission spectroscopy that can be used for the rapid analysis of geological materials in the field under ambient environmental conditions. We describe here the innovative use of handheld LIBS for the in situ analysis of rock varnish. This thinly laminated and compositionally complex veneer forms slowly over time on rock surfaces in dryland regions and is particularly abundant across the Mojave Desert climatic region of east-central California (USA). Following the depth profiling examination of a varnished clast from colluvial gravel in Death Valley in the laboratory, our in situ analysis of rock varnish and visually similar coatings on rock surfaces was undertaken in the Owens and Deep Spring valleys in two contexts, element detection/identification and microchemical mapping. Emission peaks were recognized in the LIBS spectra for the nine elements most abundant in rock varnish-Mn, Fe, Si, Al, Na, Mg, K, Ca and Ba, as well as for H, Li, C, O, Ti, V, Sr and Rb. Focused follow-up laboratory and field studies will help understand rock varnish formation and its utility for weathering and chronological studies.

@article{doi103390molecules26175200,
    author = "Harmon, Russell S. and Khashchevskaya, Daria and Morency, Michelle and Owen, Lewis A. and Jennings, M. and Knott, Jeffrey R. and Dortch, Jason M.",
    title = "Analysis of Rock Varnish from the Mojave Desert by Handheld Laser-Induced Breakdown Spectroscopy",
    year = "2021",
    journal = "Molecules",
    abstract = "Laser-induced breakdown spectroscopy (LIBS) is a form of optical emission spectroscopy that can be used for the rapid analysis of geological materials in the field under ambient environmental conditions. We describe here the innovative use of handheld LIBS for the in situ analysis of rock varnish. This thinly laminated and compositionally complex veneer forms slowly over time on rock surfaces in dryland regions and is particularly abundant across the Mojave Desert climatic region of east-central California (USA). Following the depth profiling examination of a varnished clast from colluvial gravel in Death Valley in the laboratory, our in situ analysis of rock varnish and visually similar coatings on rock surfaces was undertaken in the Owens and Deep Spring valleys in two contexts, element detection/identification and microchemical mapping. Emission peaks were recognized in the LIBS spectra for the nine elements most abundant in rock varnish-Mn, Fe, Si, Al, Na, Mg, K, Ca and Ba, as well as for H, Li, C, O, Ti, V, Sr and Rb. Focused follow-up laboratory and field studies will help understand rock varnish formation and its utility for weathering and chronological studies.",
    url = "https://doi.org/10.3390/molecules26175200",
    doi = "10.3390/molecules26175200",
    openalex = "W3197475263",
    references = "doi1018268bsgm2019v71n1a3"
}

Abstract The complete characteristics knowledge of clay minerals is necessary in the evaluation studies of hydrocarbon reservoirs. Ten samples taken from two wells in a heterogeneous clastic gas reservoir formation in NE Iran were selected to conduct the transmission Fourier transform infrared spectroscopy (FTIR) tests for the clay mineralogy studies. The FTIR analysis showed that there were clear signs of clay minerals in all samples. The wavenumber region of the clay minerals in FTIR tests was detected to be 3621, 3432, 1034, and 515 cm −1 for illite, 3567, 3432, 1613, 1088, 990, 687, 651, and 515 cm −1 for magnesium-rich chlorite, 3700, 3621, 3432, 1034, 687, and 463 cm −1 for kaolinite, and 3567, 1088, 990, and 463 cm −1 for glauconite. After screening of samples by the FTIR method, the samples were then analyzed by powder X-ray diffraction (PXRD), wavelength dispersive X-ray fluorescence (WDXRF), and scanning electron microscopy (SEM). The PXRD and SEM result showed illite was by far the most common clay present. Kaolinite, magnesium-rich chlorite, and traces of smectite and the mixed-layer clays of both the illite–smectite and chlorite-smectite types were also recognized. The combination of PXRD and WDXRF results could quantify the clay abundances in the each well too. It was concluded that the FTIR analysis successfully could show the absorption bonds of all constituent clays. However, the infrared absorption spectra of mixed-layer clays overlapped those of the respective constituents of each mixed-layer minerals. This can be considered as the evidence of the usefulness of FTIR technique in the screening of the samples for the clay mineralogy studies.

@article{doi101007s1320202101449y,
    author = "Jozanikohan, Golnaz and Abarghooei, Mohsen Nosrati",
    title = "The Fourier transform infrared spectroscopy (FTIR) analysis for the clay mineralogy studies in a clastic reservoir",
    year = "2022",
    journal = "Journal of Petroleum Exploration and Production Technology",
    abstract = "Abstract The complete characteristics knowledge of clay minerals is necessary in the evaluation studies of hydrocarbon reservoirs. Ten samples taken from two wells in a heterogeneous clastic gas reservoir formation in NE Iran were selected to conduct the transmission Fourier transform infrared spectroscopy (FTIR) tests for the clay mineralogy studies. The FTIR analysis showed that there were clear signs of clay minerals in all samples. The wavenumber region of the clay minerals in FTIR tests was detected to be 3621, 3432, 1034, and 515 cm −1 for illite, 3567, 3432, 1613, 1088, 990, 687, 651, and 515 cm −1 for magnesium-rich chlorite, 3700, 3621, 3432, 1034, 687, and 463 cm −1 for kaolinite, and 3567, 1088, 990, and 463 cm −1 for glauconite. After screening of samples by the FTIR method, the samples were then analyzed by powder X-ray diffraction (PXRD), wavelength dispersive X-ray fluorescence (WDXRF), and scanning electron microscopy (SEM). The PXRD and SEM result showed illite was by far the most common clay present. Kaolinite, magnesium-rich chlorite, and traces of smectite and the mixed-layer clays of both the illite–smectite and chlorite-smectite types were also recognized. The combination of PXRD and WDXRF results could quantify the clay abundances in the each well too. It was concluded that the FTIR analysis successfully could show the absorption bonds of all constituent clays. However, the infrared absorption spectra of mixed-layer clays overlapped those of the respective constituents of each mixed-layer minerals. This can be considered as the evidence of the usefulness of FTIR technique in the screening of the samples for the clay mineralogy studies.",
    url = "https://doi.org/10.1007/s13202-021-01449-y",
    doi = "10.1007/s13202-021-01449-y",
    openalex = "W4206177593",
    references = "doi101016jsaa200507018"
}

The interaction of similar soil forming factors with different parent materials determines soil chemical weathering, influencing soil processes and properties. In the coastal temperate rainforests of southeast Alaska, Spodosols is the dominant soil order in well-drained parent materials regardless of the lithology. Yet, the role of lithology on chemical weathering, base cation depletion, and mineral transformation during pedogenesis remains elusive. Here, we established a lithosequence comprised of soils derived from tonalite, phyllite, slate, and metavolcanic rocks to test the hypothesis that despite the soils in southeast Alaska presenting similar taxonomy and morphology, the influence of lithology in chemical weathering intensity and mineralogy can be detected. We evaluated physicochemical properties, clay mineralogy, Fe-Al oxides, chemical weathering, and elemental mass balance on eleven Spodosols sampled along the lithosequence. We also propose a new weathering index, the Weathering Index for Spodosols (WISP), suitable to evaluate the weathering degree of soils with Al mobility and base-rich parent materials. This index fills a gap in traditional weathering indices that uses Al as an immobile element and/or does not evaluate the leaching of main base cations. We found the pedons across the lithosequence expressed similar physicochemical properties, with predominantly thick profiles, presenting andic properties, relatively thick spodic horizons, and thin E horizons. Podzolization imposed similar mineral transformation trends, mainly dissolution of chlorite, mica alteration to interstratified mica-vermiculite, and formation of smectite in E horizons. Kaolinite was detected in the soils from all lithologies, but only in trace amounts in phyllite soils. Fe oxides depth distribution was similar but with a higher concentration in the slate (44.5 ± 4.2 g/kg), followed by metavolcanic (31.6 ± 4.0 g/kg), phyllite (27.9 ± 3.8 g/kg), and tonalite soils (19.9 ± 5.7 g/kg) and predominantly as organometallic complexes (extracted by Na-pyrophosphate). The stronger weathering of chlorite and mica in the E horizons was reflected in larger losses of Mg and K, while Ca and Na were more depleted in the spodic horizons, suggesting plagioclase weathering. Slate soils were the most depleted in base cations, followed by phyllite, tonalite, and metavolcanic soils. The more depleted status of slate soils was supported by the higher WISP values (WISP = 62), followed by phyllite (WISP = 55), tonalite (WISP = 53), and metavolcanic soils (WISP = 45). Our results demonstrated that lithology controlled elemental depletion intensities, Fe oxide concentrations, and partially the clay fraction mineralogy in addition to podzolization acting as a dominant pedogenic process across all sites. These findings advance our understanding of the role of lithology on soil mineralogy/geochemistry that impacts critical soil functions, such as the soil carbon cycle, elemental fluxes, organo-mineral interactions, and solid-solution reactions.

@article{doi101016jgeoderma2022116211,
    author = "Spinola, Diogo and Portes, Raquel and Fedenko, Jennifer and Lybrand, Rebecca A. and Dere, Ashlee and Biles, Frances E. and Trainor, Thomas P. and Bowden, Mark and D’Amore, David V.",
    title = "Lithological controls on soil geochemistry and clay mineralogy across Spodosols in the coastal temperate rainforest of southeast Alaska",
    year = "2022",
    journal = "Geoderma",
    abstract = "The interaction of similar soil forming factors with different parent materials determines soil chemical weathering, influencing soil processes and properties. In the coastal temperate rainforests of southeast Alaska, Spodosols is the dominant soil order in well-drained parent materials regardless of the lithology. Yet, the role of lithology on chemical weathering, base cation depletion, and mineral transformation during pedogenesis remains elusive. Here, we established a lithosequence comprised of soils derived from tonalite, phyllite, slate, and metavolcanic rocks to test the hypothesis that despite the soils in southeast Alaska presenting similar taxonomy and morphology, the influence of lithology in chemical weathering intensity and mineralogy can be detected. We evaluated physicochemical properties, clay mineralogy, Fe-Al oxides, chemical weathering, and elemental mass balance on eleven Spodosols sampled along the lithosequence. We also propose a new weathering index, the Weathering Index for Spodosols (WISP), suitable to evaluate the weathering degree of soils with Al mobility and base-rich parent materials. This index fills a gap in traditional weathering indices that uses Al as an immobile element and/or does not evaluate the leaching of main base cations. We found the pedons across the lithosequence expressed similar physicochemical properties, with predominantly thick profiles, presenting andic properties, relatively thick spodic horizons, and thin E horizons. Podzolization imposed similar mineral transformation trends, mainly dissolution of chlorite, mica alteration to interstratified mica-vermiculite, and formation of smectite in E horizons. Kaolinite was detected in the soils from all lithologies, but only in trace amounts in phyllite soils. Fe oxides depth distribution was similar but with a higher concentration in the slate (44.5 ± 4.2 g/kg), followed by metavolcanic (31.6 ± 4.0 g/kg), phyllite (27.9 ± 3.8 g/kg), and tonalite soils (19.9 ± 5.7 g/kg) and predominantly as organometallic complexes (extracted by Na-pyrophosphate). The stronger weathering of chlorite and mica in the E horizons was reflected in larger losses of Mg and K, while Ca and Na were more depleted in the spodic horizons, suggesting plagioclase weathering. Slate soils were the most depleted in base cations, followed by phyllite, tonalite, and metavolcanic soils. The more depleted status of slate soils was supported by the higher WISP values (WISP = 62), followed by phyllite (WISP = 55), tonalite (WISP = 53), and metavolcanic soils (WISP = 45). Our results demonstrated that lithology controlled elemental depletion intensities, Fe oxide concentrations, and partially the clay fraction mineralogy in addition to podzolization acting as a dominant pedogenic process across all sites. These findings advance our understanding of the role of lithology on soil mineralogy/geochemistry that impacts critical soil functions, such as the soil carbon cycle, elemental fluxes, organo-mineral interactions, and solid-solution reactions.",
    url = "https://doi.org/10.1016/j.geoderma.2022.116211",
    doi = "10.1016/j.geoderma.2022.116211",
    openalex = "W4307816463",
    references = "doi1010160033589483900650"
}

Efficient water splitting by birnessite-clay minerals in natural rock varnish, paving the way for the synthesis of a new generation of cost-effective MnO 2 electrocatalysts inspired by nature.

@article{doi101039d2se00185c,
    author = "Chaddha, Amritpal Singh and Singh, Narendra Kumar and Malviya, Manisha and Sharma, Anupam",
    title = "Birnessite-clay mineral couple in the rock varnish: a nature's electrocatalyst",
    year = "2022",
    journal = "Sustainable Energy \& Fuels",
    abstract = "Efficient water splitting by birnessite-clay minerals in natural rock varnish, paving the way for the synthesis of a new generation of cost-effective MnO 2 electrocatalysts inspired by nature.",
    url = "https://doi.org/10.1039/d2se00185c",
    doi = "10.1039/d2se00185c",
    openalex = "W4226065928",
    references = "doi101016jchemgeo201905016, doi101016jmex2021101511"
}

Abstract Bare rock surfaces in dry to semi-arid places of the world usually have a black-brown, metallic sheen covering known as "rock/desert varnish." Rock varnishes are naturally occurring surface textures rich in Mn and Fe that are deposited on rocks. The varnish surface looks to be an ideal environment for microbial development. The function of bacteria in varnish formation, on the other hand, is currently being debated. Therefore, researchers throughout the world have long been interested in the biogeochemical fingerprints of life in severe settings, and the identification of organic entities is an important aspect of the quest for early life in extreme circumstances. High-altitude Ladakh, one of the world's harshest dry deserts, was chosen as the study's sample location due to its severe climatic circumstances. The current study illustrates significant microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy, revealing the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids on the varnish layer as a result of mineral-microbial interactions. We propose for the first time that how change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) allowed microbial life to thrive in extreme environments. The current work provides evidence for a long-standing question about microbial affinity for the varnish layer and illustrates the significance of hydrophobicity in the varnish layer as a crucial component for understanding biogeochemical processes related to varnish growth.

@misc{doi1021203rs3rs2402813v1,
    author = "Chaddha, Amritpal Singh and Sharma, Anupam and Singh, Narendra Kumar and Patel, Devendra Kumar and Satyanarayana, G.N.V.",
    title = "Rock Varnish: A Nature’s Shield",
    year = "2022",
    booktitle = "Research Square",
    abstract = {Abstract Bare rock surfaces in dry to semi-arid places of the world usually have a black-brown, metallic sheen covering known as "rock/desert varnish." Rock varnishes are naturally occurring surface textures rich in Mn and Fe that are deposited on rocks. The varnish surface looks to be an ideal environment for microbial development. The function of bacteria in varnish formation, on the other hand, is currently being debated. Therefore, researchers throughout the world have long been interested in the biogeochemical fingerprints of life in severe settings, and the identification of organic entities is an important aspect of the quest for early life in extreme circumstances. High-altitude Ladakh, one of the world's harshest dry deserts, was chosen as the study's sample location due to its severe climatic circumstances. The current study illustrates significant microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy, revealing the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids on the varnish layer as a result of mineral-microbial interactions. We propose for the first time that how change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) allowed microbial life to thrive in extreme environments. The current work provides evidence for a long-standing question about microbial affinity for the varnish layer and illustrates the significance of hydrophobicity in the varnish layer as a crucial component for understanding biogeochemical processes related to varnish growth.},
    url = "https://doi.org/10.21203/rs.3.rs-2402813/v1",
    doi = "10.21203/rs.3.rs-2402813/v1",
    openalex = "W4313313557",
    references = "doi101016jmex2021101511"
}

Bare rock surfaces in dry to semiarid places of the world often host a black-brown accretion rich in Mn and Fe known as rock varnish. The varnish surface presents an ideal environment for microbial development. A burgeoning interdisciplinary arena of scholarship focuses on the biogeochemical fingerprints of life in severe settings. Given that a large number of researchers hypothesize that varnish formation is a key process by microorganisms, the high altitude Ladakh remains a largely unexplored research setting. Thus, as one of the world’s harshest dry deserts, we selected Ladakh as the focus for this investigation into the nature of organic biomarkers found in subaerial rock varnish in this severe climate. Microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy reveals the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids that we interpret as resulting from mineral–microbial interactions. We hypothesize that a newly discovered change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) might be important in facilitating the development of microbial processes that could be related to varnish formation.

@article{doi101021acsearthspacechem3c00071,
    author = "Chaddha, Amritpal Singh and Sharma, Anupam and Singh, Narendra Kumar and Patel, Devendra Kumar and Satyanarayana, G.N.V.",
    title = "Rock Varnish: Nature’s Shield",
    year = "2023",
    journal = "ACS Earth and Space Chemistry",
    abstract = "Bare rock surfaces in dry to semiarid places of the world often host a black-brown accretion rich in Mn and Fe known as rock varnish. The varnish surface presents an ideal environment for microbial development. A burgeoning interdisciplinary arena of scholarship focuses on the biogeochemical fingerprints of life in severe settings. Given that a large number of researchers hypothesize that varnish formation is a key process by microorganisms, the high altitude Ladakh remains a largely unexplored research setting. Thus, as one of the world’s harshest dry deserts, we selected Ladakh as the focus for this investigation into the nature of organic biomarkers found in subaerial rock varnish in this severe climate. Microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy reveals the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids that we interpret as resulting from mineral–microbial interactions. We hypothesize that a newly discovered change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) might be important in facilitating the development of microbial processes that could be related to varnish formation.",
    url = "https://doi.org/10.1021/acsearthspacechem.3c00071",
    doi = "10.1021/acsearthspacechem.3c00071",
    openalex = "W4385144059",
    references = "doi101016jmex2021101511"
}

Planting alfalfa in grey desert soil can have significant effects on soil nutrient levels, microbial communities, and overall soil improvement. High-throughput sequencing technology was used to explore the relationship between the rhizosphere microbial community structure of grey desert soil planted with different alfalfa varieties (Aohan, WL525HQ, Knight2, Kangsai, Victoria, and WL712), alfalfa characteristics and rhizosphere soil physicochemical properties. Alfalfa planting increased the nitrogen and organic matter in the grey desert soil, and the effects in Victoria, Kangsai, and Aohan were relatively better than those in the unplanted areas and other alfalfa areas. The Chao1 and Shannon indexes showed that the diversity and relative abundance of bacteria and fungi in Kangsai were significantly higher than those in the unplanted areas and other alfalfa areas. Redundancy analysis showed that available nitrogen and phosphorus, as well as fresh weight, significantly affected the changes in fungal and bacterial communities. Variance partitioning analysis showed that soil and alfalfa growth characteristics explained 50.04% and 51.58% of the structural changes in the bacteria and fungi, respectively. Therefore, planting alfalfa changed the community structure of bacteria and fungi, as well as the content of soil nutrients, and different varieties of alfalfa had different effects on soil improvement.

@article{doi101038s41598023410058,
    author = "Qi, Jiangjiao and Fu, Dongqing and Wang, Xuzhe and Zhang, Fanfan and Ma, Chunhui",
    title = "The effect of alfalfa cultivation on improving physicochemical properties soil microorganisms community structure of grey desert soil",
    year = "2023",
    journal = "Scientific Reports",
    abstract = "Planting alfalfa in grey desert soil can have significant effects on soil nutrient levels, microbial communities, and overall soil improvement. High-throughput sequencing technology was used to explore the relationship between the rhizosphere microbial community structure of grey desert soil planted with different alfalfa varieties (Aohan, WL525HQ, Knight2, Kangsai, Victoria, and WL712), alfalfa characteristics and rhizosphere soil physicochemical properties. Alfalfa planting increased the nitrogen and organic matter in the grey desert soil, and the effects in Victoria, Kangsai, and Aohan were relatively better than those in the unplanted areas and other alfalfa areas. The Chao1 and Shannon indexes showed that the diversity and relative abundance of bacteria and fungi in Kangsai were significantly higher than those in the unplanted areas and other alfalfa areas. Redundancy analysis showed that available nitrogen and phosphorus, as well as fresh weight, significantly affected the changes in fungal and bacterial communities. Variance partitioning analysis showed that soil and alfalfa growth characteristics explained 50.04\% and 51.58\% of the structural changes in the bacteria and fungi, respectively. Therefore, planting alfalfa changed the community structure of bacteria and fungi, as well as the content of soil nutrients, and different varieties of alfalfa had different effects on soil improvement.",
    url = "https://doi.org/10.1038/s41598-023-41005-8",
    doi = "10.1038/s41598-023-41005-8",
    openalex = "W4386097968",
    references = "doi101007s0024801200288, doi101016jecoenv201810016, doi101016jsoilbio2022108714, doi101038nmeth2604, doi101038s41598020791398, doi101093bioinformaticsbtq461, doi101093nargkm864, doi101093nargks1219, doi10109900207713444846, doi101134s106422931512011x, doi101186s1287001816002"
}

Rock varnish, a dark-coloured natural feature rich in manganese (Mn), iron (Fe), and clay minerals, is believed to provide nutritional support to microbiota.Thus, rock varnish is considered a unique substrate for potential microbial life to thrive in the extreme environments on Earth that are comparable to their planetary analogues.However, little is known about the occurrence of microbiota in rock varnish, as the microbes found on the varnish are quite diversified.We present here the new morphological and chemical results of microbial forms found in rock varnish samples from Ladakh, a potential site for hosting life in extreme environments.Our results demonstrate the presence of putative magnetofossils type biological entities in the form of nanochains present in the rock varnish layer that coincide with high magnetic susceptibility values of varnish samples.Further, the higher concentrations of oxidised fractions of Mn 4+, and carboxylic acid functionality on the varnish surface revealed the signatures of organic entities.These collective results point towards the enriched concentration of magnetic minerals on the varnish layer that are possibly sourced through biotic forms.Consequently, the rock varnish can serve as a "black box" of ancient environmental records, as well as a potential geomaterial for astrobiological studies from the Martian analogue field location of Ladakh, which needs to be explored further for extensive biogeochemical studies.

@misc{doi1022541essoar16986766772451689v1,
    author = "Chaddha, Amritpal Singh and Sharma, Anupam and Singh, Narendra Kumar and Ali, Nawaz and Das, Payal and Pandey, S. K. and Phartiyal, Binita and Kumar, Subodh",
    title = "Ladakh's Rock Varnish: A potential Geomaterial for astrobiological studies",
    year = "2023",
    abstract = {Rock varnish, a dark-coloured natural feature rich in manganese (Mn), iron (Fe), and clay minerals, is believed to provide nutritional support to microbiota.Thus, rock varnish is considered a unique substrate for potential microbial life to thrive in the extreme environments on Earth that are comparable to their planetary analogues.However, little is known about the occurrence of microbiota in rock varnish, as the microbes found on the varnish are quite diversified.We present here the new morphological and chemical results of microbial forms found in rock varnish samples from Ladakh, a potential site for hosting life in extreme environments.Our results demonstrate the presence of putative magnetofossils type biological entities in the form of nanochains present in the rock varnish layer that coincide with high magnetic susceptibility values of varnish samples.Further, the higher concentrations of oxidised fractions of Mn 4+, and carboxylic acid functionality on the varnish surface revealed the signatures of organic entities.These collective results point towards the enriched concentration of magnetic minerals on the varnish layer that are possibly sourced through biotic forms.Consequently, the rock varnish can serve as a "black box" of ancient environmental records, as well as a potential geomaterial for astrobiological studies from the Martian analogue field location of Ladakh, which needs to be explored further for extensive biogeochemical studies.},
    url = "https://doi.org/10.22541/essoar.169867667.72451689/v1",
    doi = "10.22541/essoar.169867667.72451689/v1",
    openalex = "W4388097517",
    references = "doi101016jmex2021101511"
}

Investigators seeking to solve the mystery of rock varnish formation have yet to embrace hypothesis testing. Thus, this paper presents nine tests that assess the validity of the eight hypotheses proposed over the last four decades to explain varnish formation. An almost singular focus on manganese (Mn)-enrichment by many in varnish research may have led to six of the eight hypotheses failing to explain iron (Fe)-enrichment, as well as why clay minerals dominate the composition of rock varnish. Many varnish researchers displayed a sampling bias by collecting samples only from hot and dry deserts; thus, it should be of no surprise that four hypotheses failed a test of explaining varnishes in different climates; five hypotheses failed to explain varnishes in subsurface locations; and seven hypotheses failed to explain differences in varnish growth rates in hot deserts versus wetter locations. In the end, seven of eight proposed hypotheses to explain varnish formation failed more than five tests, any one of which would falsify the hypothesis. Only one hypothesis “passed” all nine tests.

@article{doi10117703091333241248038,
    author = "Dorn, Ronald I.",
    title = "Rock varnish revisited",
    year = "2024",
    journal = "Progress in Physical Geography Earth and Environment",
    abstract = "Investigators seeking to solve the mystery of rock varnish formation have yet to embrace hypothesis testing. Thus, this paper presents nine tests that assess the validity of the eight hypotheses proposed over the last four decades to explain varnish formation. An almost singular focus on manganese (Mn)-enrichment by many in varnish research may have led to six of the eight hypotheses failing to explain iron (Fe)-enrichment, as well as why clay minerals dominate the composition of rock varnish. Many varnish researchers displayed a sampling bias by collecting samples only from hot and dry deserts; thus, it should be of no surprise that four hypotheses failed a test of explaining varnishes in different climates; five hypotheses failed to explain varnishes in subsurface locations; and seven hypotheses failed to explain differences in varnish growth rates in hot deserts versus wetter locations. In the end, seven of eight proposed hypotheses to explain varnish formation failed more than five tests, any one of which would falsify the hypothesis. Only one hypothesis “passed” all nine tests.",
    url = "https://doi.org/10.1177/03091333241248038",
    doi = "10.1177/03091333241248038",
    openalex = "W4395013421",
    references = "doi10100797814020571997, doi101016jmex2021101511, doi101086691147, doi1018268bsgm2019v71n1a3"
}

underwater on three different clay sediments. Samples were then analyzed using 16S ribosomal RNA sequencing to identify the bacterial communities associated with each clay system. Results show that samples decaying on the surface of kaolinite have a lower bacterial diversity than those decaying on the surface of bentonite and montmorillonite, which could explain the limited decay of carcasses deposited on this clay. However, this is not the only role played by kaolinite, as a greater proportion of gram-negative over gram-positive bacteria is observed in this system. Gram-positive bacteria are generally thought to be more efficient at recycling complex polysaccharides such as those forming the body walls of arthropods. This is the first experimental evidence of sediments shaping an entire bacterial community. Such interaction between sediments and bacteria might have contributed to arthropods' exquisite preservation and prevalence in kaolinite-rich Lagerstätten of the Cambrian Explosion. Supplementary Information: The online version contains supplementary material available at 10.1186/s13358-024-00324-7.

@article{doi101186s13358024003247,
    author = "Corthésy, Nora and Saleh, Farid and Thomas, Camille and Antcliffe, Jonathan B. and Daley, Allison C.",
    title = "The effects of clays on bacterial community composition during arthropod decay",
    year = "2024",
    journal = "Swiss Journal of Palaeontology",
    abstract = "underwater on three different clay sediments. Samples were then analyzed using 16S ribosomal RNA sequencing to identify the bacterial communities associated with each clay system. Results show that samples decaying on the surface of kaolinite have a lower bacterial diversity than those decaying on the surface of bentonite and montmorillonite, which could explain the limited decay of carcasses deposited on this clay. However, this is not the only role played by kaolinite, as a greater proportion of gram-negative over gram-positive bacteria is observed in this system. Gram-positive bacteria are generally thought to be more efficient at recycling complex polysaccharides such as those forming the body walls of arthropods. This is the first experimental evidence of sediments shaping an entire bacterial community. Such interaction between sediments and bacteria might have contributed to arthropods' exquisite preservation and prevalence in kaolinite-rich Lagerstätten of the Cambrian Explosion. Supplementary Information: The online version contains supplementary material available at 10.1186/s13358-024-00324-7.",
    url = "https://doi.org/10.1186/s13358-024-00324-7",
    doi = "10.1186/s13358-024-00324-7",
    openalex = "W4400503207",
    references = "doi101016jearscirev2020103464, doi101017pab201942"
}

@article{doi101016jquascirev2024109146,
    author = "Liu, Tanzhuo and Broecker, Wallace S. and Hemming, Sidney R. and Roth, Helena and Dunseth, Zachary C. and Stiebel, Guy D. and Stein, Mordechai",
    title = "Holocene rock varnish microstratigraphy in the Dead Sea basin and Negev Desert: Chronometric application and climatic implication",
    year = "2025",
    journal = "Quaternary Science Reviews",
    url = "https://doi.org/10.1016/j.quascirev.2024.109146",
    doi = "10.1016/j.quascirev.2024.109146",
    openalex = "W4407030828",
    references = "doi101086691147, doi1018268bsgm2019v71n1a3"
}

Abstract Desert varnish, or rock varnish, is a thin reddish-black microlayer that covers rocks and consists mainly of clay minerals, as well as oxides of iron and manganese. In this paper, both terms—desert and rock varnish—are used interchangeably. Rock varnish has been an object of study and interest for more than two centuries. As it develops mainly in arid and desert areas, the interactions between solar radiation, wind, humidity, and microbial activity are determining factors in its formation. The growth rate of a few microns per millennium suggests that rock varnish can function as a tool for recording past environments. The current work presents the state of knowledge of rock varnish, focusing mainly on inorganic studies. The formation theories, geographical location, magnetic properties, lithodiversity, climatic variability, dating insights, pedofeatures, and some historical aspects are discussed. In addition, the different analytical techniques that have been used to study varnish chemical and mineralogical composition are grouped.

@article{doi101134s1064229325600447,
    author = "Martínez-Pabello, Pável U. and Mergelov, N. S. and Bronnikova, M. A. and Sedov, Sergey and Лебедева, М. П. and Golovanov, D. L. and Trejo-Martin, P.",
    title = "Desert Varnish—a “Microsoil” of the Arid Environments: Origin, Geographical Variability, and Paleoecological Significance (Review)",
    year = "2025",
    journal = "Eurasian Soil Science",
    abstract = "Abstract Desert varnish, or rock varnish, is a thin reddish-black microlayer that covers rocks and consists mainly of clay minerals, as well as oxides of iron and manganese. In this paper, both terms—desert and rock varnish—are used interchangeably. Rock varnish has been an object of study and interest for more than two centuries. As it develops mainly in arid and desert areas, the interactions between solar radiation, wind, humidity, and microbial activity are determining factors in its formation. The growth rate of a few microns per millennium suggests that rock varnish can function as a tool for recording past environments. The current work presents the state of knowledge of rock varnish, focusing mainly on inorganic studies. The formation theories, geographical location, magnetic properties, lithodiversity, climatic variability, dating insights, pedofeatures, and some historical aspects are discussed. In addition, the different analytical techniques that have been used to study varnish chemical and mineralogical composition are grouped.",
    url = "https://doi.org/10.1134/s1064229325600447",
    doi = "10.1134/s1064229325600447",
    openalex = "W4410548210",
    references = "doi101007bf00378791, doi101016jatmosenv201710022, doi101016jcatena200703007, doi101016jmex2021101511, doi101016s0166248108x70102, doi101029rf003p0189, doi101086691147, doi101089153110701750137413, doi101134s106422931512011x, doi101146annurevmicro541827, doi1018268bsgm2019v71n1a3, doi102138am20015611, doi1029003m41749785317072353, doi1058799m39, potter1977desert"
}

@article{doi102138am20259768,
    author = "Dorn, Ronald I.",
    title = "Revisiting the importance of clay minerals in rock varnish",
    year = "2025",
    journal = "American Mineralogist",
    url = "https://doi.org/10.2138/am-2025-9768",
    doi = "10.2138/am-2025-9768",
    openalex = "W4407467258",
    references = "doi101016jmex2021101511"
}