Diversity of clay minerals in soils of solonetzic complexes in the southeast of Western Siberia

Data on the mineralogical composition of clay in soils of solonetzic complexes of the Priobskoe Plateau and the Kulunda and Baraba lowlands have been generalized. The parent materials predominating in these regions have loamy and clayey textures and are characterized by the association of clay minerals represented by dioctahedral and trioctahedral mica–hydromica, chlorite, kaolinite, and a number of irregular interstratifications. They differ in the proportions between the major mineral phases and in the qualitative composition of the minerals. Mica–hydromica and chlorites with a small amount of smectitic phase predominate on the Priobskoe Plateau and in the Kulunda Lowland; in the Baraba Lowland, the portion of mica–smectite interstratifications is higher. An eluvial–illuvial distribution of clay fraction in solonetzes is accompanied by the acid–alkaline destruction and lessivage of clay minerals, including the smectitic phase in the superdispersed state. This results in the strong transformation of the mineralogical composition of the upper (suprasolonetzic) horizons and in the enrichment of the solonetzic horizons with the products of mineral destruction; superdispersed smectite; and undestroyed particles of hydromica, kaolinite, and chlorite from the suprasolonetzic horizons. A significant decrease in the content of smectitic phase in the surface solodic horizons of solonetzic complexes has different consequences in the studied regions. In the soils of the Priobskoe Plateau and Kulunda Lowland with a relatively low content (10–30%) of smectitic phase represented by chlorite–smectite interstratifications, this phase virtually disappears from the soils (there are only rare cases of its preservation). In the soils of the Baraba Lowland developed from the parent materials with the high content (30–50%) of smectitic phase represented by mica–smectite interstratifications, the similar decrease (by 10–20%) in the content of smectitic phase does not result in its complete disappearance. However, the smectitic phase acquires the superdispersed state and the capacity for migration.     

The behavior of the clay fraction in model ecosystems of soil lysimeters

Changes in the mineralogical composition of the clay fraction (<0.001 mm) sampled from soils of the model lysimetric experiment of Moscow State University have been studied. The mineralogical composition of clay is represented by the paragenetic association of minerals typical of noncalcareous mantle loams in the center of the Russian Plain. The predominant smectitic phase consists of complex mixed-layered minerals (mica-smectite with high and low contents of the smectitic layers, chlorite-smectite with different ratios between the chloritic and smectitic layers) and individual smectites. Tri-and dioctahedral hydromica, kaolinite, chlorite, and clay-sized quartz are present in lower amounts. At the early stages of the experiment, the distribution of the smectitic phase in the soil profile is more contrasting than the distribution of the clay fraction. Under the impact of artificially planted meadows, forests, and agrocenoses, soil profiles with different distribution patterns of the clay fraction are formed. The weakly pronounced eluvial distribution pattern of the clay fraction has been registered. Under spruce and mixed stands, the loss of the clay fraction from the upper horizons is due to the hydrolysis of smectitic minerals in the acidified medium. Under broad-leaved stands, perennial herbs, agroecenoses, and fallow, the depletion of smectites from the upper horizons is due to lessivage. The relative accumulation of hydromica and kaolinite is observed in the uppermost soil layer.     

Mineralogical composition of the clay fraction and micromorphology of the Middle and Late Pleistocene loesses and paleosols in the center of the East European Plain

The mineralogical composition of the clay fraction and microfabrics of the cryogenic soil-loess sequences of the Middle and Late Pleistocene ages have been studied near the northern boundary of loess sediments on the East European Plain. Poorly ordered mixed-layered mica-smectitic minerals with different portions of smectitic layers predominate in the clay fraction; di-and trioctahedral hydromicas occupy the second place. The clay fraction also contains chlorite, clay-size quartz grains, and feldspars. Individual smectite is present in some of the samples. Interstadial chernozem-like paleosols are specified by the higher content of clay, the maximum concentration of smectitic layers in the mixed-layered minerals, and the presence of individual smectite. The clay fraction in the profiles of interglacial paleosols is sharply differentiated: in the eluvial part, it is depleted of smectite and enriched in kaolinite, hydromica, and clay-size quartz. These features allow us to suppose that interglacial paleosols were subjected to podzolization processes. According to the mineralogical indices, Middle Pleistocene paleosols can be differentiated into those subjected to lessivage (the Kamenskii interglacial paleosol) and podzolization (the Inzhavin interglacial paleosol).     

Differentiation of layered silicates and biogenic silica in meadow podbel soils of the Central Amur Lowland

Mineralogy of the fine component of meadow podbel soil in the Central Amur Lowland significantly varies depending on texture differentiation within the profile and clay categories with different binding strengths (water-peptized and aggregated clay). In the eluvial part of the profile, hydromicas are predominant, which are accompanied by kaolinite and mica-smectites with a low content of smectite layers; there are many finely dispersed quartz and feldspars; plagioclases are less abundant. The illuvial part of the profile is characterized by a high content of smectite minerals (mica-smectite and kaolinite-smectite interstratifications). Kaolinite, chlorite, and chlorite-vermiculite are also found. Fragmentary components pass into a peptized state: micas-hydromicas, kaolinite, finely dispersed quartz, feldspars, plagioclases, amphiboles, and diatom skeletons (mainly in the illuvial part of the profile). Aggregated clays are characterized by a high content of interstratifications with smectite layers. The mineral composition of two clay categories is strongly differentiated according to eluvial-illuvial type. The bulk chemical composition confirms the textural differentiation of the finely dispersed component within the profile. The chemistry of silty sand cutans on the faces of structural units in the illuvial part of the profile significantly differs from the chemistry of the enclosing horizon and is analogous to that of the eluvial part of the profile. The involvement of silica in the meadow podbel fractions with different binding strengths has been revealed.     

The nature of smectites and associated interstratified minerals in soils of the Gharb plain of Morocco

The mineralogies of ‘Tirs’ (Typic Pelloxererts), and ‘Debs’ (Typic Haploxerolls and Typic Xerochrepts) soils of the Gharb plain in north-western Morocco are investigated, with special attention given to the determination of the nature of the smectitic phase using the lithium test (Li test) and the alkylammonium method. The sand and silt mineralogy of Tirs soils is dominated by quartz with small amounts of feldspars and kaolinite. The sand and silt fractions of Dehs soils also contain significant amounts of mica, chlorite, and interstratified phyllosilicates. The clay minerals of Tirs soils are predominantly a high-charge smectite. The estimated interlayer charge for this phase is 0.61 mol(c)/O₁₀(OH)₂ and the fraction of tetrahedral charge varies from 38 to 44%. Although the percentage tetrahedral charge is less than 50%, the smectitic phase behaves as beidellite with the Li test. Dehs clays are more heterogeneous, consisting of smectite, vermiculite, illite, kaolinite, chlorite, and interstratified illite/smectite and illite/vermiculite. The Li test and the alkylammonium method demonstrate that a high-charge smectite or vermiculite is interstratified with illite. A low-charge montmorillonite is also present both in Tirs and in Dehs soils. The high-charge beidellitic phase is believed to be a transformation product of mica, whilst the low charge montmorillonite is thought to be inherited from the parent material.     

Spatial distribution of mineral components in microcombinations of agrogrey soils with the second humus horizon in the Vladimir opolie area

Mineralogical composition of silt and clay fractions (<1.1–5 and 5–10 µm) in heavy loamy agrogrey soils (Luvic Retic Phaeozems) considerably changes both in the vertical (along the soil profile) and horizontal (along soil microcatenas) directions. The eluvial–illuvial distribution pattern of the clay fraction in the podzolized agrogrey soils with the second humus horizon is replaced by the homogeneous distribution in the agrogrey soils with residual carbonates. The distribution of silt fractions in the soil profiles is relatively homogeneous. The clay (<1 µm) fraction of the parent material is represented by the poorly ordered micasmectite interstratifications minerals, the proportion between which changes in the soil profiles in dependence on the particular pedogenetic processes. Hydromicas represent the second important component of the clay fraction. They consist of di- and trioctahedral varieties, the proportion between which changes in the soil profiles. Kaolinite and iron–magnesium chlorite are present in smaller amounts. The second humus horizon is characterized by the lowest content of mica-smectite interstratifications minerals with the high content of smectitic layers and by the lowest content of the clay fraction. Silt fractions are composed of quartz, micas, potassium feldspars, and plagioclases.     

Genesis and micromorphology of loess-derived soils from central Kansas

The genesis and micromorphology of three Harney soils from different precipitation regions (from 540 mm to 715 mm) (fine, smectitic, mesic Typic Argiustolls) in the Smoky Hills of central Kansas were investigated. The objectives were to (1) examine the morphological, chemical, physical and mineralogical characteristics of Harney soils formed in loess; (2) determine the clay mineral distribution with depth and the origin of the clay minerals present; and (3) investigate the relationship between the clay mineralogy and other soil properties such as soil plasmic fabric, COLE values and fine clay/total clay ratios. Mineralogical and micromorphological techniques were used to evaluate the characteristics of the loess-derived soils. The first pedon was formed in 88 cm of Bignell loess over Peoria loess and the other two pedons were formed from Peoria loess. The chemical properties were similar for the pedons studied. Differences were observed in physical properties, especially in particle size distribution, oven-dry bulk density and coefficient of linear extensibility values. Although the soils were mapped in the same soil series, the geomorphic positions of the pedons and the nature of the parent material affected the characteristics of the soils. Smectite was the predominant clay mineral, especially in the fine clay fraction, regardless of the location in the precipitation gradient. The dominance of smectite increased in the C-horizons. This implies a detrital source of smectite in the B-horizons formed in both Bignell and Peoria loess units. The presence of randomly interstratified mica-smectite and the micromorphological observations of weathering biotite indicate that weathering also plays an important role in the mineralogy of Harney soils. The high content of clay mica in the surface horizons was caused by dust fall in the study area. Thick and continuous argillans were observed when FC/TC and COLE values were low and crystalline smectite was present. In the lower part of the soil profiles, the plasmic fabric was mostly ma-skelsepic (granostriated b-fabric) and smectite was more crystalline as indicated by sharper X-ray diffraction peaks.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Specific Features of Profile Distribution and Crystallochemistry of Phyllosilicates in Soils of the Cisbaikal Forest-Steppe

The mineralogical composition of agrogray, dark gray, and agro-dark gray soils (Luvic Greyzemic Retic Phaeozems); agro-dark gray residual-calcareous soils (Calcaric Cambic Phaeozems); clay-illuvial agrochernozems (Luvic Chernic Phaeozems); and agrochernozems with migrational–mycelial carbonates (Haplic Chernozems) developed in the forest-steppe of Central Siberia within the Irkutsk Depression has been studied. The clay (<1 μm) fraction separated from these soils consists of mixed-layer minerals with alternating layers of hydromica, smectite, vermiculite, and chlorite; the proportions between them change within the soil profiles. The clay fraction also contains hydromicas, kaolinite, chlorite, and some admixture of the fine-dispersed quartz. Each type of the soils is characterized by its own distribution pattern of clay material with specific alternation of layers in the mixed-layer formations. Mixed-layer minerals of the chlorite–vermiculite type predominate in the upper horizons of texture-differentiated soils. Down the soil profile, the content of mixed-layer mica–smectitic minerals increases. In the clay fraction of arable dark gray-humus soils with residual carbonates, the distribution of the clay fraction and major mineral phases in the soil profile is relatively even. An increased content of well-crystallized kaolinite is typical of these soils. The parent material of agrochernozems has a layered character: the upper horizons are generally depleted of clay, and the middle-profile and lower horizons are characterized by the considerable kaolinite content. In general, the clay material of soils of the Tulun–Irkutsk forest-steppe differs considerably from the clay material of foreststeppe soils developed from loesslike and mantle loams in the European part of Russia. In particular, this difference is seen in the proportions between major mineral phases and between biotitic and muscovitic components, as well as in the degree of crystallinity and behavior of kaolinite and chlorite.     

Amount and composition of clay-associated soil organic matter in a range of kaolinitic and smectitic soils

In the global carbon cycle, soil organic matter (SOM) is a major source/sink of atmospheric carbon. Clay minerals stabilize part of the SOM through mineral–organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor decomposition of organic matter, tropical soils would be very poor in organic matter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic matter that is bound to the mineral surface. We focused on organic matter that is associated with kaolinite and smectite. We characterized kaolinite- and smectite-associated SOM in soils from seven countries, employing ¹³C NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggests that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. We first extracted the clay fraction with NaOH and thereafter with Na₄P₂O₇. About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicate that kaolinite-associated SOM is enriched in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. We suggest that different clay minerals use different binding mechanisms to complex SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominantly present in the soil.     

Advances in characterization of soil clay mineralogy using X‐ray diffraction: from decomposition to profile fitting

Structural characterization of soil clay minerals often remains limited despite their key influence on soil properties. In soils, complex clay parageneses result from the coexistence of clay species with contrasting particle sizes and crystal chemistry and from the profusion of mixed layers with variable compositions. The present study aimed to characterize the mineralogy and crystal chemistry of the <2 μm fraction along a profile typical of soils from Western Europe and North America (Neo Luvisol). X‐ray diffraction (XRD) patterns were interpreted using: (i) the combination of XRD pattern decomposition and indirect identification from peak positions commonly applied in soil science; and (ii) the multi‐specimen method. This latter approach implies direct XRD profile fitting and has recently led to significant improvements in the structural characterization of clay minerals in diagenetic and hydrothermal environments. In contrast to the usual approach, the multi‐specimen method allowed the complete structural characterization of complex clay parageneses encountered in soils together with the quantitative analysis of their mineralogy. Throughout the profile, the clay paragenesis of the studied Neo Luvisol systematically includes discrete smectite, illite and kaolinite in addition to randomly interstratified illite‐smectite and chlorite‐smectite. Structural characteristics of the different clay minerals, including the composition of mixed layers, did not vary significantly with depth and are thus indicative of the parent material. The relative proportion of the <2 μm fraction increased with increasing depth simultaneously with smectite relative proportion. These results are consistent with the leaching process described for Luvisols in the literature.     

Mineralogical Composition of Particle-Size Fractions of Solonetzes from the North Crimean Lowland

Data on the mineralogical composition of clay (<1 μm), fine silt (1–5 μm), medium silt (5–10 μm), and coarser (>10 μm) fractions of meadow solonchakous solonetzes (Calcic Gypsic Salic Stagnic Solonetz (Albic, Siltic, Columnic, Cutanic, Differentic)) developing from loesslike loam and clay in the North Crimean Lowland are presented. Fractions >5 μm constitute nearly 50% of the soil mass and are characterized by the same mineralogical composition in the entire profile; they consist of quartz, plagioclases, potassium feldspars, and micas (biotite and muscovite). The eluvial-illuvial redistribution of clay in the course of solonetzic process is accompanied by changes in the portion of mixed-layer minerals and hydromicas in the upper part of the profile; a larger part of the smectitic phase is transformed into the superdisperse state. In the eluvial SEL horizon and in the illuvial BSN horizon, the clay fraction is impoverished in smectitic phase and enriched in trioctahedral hydromicas. Upon calculation of the content of clay minerals per bulk soil mass, the distribution of mixed-layer minerals is either eluvial, or eluvial-illuvial, whereas the distribution of hydromicas has an illuvial pattern without distinct eluvial minimum in the SEL horizons. The eluvial-illuvial distribution pattern of clay minerals in solonetzes of the North Crimean Lowland is compared with the distribution pattern of clay minerals in solonetzes of the West Siberian Lowland. Coefficients characterizing differentiation of solonetzes by the contents of particular mineral components are suggested.     

Clay mineralogy in agrochernozems of western Ukraine

The mineralogy of clay fractions separated from deep low-humus deep-gleyic loamy typical agrochernozems on loess-like loams of the Upper Bug and Dniester uplands in the Central Russian loess province of Ukraine consists of complex disordered interstratifications with the segregation of mica- and smectite-type layers (hereafter, smectite phase), tri- and dioctahedral hydromicas, kaolinite, and chlorite. The distribution of the clay fraction is uniform. The proportions of the layered silicates vary significantly within the profile: a decrease in the content of the smectite phase and a relative increase in the content of hydromicas up the soil profile are recorded. In the upper horizons, the contents of kaolinite and chlorite increase, and some amounts of fine quartz, potassium feldspars, and plagioclases are observed. This tendency is observed in agrochernozems developed on the both Upper Bug and Dniester uplands. The differences include the larger amounts of quartz, potassium feldspars, and plagioclases in the clay material of the Upper Bug Upland, while the contents of the smectite phase in the soil profiles of the areas considered are similar. An analogous mineral association is noted in podzolized agrochernozems on loess-like deposits in the Cis-Carpathian region of the Southern Russian loess province developed on the Prut–Dniester and Syan–Dniester uplands. The distribution of particle-size fractions and the mineralogy of the clay fraction indicate the lithogenic heterogeneity of the soil-forming substrate. When the drifts change, the mineral association of the soils developed within the loess-like deposits gives place to minerals dominated by individual smectite with some mica–smectite inter stratifications, hydromicas, and chlorite.     

Characteristics of Clay Minerals in Podzols and Podzolic Soils

The clay minerals in Podzols and podzolic soils developed under coniferous forests in the Subarctic and Cool-temperate zones are characterized by the predominance of smectite and/or mica-smectite interstratified minerals in the eluvial horizons and chlorite-vermiculite intergrade in the illuvial horizons. A large amount of vermiculite is present in the eluvial horizons of some podzolic soils in the Cool-temperate zone. The illuvial horizons of these soils also contain free iron oxides such as goethite. Imogolite and allophane are present in the illuvial horizons of several soils derived from volcanic ashes. It is suggested that the critical bioclimate for the release of interlayered aluminum from the 2:1-type minerals lies between the Cool- and Warm-temperate zone. In the eluvial horizons of Podzols and podzolic soils, mica minerals and chlorite, as primary minerals, have been transformed to smectite through the pedogenic process. Based on previous studies on the structure and degradation of the dioctahedral mica minerals, it is considered that smectite is transformed from 1M-type mica minerals directly, and from 2M-type mica minerals via mica-smectite interstratifled minerals. The formation of a smectite lattice in the eluvial horizon should be a clay-mineralogical indicator of podzolization.     

Minerals in the three-component combination of agrochernozems in the Kamennaya Steppe

Properties and mineralogy of fine fractions separated from agrochernozems forming a three-component noncontrasting soil combination in the Kamennaya Steppe have been characterized. The soil cover consists of zooturbated (Haplic Chernozems (Clayic, Aric, Pachic, Calcaric)), migrational-mycelial (Haplic Chernozems (Clayic, Aric, Pachic)), and clay-illuvial (Luvic Chernozems (Clayic, Aric, Pachic)) agrochernozems. All the soils are deeply quasi-gleyed because of periodical groundwater rise. The mineralogy of the fraction <1μm includes irregular mica–smectite interstratifications, di- and trioctahedral hydromicas, imperfect kaolinite, and magnesium–iron chlorite. The profile distribution of these minerals slightly varies depending on the subtype of spot-forming soils. A uniform distribution of clay minerals is observed in zooturbated agrochernozem; a poorly manifested eluvial–illuvial distribution of the smectite phase is observed in the clay-illuvial agrochernozem. The fractions of fine (1–5 μm) and medium (5–10 μm) silt consist of quartz, micas, potassium feldspars, plagioclases, kaolinite, and chlorite. There is no dominant mineral, because the share of each mineral is lower than 35–45%. The silt fractions differ in the quartz-to-mica ratio. The medium silt fraction contains more quartz, and the fine silt fraction contains more micas.     

Spatial distribution of clay minerals in agrochernozems of erosional and denudational plains in the Stavropol region

The distribution pattern of the fine fractions (<1.0 and 1?C5 ??m) and the mineralogical composition of the agrochernozems formed on the erosional-denudational plains of the Stavropol region have been studied. Erosion and denudation caused the redistribution of the fine material within the catena with its maximal accumulation on the lowermost part of the slope. The same processes favored the formation of surface deposits slightly differing in the composition of the principal mineral phases, i.e., complex disordered mixedlayered micas-smectites with varying combinations of micaceous and smectite layers in crystallites and di- and trioctahedral hydromicas. Imperfect kaolinite and magnesium-ferric chlorite are accompanying minerals. An increase in the amount of mixed-layered minerals with smectite layer is observed down the profile. In addition to the mentioned minerals, the individual smectite and clinoptilolite, which are components of Tertiary deposits, are identified in the lower parts of the agrochernozem profiles. The fine-silt fractions consist of (in decreasing order) di- and tri-octahedral micas, quartz, feldspars, plagioclase, and an admixture of phyllosilicates (kaolinite, chlorite, and mixed-layered chlorite-smectites). The maximal amount of the fine fraction, as well as the maximal amount of mica in it, is registered in the soils in the lower part of the slope. The phyllosilicates are decomposed in this fraction in the upper horizons. The seven-year-long application of mineral fertilizers intensified the peptization of the soil mass in the arable horizons, which increased the content of clay particles in them. A more contrasting distribution of the mixed-layered formations in the profiles, a considerable decrease in their reflection intensities, an increase in the structural disorder of the minerals, and a certain increase in the content of the fine-dispersed quartz are observed.     

The influence of weathering processes on labile and stable organic matter in Mediterranean volcanic soils

The relationship and mechanisms among weathering processes, cation fluxes, clay mineralogy, organic matter composition and stability were studied in soils developing on basaltic material in southern Italy (Sicily). The soils were transitions between Phaeozems and Vertisols. Intense losses of the elements Na, Ca and Mg were measured indicating that weathering has occurred over a long period of time. The main weathering processes followed the sequence: amphibole, mica, volcanic glass or if ash was the primary source → smectite → interstratified smectite–kaolinite → kaolinite. Kaolinite formation was strongly related to high Al, Mg and Na losses. The good correlation between oxyhydroxides and kaolinite in the soils suggests that (macro)aggregates have formed due to physical or electrostatic interactions between the 1:1 clay minerals and oxides. The stability of organic matter was investigated with a H₂O₂-treatment that assumes that chemical oxidation mimics the natural oxidative processes. The ratio of C after the H₂O₂ treatment to the total organic C ranged from 1–28%. No correlation between clay content and organic matter (labile or stable fraction) was found. The refractory organic fraction was enriched in aliphatic compounds and did not greatly interact with the kaolinite, smectite or poorly crystalline Fe or Al phases. A part of this fraction (most probably proteins) was bound to crystalline Fe-oxides. In contrast, the oxidisable fraction showed a strong relationship with poorly crystalline oxyhydroxides and kaolinite. Surprisingly, smectite did not contribute to the stabilisation of any of the organic C fractions. The stabilisation of organic matter in the soils has, therefore, two main mechanisms: 1) the protection of labile (oxidisable with H₂O₂) organic matter, including also aromatic-rich compounds such as charcoal, by the formation of aggregates with oxyhydroxides and kaolinite and 2) the formation of a refractory fraction enriched in aliphatic compounds.     

Clay mineral evolution along a soil chronosequence in an Alpine proglacial area

As a consequence of global warming, additional areas will become ice-free and subject to weathering and soil formation. The most evident soil changes in the Alps will occur in proglacial areas where young soils will continuously develop due to glacier retreat. Little is known about the initial stages of weathering and soil formation, i.e. during the first decades of soil genesis. In this study, we investigated clay minerals formation during a time span 0-150 years in the proglacial area of Morteratsch (Swiss Alps). The soils developed on granitic till and were Lithic Leptosols.Mineralogical measurements of the clay (< 2 μm) and fine silt fraction (2-32 μm) were carried out using XRD (X-ray Diffraction) and DRIFT (Diffuse Reflectance Infrared Fourier Transform). Fast formation and transformation mechanisms were measured in the clay fraction. The decreasing proportion of trioctahedral phases with time confirmed active chemical weathering. Since the start of soil formation, smectite was actively formed. Some smectite (low charge) and vermiculite (high charge) was however already present in the parent material. Main source of smectite formation was biotite, hornblende and probably plagioclase. Furthermore, irregularly and regularly interstratified clay minerals (mica-HIV or mica-vermiculite) were formed immediately after the start of moraine exposure to weathering. In addition, hydroxy-interlayered smectite (HIS) as a transitory weathering product from mica to smectite was detected. Furthermore, since the start of soil evolution, kaolinite was progressively formed. In the silt fraction, only little changes could be detected; i.e. some formation of an interstratified mica-HIV or mica-vermiculite phase.The detected clay mineral formation and transformation mechanisms within this short time span confirmed the high reactivity of freshly exposed sediments, even in a cryic environment.     

Bilanzierung der Tonbildung und -verlagerung sowie der Tonmineralumwandlung in Löß-Parabraunerden

Balances of alteration and migration of clay fractions and clay minerals in Gray Brown Podzolic Soils from Loess Balances of alteration and migration of clay fractions, clay minerals, K, Mg and Fe bound in clay silicates were calculated for Gray Brown Podzolic Soils (Parabraunerden) derived from Young Pleistocene Loess below arable and forest land in the district south of Würzburg. The extent of clay formation lies between 63 and 98 kg/m², of which 60–80 % belong to the fraction <0.1 μm. The clay migration varies between 35 and 51 kg/m², the fractions < 0.2 μm (especially the fraction <0.1 μm) predominating, the coarse clay being involved only to a smaller extent. During Holocene soil formation 121 kg illite, 16 kg vermiculite and 11 kg kaolinite per m² are formed in the clay fraction (< 2 pm). The loss of smectite amounts to 68 kg/m². The biggest alterations of the clay mineral quantities occur in the fraction <0.1 pm; they indicate a smectite-illite transformation. Illite, at 23 kg/m², accounts for half of the clay migration, followed by smectite and vermiculite each at 9 kg/m² and kaolinite at 5 kg/m². When comparing the migrated with the present amounts no preference of certain clay minerals during clay migration can be determined. In the clay fractions the gained Fe (3.07?4.32 kg/m²) and K (2.75?3.84 kg/m²) predominate over Mg (0.57?1.15 kg/m²). The three elements accumulate to the greatest extent in the fine clay fraction. The element migration parallels the gain. The balances of the elements are discussed in connection with pedogenic illite formation as well as biotite and vermiculite disintegration.     

Effect of Clay Minerals and Organic Matter on the Cation Exchange Capacity of Silt Fractions

The cation exchange capacity (CEC) and specific surface properties were investigated in four particle‐size fractions < 50 μm from three loess (one Kastanozem and two Phaeozems), a holocene (Fluvisol) and a basalt soil (Nitisol) before and after destruction of organic matter. Particle‐size fractions were separated by sedimentation after chemical and physical dispersion of the soil samples. Illite, amorphous minerals, mixed layers, smectite and kaolinite were the predominant clay minerals. They were detected in all size fractions. The CEC increased with increasing organic matter contents and this effect was more pronounced in coarser fractions. The organic matter content per unit surface area was two or three times larger in coarse silt than in clay, irrespective of the soil type.