Mineral Composition and Weathering of Soils Derived from Xiashu Loess

Mineralogical, physical and chemical analyses of the soils derived from Xiashu loess were carried out. The primary minerals of these soils were found to be mainly composed of light minerals, such as quartz, feldspar and mica, with traces of heavy minerals. Clay minerals, more complicate in composition, were dominated by hydromica, accompanied by smectite, vermiculite, chlorite, kaolinite, 2:1/1:1 randomly interstratified minerals and small amounts of quartz, goethite, lepidocrocite and hematite, Clay minerals were characterized by low crystallinity and fine particle size. In light of the quartz/feldspars ratio of the 0.01-0.05mm silt fraction, and the clay mineral composition, the freeness of iron oxide, and the silica/sesquioxide and silica/alumina ratios in < 0.002mm clay fraction, it is concluded that the weathering intensity of these soils was lower than those of red soil and yellow earth, but higher than that of brown earth, and that the soil allitization, depotassication and hydroxylation of cl     

Biotite weathering in podzolic soil under conditions of a model field experiment

The biotite changes in the 1–5 μm fraction after its occurrence in the F, H, AE, and E horizons of a pale-podzolic soil for five years under conditions of a model field experiment were assessed by X-ray diffraction analysis. It was found that the main changes of the biotite in all the horizons included the degradational transformation of its crystal lattice to interstratified mica-vermiculite structures and vermiculite. The intensity of this process gradually decreased from the F horizon down the profile in parallel with the decrease in the amount of roots and the abundance and activity of microbiota. Chloritized structures were present among the products of the biotite weathering in the H, AE, and E horizons; the degree of chloritization gradually increased from the H horizon to the E horizon. The main identified products of the biotite weathering in the AE and E horizons formed during the 5 years of the model experiment were identified in the clay and fine-silt fractions from these horizons of the native pale-podzolic soils. Therefore, the vermiculite, soil chlorite, and mixed-layer illite-vermiculite minerals in the soils studied could be considered as products of the recent soil functioning. The obtained results and literature data showed that the weathering of biotite resulted in the formation of K- and Al-buffer systems.     

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.     

Biotite weathering in peaty-podzolic gleyic soil under conditions of a model field experiment

Changes in biotite (fraction 1–5 μm) after exposure in the T2, H, Eih, and E horizons of peatypodzolic gleyic soil under conditions of a model field experiment were studied by X-ray diffraction. It was found that the main transformations of the biotite in all horizons included the degradation of its crystal lattice into regularly interstratified biotite-vermiculite and randomly interstratified biotite-smectite structures and vermiculite. The transformation intensity decreased down the profile simultaneously with a reduction in the content of organic matter, roots, and microbiota population and activity. Chloritized structures were also present among the biotite weathering products in the E horizon. The main identified products of biotite weathering formed in horizons Eih and E over a five-year period of the model experiment were detected in the clay and fine silt fractions of these horizons and in native peaty-podzolic gleyic soils. This suggests that vermiculite and soil chlorite in the soils studied are products of soil functioning. It follows from the results, with consideration for literature data, that the weathering of biotite results in the formation of a potassium-buffering system.     

Mineralogical and geo-chemical characterization of a diapiric formation in the North of Spain

We have selected seven profiles located in a diapiric formation in the North of Spain. The profiles have been analyzed for the mineralogy and the chemical composition of original materials, soils developed above them and clay fractions. Three soils formed on basic rock of volcanic origin (ophite) and rich in alterable minerals, three others formed on clay marl and one soil formed on gypsiferous marl. Plagioclases, pyroxenes, vermiculites, and biotites are the main minerals found in the soil samples and ophitic rocks. Biotite, smectite, chlorite and interstratified chlorite–vermiculite make up the predominant mineralogical association in the clay fraction of the soils. Calcite, biotite and on top of all chlorite are the main minerals in the marls and the soils developed on them, with gypsum predominant in the gypsiferous marl. The mineralogy of its clay fraction is comprised mainly of chlorite and biotite. The variations in content of Al₂O₃, TiO₂ and Na₂O in the ophites are considered to be associated with the differences in the evolution of the pyroxenes. The variability of the chemical composition of the Keuper sediments and the soils is attributed more to the chaotic disposition of the Triassic materials in the formation of the diapir than to intense chemical weathering. The low concentrations of silica, iron, and aluminum extractable with ammonium oxalate indicate the low proportion of non-crystalline products. Fundamentally, it is the semiarid conditions in the study zone, together with the processes of extrusion and hydrothermal activity affecting the formation of the diapir, that are responsible for the genesis of the minerals.     

Soil organic matter formation along a chronosequence in the Morteratsch proglacial area (Upper Engadine,Switzerland)

Global warming leads to the melting of ice caps and glaciers and, consequently, the exposure of new areas of land to the atmosphere and weathering. These areas usually have a high reactivity to both biotic and abiotic changes. Proglacial areas in the Alps usually have a deglaciation time span of around 150 years (time since the end of the “Little Ice Age” in the 1850's). We investigated a chronosequence of very young soils in the proglacial area Morteratsch (Swiss Alps) to derive time-trends of soil organic matter accumulation and evolution. Total organic C and N contents, C and N contents of the various organic matter (OM) density fractions and of the labile (oxidised by H₂O₂) and stable (H₂O₂-resistant) fractions were measured. Further characterisation of OM and the various fractions was performed using Diffuse Reflection Infrared Fourier Transform (DRIFT). Soil organic matter has been accumulated over 150 years at very high rates, values lay between 7 and 36 g C/m²/year. This led to a soil organic matter abundance of about 1–5.5 kg C/m² after 140 years. Even at the start of soil formation, a very stable fraction of soil organic matter was detectable. Stable organic matter (resistant to the H₂O₂ treatment) comprised about 6% of the total soil organic carbon and 10% of the total nitrogen. At the start of soil formation, a very high proportion of soil organic matter was present in the density fractions < 1.6 g/cm³. After about 140 years, 15% of soil organic carbon and 35–40% of the nitrogen was already present in the highest density fraction (> 2 g/cm³). With time, the quality of soil organic matter changed: a decrease of hydrophobicity, an increase in aromatic compounds in the bulk soil and a decrease in phenolic functional groups in the heaviest density fraction were detectable with increasing age. In general, stable organic matter as well as the density fraction > 2 g/cm³ had a low C/N ratio and were enriched in proteinaceous materials. The adsorption of proteinaceous materials points to a strong organo-mineral association. This process has existed since the very beginning of soil formation.     

Weathering of chlorite in a soil derived from a chloritoschist under humid tropical conditions

Eight samples from a soil derived from greenschist were chosen to represent the weathering transformation of chlorite and were subjected to chemical and mineralogical analyses by X-ray, D.T.A., selective and acid dissolution techniques. Regularly interstratified chlorite-vermiculite is the major weathering product in the sand and silt fractions, whereas nontronite predominates in the clay fraction. Kaolinite and a “chlorite-like” mineral progressively appear in the clay fraction as the soil surface is approached. The results show that the genetic link suspected between the clay chloritic component and the kaolinite does not withstand a detailed examination. The clay chloritic mineral is a disordered remnant of the regularly interstratified chlorite-vermiculite and it appears in the clay fraction of the solum concommitantly with the argillitization of the silt fraction. Kaolinite occurs in close association with nontronite and would perhaps form with this mineral a true mixed-layer phase. The unequal distribution of aluminium, iron and magnesium amongst the available sites in the parent chlorite appears to be a factor that strengthens the divergent behaviour of these elements during the weathering.     

辽西褐土的细粒矿物组成

辽宁省西部是低山丘陵区,年平均温度7.1℃,年平均降雨量400—500毫米,降雨集中在夏季,具有明显的大陆性气候特征,干燥度小于1,属半干旱类型.植被为油松柞木和草原灌木丛林.土壤属棕色森林土向栗钙土过渡的褐土地带.母质主要为花岗片麻岩风化物.山麓缓坡和河谷两岸为黄土丘陵.在黄土沉积物覆盖层下,常见红色风化壳露头,即红色粘土层.此外,并有松软易风化岩层,如砂岩、页岩和变质岩等.本区近百年来,由于自然植被受到严重破坏,大量水土流失,土壤侵蚀严重,土壤有机质含量低,成为辽宁省的低产区.因此,鉴定本区不同母质的土壤矿物胶体组成及其特征,不仅可以研究这一特定自然条件下土壤矿物的转化和形成,并且有助于了解土壤特性和肥力特征.     

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.     

Mineralogic evolution in hydromorphic sandy soils and podzols in “landes du médoc”, France

Soils of the nearly level “Landes du Médoc” in southwestern France have a pattern of alternating bodies of hydromorphic podzols (Haplaquods) and low humic hydromorphic soils (Psammaquents). The soils are formed in a sedimentary mantle of coarse, quartzose sands with a slight microrelief consisting of low, elongated ridges and shallow, intervening troughs. The water table is at shallow depths throughout the plain, even at the surface in places. The podzols on the crests of the low ridges have distinct A₂ and cemented B₂ h horizons. Podzols persist down the sides of ridges but going downslope first lose the A₂ horizon and then the cementation of the Bh horizon. Soils in the shallow troughs have A₁ and Cg horizons without B horizons.The fine silt (2–20 μm) and clay (0–2 μm) fractions of the parent sand contain primary trioctahedral chlorite, mica, feldspars, and quartz, with the last mineral predominant. During soil development, the first three minerals undergo weathering at different rates and to different extents. Chlorite is most strongly weathered, followed in order by plagioclases and K-minerals. In the fine silt fraction, weathering seems to occur mostly by fragmentation of particles. In the clay fraction, the phyllosilicates successively form irregularly interstratified minerals with contractible but not expandable vermiculitic layers, interstratified minerals with contractible and expandable smectitic layers, and finally smectites.The extent to which the silicate minerals are weathered becomes progressively greater from the low humic hydromorphic soils to the podzols with friable Bh horizons to the podzols with cemented Bh horizons. Smectite is present only in the A₂ horizons of these last podzols.The aluminum release by weathering of silicate minerals is translocated in part in the form of organo-metal complexes into the Bh horizons of the podzols. Greatest concentrations of Al are associated with coatings of monomorphic organic matter on mineral grains in the cemented Bh horizons, in which some Al has also crystallized into gibbsite. That mineral was not detected in friable B horizons of podzols nor in the low humic hydromorphic soil. Contrary to expectations, the mobile Al did not enter interlayer spaces of expanding 2:1 clay minerals.