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The presence and formation of hydroxy-interlayered minerals in the Holocene paleosol at the Wugong profile on the southernmost Chinese Loess Plateau were investigated by examining the < 1 μm and 1–2 μm clay fractions of the 50–150 cm horizon by Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Prior to the analyses free Al and Fe phases were removed by dithionite-citric-bicarbonate (DCB) extraction. XRD patterns were obtained with air-dried, Mg-saturated, Mg-saturated with glycerol solvation (Mg-glycerol) and K-saturated and heated at 110, 250, 350, 450 and 550 °C samples both before and after hydroxy-interlayered materials removal by sodium citrate extraction. The absorption bands at 3699 and 3430 cm − 1 in the FTIR spectra of both sub-fractions indicated the presence of hydroxy-interlayered minerals. The K-saturated and heated XRD patterns confirmed this by the progressive collapse of the 1.4 nm reflection to a 1.0 nm reflection. After removal of the hydroxy-interlayered materials no obvious increase of the relative XRD intensity of the 1.8 nm reflection in Mg-glycerol pattern was observed and the 1.4 nm reflection exhibited a stronger collapse to 1.0 nm after K-saturation and increasing temperature treatment. This indicated that the hydroxy-interlayered minerals in the Holocene paleosol were mainly composed of hydroxyl-interlayer vermiculite. The hydroxyl-interlayer vermiculite was formed by the polymerization of Al/Fe-hydroxy materials within the interlayer space of vermiculite. Shoulder-like reflection at 1.2 nm in the K-550 °C pattern of the 1–2 μm fraction indicated a higher degree of Al/Fe-hydroxylation in the coarse fraction than in the fine fraction, which was consistent with the lower extractable Al and Fe in the coarse fraction. The reflection at 1.38 nm in the K-550 °C pattern could be interpreted as pedogenic chlorite. Thus, transformation of vermiculite into hydroxy-interlayered vermiculite and finally into a chlorite-like mineral (pedogenic chlorite) has occurred in the Holocene paleosol on the southernmost Chinese Loess Plateau. [ABSTRACT FROM AUTHOR]

Abstract: Soils derived from the Loess Plateau of China are regionally important and expression of the soil properties along the soil profile may be directly related to climate changes. The objective of this research was to analyze the clay mineral transformation of loess from the southernmost Loess Plateau, in relation to the pedogenesis and the regional climate changes. The mineralogy of the soil profile at the site was studied by using X-ray diffraction. The results showed that 2:1 layer type minerals dominated with the illite in the majority throughout the profile. The changes of the soil minerals were consistent with the soil genetic horizons and with the variable CaCO3 content, particle size distribution, and variations of magnetic susceptibility. A relatively high vermiculite content and the presence of hydroxyl-interlayered mineral (HIM) occurred in the most weathered horizon (50–150cm) corresponding to the buried palaeosol (S0) formed during the ‘optimum’ Holocene (8400–3100B.P.yr) which indicated the acidic conditions with low organic matter in this period. During the formation of palaeosol, the expected transformation of illite and chlorite into vermiculite through the formation of mixed layers has occurred. The distribution of kaolinite was uniform with depth in the palaeosol suggesting inheritance from the original eolian deposition materials. Generally, the depotassication of illite and the degradation of chlorite were the major mineral transformation processes that occurred with soil-formation. Calculation of the total amount of carbonate leaching of the palaeosol suggested that the annual precipitation during the ‘optimum’ Holocene was probably approximately to 880mm. The regional climate during the ‘optimum’ Holocene on the southernmost Loess Plateau was analogous with the modern climate conditions in the northern subtropical zone (the southern slope of Qingling Mountains) for the formation of Udic Luvisols (Brown Soil). However, as a result of its specific pedogenic pattern, the pedogenic strength of the palaeosol did not reached the level of Udic Luvisols (Brown Soil). The hydroxyl-interlayered mineral presence in the 50–150cm horizon only, also illuminated that the palaeosol was buried rapidly by post depositional loess during the abrupt cold-dry climate in the late Holocene. The contradiction between the type clay minerals present and the measured alkaline soil pH values in the palaeosol could be understood by the recalcification caused by the post-pedogenic leaching from the overlying loess. [Copyright &y& Elsevier]