Clay Mineral Compositions And Crystalline Degree Of Illite In The Quaternary Red Clay In Southern Anhui Province, Southeast China

Anhui province is located in the east China, the climate is in a transition zone between warm temperate zone and subtropical zone, and is the transition zone of red earth and loess, both the quaternary red clay exposed, and the Xia-shu loess distribution, is revealed the sensitive area of the loess deposit and red soil development. Clay mineral composition, weathering degree, element geochemical characteristic, chromaticitand, illite crystallinity (IC) of the Quaternary Red Clay (QRC) in Xuanzhou District (XZ) and Langxi County (LX) of Xuancheng City, Anhui Province, Southeast China, were studied to explore relationship and potential paleo-environmental implications of the Quaternary Red Clay. The main results as follows:(1) The clay minerals in different layers of Profiles XZ and LX are quite similar in composition, despite some slight differences. The layer of Yellow-brown Earth (YBE) in the upper part of the profiles is identified as Xiashu Loess formed during the Last Glacial Period, and its clay minerals consist mainly of illite, kaolinite and 2:1 type vermiculite, but in the Uniform Red Clay (URC) and Reticulate Red Clay (RRC), the clay minerals are dominated with illite and kaolinite, and nil of vermiculite. However, in the soil below the reticulated mottling horizon, the peak of illite has become wide and flat, and illite-montmorillonite mixed-layer minerals are formed of weathered illite, reflect the research profile of red clay weathering intensity strengthened with the increase of depth of profile.(2) Mineral composition of the clay fraction was analyzed using the X-ray diffraction (XRD) method, the width of half maximum height of d001 (1.0nm) peak of illite was measured to calculate IC value of illite. Results show that IC values of the two profiles range between 0.4 and 0.7, suggesting that crystallization degree of the illite (IC) lingers between the high and moderate levels. In Profiles XZ and LX IC increases steadily, but crystallization degree declines from YBE to URC and to RRC. Weathering and decomposition of illite in the RRC may also be related to long-term intensive groundwater activities during the formation of vermiculate. In the QRC profiles, the rising trend of IC with soil depth coincides with the variation of weathering degree with soil depth. The QRC in this study is significantly higher than the loess in the Loess Plateau (0.3～0.4), paleosols (0.35～0.5), and Tertiary Red Clay (0.4～0.5), suggesting that the QRC in South China is highly weathered, and consequently low in illite crystallinity.(3) In Profile XZ IC is negatively related to Ba, K2O/TiO2, Na2O/TiO2 and MgO/TiO2, to a significant extent (p<0.05), and in Profile LX IC is negatively related to Saf, Ba, K2O/TiO2 and Na2O/TiO2, to a significant extent, too (p<0.05). The correlations between K2O/TiO2 and IC in Profiles XZ and LX are the most significant, which may be closely related to the high volume of K existing in the interlayer of illite, and K loss from weathering illite and decomposition of illite crystallines of the QRC. Soil colors of XZ and LX profile were measured, The results show that L* in the profile generally decreases downwards, but on the contrary, a* and b* are increase downwards. But at the bottom of the research section, reticulate layer due to the iron loss, soil pale, L* values on the high side; a*and b* values also appear trough wave. In the QRC profiles, the rising trend of IC with soil depth coincides with the variation of weathering degree with soil depth. So like particle size of red soil and elemental geochemistric indices, IC value of QRC, possesses some similar paleoclimatic implications. Therefore, IC of the QRC can be regarded as one of the effective paleoclimatic indicators, reflecting evolution of the paleoclimate during the period when QRC was forming.