皖南第四纪红土伊利石结晶度值与风化强度的关系

对安徽宣城市宣州和郎溪两地第四纪红土黏土矿物组合和伊利石结晶度进行研究,探讨其对第四纪红土形成环境的指示意义。结果表明,两地第四纪红土黏土矿物的组合基本相似:剖面上部末次冰期下蜀黄土层(黄棕色土层)黏土矿物主要为伊利石、高岭石和2∶1型的蛭石;均质红土和网纹红土以伊利石和高岭石为主,无蛭石;网纹层下部出现伊利石-蒙脱石混层矿物。根据伊利石的X射线衍射(XRD)峰,获得伊利石结晶度(Illite crystallinity,IC)值,可以反映伊利石结晶程度。两地第四纪红土同类层次的IC值较接近。宣州和郎溪剖面黄棕色土的IC值平均为0.463,均质红土为0.599,网纹红土为0.726。全剖面样品IC值与风化强度指标呈显著负相关,充分说明红土IC值可有效地反映红土的风化强度。第四纪红土剖面从黄棕色土→均质红土→网纹红土,IC值升高,伊利石结晶变差,反映了红土风化强度增加,形成的气候环境更加湿热。

Relationship between illite crystallinity (IC) value and weathering degree of quaternary red clay in Southern Anhui Province, Southeast China

Clay mineral composition and illite crystallinity (IC) value 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 between the two and potential paleo-environmental implications of IC value for formation of the QRC. Clay fraction of the QRC was extracted and then treated with citrate-bicarbonate-dithionite (CBD) to remove free Fe. Mineral composition of the clay fraction was analyzed using the X-ray diffraction (XRD) method, and the width of half maximum height of d001 (1.0 nm) peak of illite was measured to calculate IC value. Contents of macro elements of the QRC were analyzed by the X-ray fluorescence (XRF) method, and SiO2/Al2O3 (Sa), SiO2/(Al2O3 Fe2O3) (Saf), Ba value as well as indicators of weathering degree of the QRC, such as CaO/TiO2, MgO/TiO2, K2O/TiO2 and Na2O/TiO2 were calculated. Results show that 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 lower part of the RRC, the peak of illite has become wide and flat, and illite-montmorillonite mixed-layer minerals are formed from weathered illite. IC values of the two profiles range between 0.4 and 0.7, suggesting that crystallization degree of the illite lingers between the high and moderate levels. IC value in the profiles varies sharply between layers. It is 0.482, 0.578 and 0.735 on average, respectively, in the YBE, URC and RRC of Profile XZ and 0.454, 0.628 and 0.717 on average, respectively, in the YBE, URC and RRC of Profile LX. The QRC in this study is significantly higher than the loess (0.3-0.4), paleosols (0.35-0.5) and Tertiary Red Clay (0.4-0.5) in the Chinese Loess Plateau, suggesting that the QRC in Southern China is highly weathered, and consequently low in illite crystallinity. In Profile XZ, IC value is negatively correlated with Ba, K2O/TiO2, Na2O/TiO2 and MgO/TiO2 to a significant extent (p<0.05), and in Profile LX, IC is negatively correlated with Saf, Ba, K2O/TiO2 and Na2O/TiO2 to a significant extent, too (p<0.05). The correlations between K2O/TiO2 and IC value in Profiles XZ and LX are the most significant, with correlation coefficient (r) being -0.818 and -0.929, respectively, 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. This finding fully demonstrates that IC value can be used as an effective indicator of weathering degree of the QRC in Southern China. In Profiles XZ and LX, IC value increases steadily, indicating that the crystallization degree declines from the 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 value with soil depth coincides with the variation of weathering degree with soil depth. So like particle size of red soil and elemental geochemical indices, IC value of QRC possesses some similar paleoclimatic implications. Therefore, IC value 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.