安徽宣城红土剖面中粘土矿物过渡相及其意义

为研究长江中下游红土剖面中粘土矿物的特征及其成因意义, 对安徽宣城红土剖面中粘土矿物进行深入、系统的X射线衍射分析.结果表明, 宣城剖面各土壤层中粘土矿物成分基本一致, 主要为蛭石、伊利石、高岭石, 以及粘土矿物过渡相. 由采自剖面上部样品的X射线衍射图可知, 经乙二醇饱和后7 ?衍射峰可分解为7.15、7.60和7.92 ?三部分, 表明除了高岭石(7.15 ?)外, 还存在高岭晶层含量分别为~80%和~95%的2种高岭-蒙脱石过渡相, 并以前者为主; 剖面下部样品在乙二醇饱和后, 7 ?衍射峰可分解为7.16、7.79和8.35 ?等3个衍射峰, 其中8.35 ?峰衍射强度很小, 表明除了高岭石外, 样品中存在高岭晶层含量为~90%和~43%的高岭-蒙脱石过渡相, 后者含量甚少.甲酰胺饱和结果表明, 高岭-蒙脱石混层粘土矿物相中高岭晶层为埃洛石相.加热试验的衍射图中10 ?衍射峰强度明显增强, 证实高岭相中含有一定数量的来源于绿泥石风化的蒙脱石间层; 而10 ?衍射峰的低角度一侧没有出现拖尾现象, 则指示高岭-蒙脱石混层矿物中的蒙脱石不是简单的羟基间层蒙脱石.此外, 红土剖面中还普遍出现过渡性粘土矿物伊利石-蒙脱石混层和伊利石-蛭石混层粘土矿物.大量过渡性粘土矿物相的出现, 从成土作用的角度上说明红土沉积物经历了沉积-风化、以及多期风化作用叠加, 而且在沉积-风化成土过程中, 气候环境变化于强烈化学风化的温暖、季节性干旱和强烈风化淋滤的温暖而更加潮湿的条件.蛭石-伊利石混层粘土矿物仅发育于红土剖面上部, 表明总体上剖面上部的化学风化程度低于剖面下部. 

Mixed-Layer Clay Minerals in the Xuancheng Red Clay Sediments,Xuancheng, Anhui Province

Clay species of the Xuancheng red clay sediments were investigated using X-ray diffraction (XRD) method to understand the clay mineralogy and its genesis significance of the sediments in the middle to lower reaches of the Yangtze River, South China. Our results show that soil layers of the laterite profile have similar clay mineral compositions of mainly vermiculite, illite, kaolinite, and mixed-layer clays. In the XRD patterns of clay separates, there was weak peak in lower angle side of 7 ? peak, which moved towards lower angle after glycolated treatment, indicating the presence of kaolin-smectite mixed-layer clays. The 7 ? peak of the representative sample (X-18) of the upper soil profile was decomposed into three components of 7.15, 7.60, and 7.92 ? respectively, suggesting that there exists kaolinite, as well as two kaolin-smectite mixed-layer clays, with ~80% and ~95% kaolin layers respectively, and with the former in abundance and the latter in relatively less abundance. The 7 ? peak of sample (X-160) of the lower soil profile contains three components of 7.16, 7.79, and 8.35 ? respectively, with the notably weak peak of 8.35 ? suggesting that there are kaolinite and two kaolin-smectite mixed-layer clays of ~90% and ~43% kaolin layers respectively, with the latter only present in trace amounts. The intensity of the 10 ? peak increased notably with a decrease of the low-angle shoulder on the kaolinite (001) peak after formamide treatment, suggesting that kaolin in the mixed-layer kaolin-smectites is halloysite. On being heated to 400 ℃, the 10 ? peak became much more intense, indicating that certain amounts of kaolin phase derived from smectite layers with chlorite origin. No low-angle tail was observed on the 10 ? peak when heated to 600 ℃, indicating that the interstratified smectite component was not hydroxy interlayered. Mixed-layer illite-smectite and illite-vermiculite were also observed in the Xuancheng red clay sediments. The occurrence of abundant mixed-layer clays in the red clay profile suggests that the red earth sediments underwent overlap of deposit-weathering and multi-staged weathering processes. In the deposit-weathering pedogenesis process, the climate fluctuated between warm/seasonally dry and warm/humid conditions. The presence of mixed-layer illite-vermiculite in upper section indicates that the weathering degree of upper section is generally lower than that of the lower section.