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 Å peak, which moved towards lower angle after glycolated treatment, indicating the presence of kaolin-smectite mixed-layer clays. The 7 Å 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 Å 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 Å peak of sample (X-160) of the lower soil profile contains three components of 7.16, 7.79, and 8.35 Å respectively, with the notably weak peak of 8.35 Å 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 Å 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 Å 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 Å 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.