Kaolinite-smectite mixed-layer clays in the Jiujiang red soils and their climate significance

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• 作者：Hanlie Hong^a ; ^b ; ^{honghl8311@yahoo.com.cn} ; G. Jock Churchman^c ; Yansheng Gu^a ; Ke Yin^b ; Chaowen Wang^b
• 关键词：Red earth ; Clay minerals ; Mixed-layer kaolinite&ndash ; smectite (K/S) ; Pedogenesis ; Palaeoclimate
• 刊名：Geoderma
• 出版年：2012
• 期刊代码：28_00167061
• 类别：ear
• 出版时间：March, 2012
• 卷：173-174
• 期：Complete
• 页码：75-83
• 文件大小：1578 K

摘要

Clay mineralogy of the Jiujiang soils, classified as red earth (on summit and slope), were investigated using X-ray diffraction (XRD), cation exchange capacity (CEC) determination, and high-resolution transmission electron microscopy (HRTEM). The XRD results show that the soils included kaolinite, illite, vermiculite, and mixed-layer I/S and K/S as clay minerals. In XRD patterns of clay separates the low-angle shoulder on the kaolinite (001) peak shifted to ~ 8 脜 on glycolation, indicating the presence of mixed-layer K/S clays in the soils. Kaolinite content in the interstratified kaolinite-smectites was estimated to be ~ 95%for samples of the middle to lower soil profile and ~ 60%for that of the upper portion, respectively. A broad reflection for 10-14 脜 disappeared and an 18.7 脜 peak occurred in the XRD patterns after formamide treatment. The 10 脜 peak became much more intense on heating to 400 掳C, indicating that the 10-14 脜 peak was expandable and easily contractible and thus originated from mixed-layer illite-smectite (I/S) clays. After heating to 550 掳C, the 7 脜 peak disappeared while the 10 脜 peak became more intense and showed a low-angle tail, indicating the interstratified smectite component was hydroxy interlayered. The higher CEC values of the Jiujiang clay separates (22.2-27.3 cmol/kg) also pointed to the occurrence of smectite in the soils. HRTEM analysis showed different lattice fringes for clay particles in the samples; a great majority of layer spacings were in the range 10-20 脜, with a minority of ~ 7 and ~ 25 脜. Particles with ~ 7 脜 fringes showed a lateral transition of two 7 脜 kaolinite layers merging into one 12 脜 smectite layer. This can be interpreted as the partial intercalation of one smectite layer in kaolinite. There was no long-range order in the stacking sequence of K/S layers, and the interstratification was of a random nature. In the uniform soil horizon of the upper profile, there was more vermiculite and illite, but less I/S and K/S, while in the net-like soil horizon of the middle-lower profile there was more kaolinite, I/S, and K/S, with only a trace of vermiculite. The occurrence of K/S clays confirmed that the Jiujiang red earth was derived from syndepositional pedogenesis and the climates changed from warm and seasonally dry to warm and more humid conditions during the soil-forming period.