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

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 similar to 8 angstrom 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 similar to 95% for samples of the middle to lower soil profile and similar to 60% for that of the upper portion, respectively. A broad reflection for 10-14 A disappeared and an 18.7 angstrom peak occurred in the XRD patterns after formamide treatment. The 10 angstrom peak became much more intense on heating to 400 degrees C, indicating that the 10-14 angstrom peak was expandable and easily contractible and thus originated from mixed-layer illite-smectite (VS) clays. After heating to 550 degrees C, the 7 angstrom peak disappeared while the 10 angstrom 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 angstrom, with a minority of similar to 7 and similar to 25 angstrom. Particles with similar to 7 angstrom fringes showed a lateral transition of two 7 angstrom kaolinite layers merging into one 12 angstrom 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. (C) 2011 Elsevier B.V. All rights reserved.