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 ~8Å 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 10–14Å disappeared and an 18.7Å peak occurred in the XRD patterns after formamide treatment. The 10Å peak became much more intense on heating to 400°C, indicating that the 10–14Å peak was expandable and easily contractible and thus originated from mixed-layer illite–smectite (I/S) clays. After heating to 550°C, the 7Å peak disappeared while the 10Å 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 (22.2–27.3cmol/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 10–20Å, with a minority of ~7 and ~25Å. Particles with ~7Å fringes showed a lateral transition of two 7Å kaolinite layers merging into one 12Å 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.