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

Abstract

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. Å 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. Å disappeared and an 18.7. Å peak occurred in the XRD patterns after formamide treatment. The 10. Å peak became much more intense on heating to 400 °C, indicating that the 10-14. Å peak was expandable and easily contractible and thus originated from mixed-layer illite-smectite (I/S) clays. After heating to 550 °C, the 7. Å peak disappeared while the 10. Å 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. Å, with a minority of ~ 7 and ~ 25 Å Particles with ~ 7. Å fringes showed a lateral transition of two 7. Å kaolinite layers merging into one 12. Å 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. © 2011 Elsevier B.V.