Pedogenic alteration of illite in subtropical China

Pedogenic alteration of illite from red earth sediments in Jiujiang in subtropical China was investigated using X -ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Illite, hydroxy-interlayered vermiculite (HIV), kaolinite and mixed-layer illite-HIV (I-HIV) are present in the soils. The characteristic reflections of the clay phases were 14 Aa, 10-14 Aa, 10 Aa, and 7 Aa, respectively. After Mg-glycerol saturations, the 14 Aa peak of the samples did not expand, and after heating at 350 degrees C and 550 degrees C it shifted to 13.8 Aa and 12 Aa respectively, with no residual 14 Aa reflection, suggesting the occurrence of hydroxy-interlayered vermiculite. The randomly interstratified I-HIV clays were characterized by a broad peak at 10-14 Aa, which did not change its position after Mg-glycerol saturation, but collapsed to 10 Aa after heating at 350 degrees C and 550 degrees C. HRTEM analysis showed different lattice fringes of 12 Aa, 10 Aa and 7 Aa. Mixed-layer I-HIV, HIV-K and illite-kaolinite (I-K) were observed in the HRTEM images which represented the intermediate phases during illite alteration. The merging of two 10 Aa illite layers into a 12 Aa HIV layer, lateral transformation of one HIV layer into one kaolinite layer and alteration of one illite layer into two kaolinite layers illustrated the mechanisms of illite-to-HIV, HIV-to-kaolinite and illite-to-kaolinite transformation, respectively. The proposed pedogenic alteration of illite and the weathering sequence of the clay minerals in Jiujiang is illite-->I-HIV-->HIV-->HIV-K-->kaolinite. In addition, illite may transform directly to kaolinite.