Weathering of Fe-Mg Rich Smectite in Red Soils Developed on Tuffaceous Sandstone and Andesite

The sequential process of weathering of Fe-Mg rich smectite is described for 4 Red soil profiles developed on tuffaceous sandstone and andesite. The weathering index (WI) which numerically evaluated the position and order of each sample in the weathering sequence, showed a close correlation with the changes in the chemical and clay mineralogical characteristics of the samples. This observation indicates that WI may provide an effective means to analyze the processes occurring in soils. The whole process covered by the 4 profiles was considered to correspond to the kaolinization of smectite. XRD showed 3 distinct stages, dominated. respectively by 1) smectite, 2) a component giving apparently a very broad band between 7.5 and 14 Å, and 3) halloysite (7 Å). Halloysite (7 Å) may be interstratified with smectite, for it does not give a 7.2 Å peak after 350°C heating. However, it is not certain whether this interstratification occurred in association with subaerial weathering of smectite, for the presence of a halloysitic component is suggested in the smectitic parent materials.