Properties of kaolins in red Oxisols and red Ultisols in Thailand

Purified samples of kaolin group minerals [kaolins] from red Oxisols and red Ultisols from 14 sites on diverse parent materials and under various climatic conditions in Thailand were characterized using analytical transmission electron microscopy, X-ray powder diffraction and chemical analysis. The kaolins show a variety of crystal morphologies including euhedral hexagonal to subhedral platy crystals, tubes and partly rolled tubes. Tubes and partly rolled tubes occur in soils under both udic and ustic moisture regimes (i.e. Rhodic Kandiudox, Typic Kandiudox, Typic Kandiudults, Rhodic Kandiustox). The lowest kaolin HB crystallinity index (3.3–5.3) and smallest size (9–11 nm) are for kaolins consisting of anhedral plates, tubes and partly rolled tubes in a soil on basalt under very high rainfall. This kaolin has the highest specific surface area (SSA) (35–64 m2 g− 1), cation exchange capacity (CEC) (16–23 cmol kg− 1) and Fe2O3 content (1.99–3.62%). The trend of increasing kaolin crystal size with associated decreasing CEC and SSA in these soils is in the sequence red Oxisols on limestone under ustic moisture regime < red Ultisols on old alluvium < red Oxisols on basalt under ustic moisture regime < red Oxisols on limestone under udic moisture regime < red Oxisols on limestone under udic moisture regime < red Ultisols on sedimentary rock under udic moisture regime.