Clay mineralogy of Thai soils

The wide variety of soils in Thailand results of the interaction between climate, parent rocks, and physiography of the country. This is reflected in the wide variety of clay minerals. The complete country can be divided into five regions: the northern, central, north-east, south-east coast and peninsular region. The soil map of Thailand has been used to define the clay mineral provinces. From a mineralogical point of view eight clay mineral and three other assemblages can be distinguished. The clay mineral assemblages concern: kaolinite; kaolinite and illite; kaolinite and smectite; kaolinite, smectite and illite; kaolinite, illite and smectite; smectite and kaolinite; smectite, kaolinite and illite; kaolinite and gibbsite. The three other land types comprise clay-free/quartz-dominated soils, organic soils and slope complexes. Kaolinite is present in all soils. In oxisols, kaolinite is associated with gibbsite, goethite and hematite. In areas where the parent rock is mica-rich, illite is the dominant clay mineral. Smectite is the dominant clay mineral in areas where brackish water conditions prevailed during soil formation and where high amounts of dissolved ions were present.