Clay mineralogy and its relationship to soil solution composition in soils from different weathering environments of humid Asia: Japan, Thailand and Indonesia

We studied the relationship between the clay mineralogy of subsurface soils from 204 sites and their water extracts in upland soils of humid Asia (Japan, Thailand and Indonesia) to understand their formation. The clay minerals were identified by XRD and the kaolin minerals and gibbsite were quantified with DTA. Their thermodynamic stability was related to the ion activity of soil water extracts. In soils on mica-free andesitic or mafic parent material, kaolin minerals and smectite dominated in all regions. Among mica-containing soils derived from felsic or sedimentary rocks, mineralogical and soil solution compositions indicate that, in Thailand, mica was stable at high pH (5.4-6.5), whereas in soils from Japan and Indonesia mica turned into hydroxy-Al interlayered vermiculite (HIV) or vermiculite at lower pH (4.3-5.5). In Japanese soils, HIV and gibbsite form under high activity of Al-OH species. In contrast, the low activity of Al-OH species in Indonesian soils resulted in the dissolution of Al hydroxides between 2:1 layers and gibbsite and the dominance of kaolin minerals and vermiculite. We conclude that three factors play an important role for the clay mineralogy in humid Asia: the nature of parent material (i.e. presence or absence of mica), soil pH (as affected by precipitation and evapotranspiration) and activity of Al-OH species in soil solution (reflecting soil age).