『Abstract
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).
Keywords: 2:1 type clay minerals; Hydroxy-Al interlayered vermiculite;
Vermiculite; Gibbsite; Thermodynamic analysis; Stability diagram』
1. Introduction
2. Materials and methods
2.1. Soils
2.2. Analytical methods
3. Results
3.1. Mineralogy of the silt and clay fractions
3.2. Chemical composition of soil water extracts
4. Discussion
4.1. pH and activities of Al-OH species in the soil water
extracts
4.2. Neoformation and dissolution of gibbsite, kaolin minerals
and smectite
4.3. Transformation of 2:1 type clay minerals
4.4. Weathering sequence in upland soils of humid Asia
References