The mineralogical and geochemical composition of the soils of
three representat ive pedons fanned on basaltic andesite, andes itic basalt and
phyllite were investigated. Results by XRF showed that progressive
weathering of rocks have been marked by gradual accumulation of AI, Fe,
Ti, Mg. H30+ and depletion of Na, K, Ca and Si in the soi l; although,
predominant clay, loss and gain trend of elements was different on the
various rocks. Based on x·ray diffraction analysis, minerals in basaltic
andesite and andesitic basalt were similar but, the intensity of mica to
smectite or vermiculite transformation for latter was relative ly higher than
the former. This process revealed the degradation mineral because of two
reasons: ( i) - smectite and vermiculite increased whereas mica decreased in
surface horizons. (ii}-Irregular mixed layer of mica-5mectite or vermicu lite
was present in deeper part (170 cm) of the soils from andesitic basalt but
shallower depth (75 cm) of the soi ls from basaltic andes ite. Clay minerals in
phyllite were mica and chlorite that stratified with vermiculite. However, the
absence of smectite in phyllite might be attributed to more acidic condition
or position of the Fe ion in the mineral lattice of chlorite (higher Fe in the
interlayer hydroxide sheet)