Clay mineralogy and geochemistry of the soils derived frommetamorpbic and mafic igneous parent rocksin Lahijao area

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)