Mineral alteration of two different bedrocks and soil derived from them in Lahijan

Microscopical studies were carried out, in addition to physico-chemical and mineralogical analysis, in order to evaluate the effect of parent material and weathering on soil genesis in forest regions of Lahijan. Two representative soil pedons in mountain landform developed on granite and andesitic basalt located about 8 km southeast of Lahijan were selected and sampling were taken from each horizon by Kubiena boxes or clod. Mineral and their weathering were studied by a polarized microscope under plain and cross light and comparison was made with total elemental analysis by XRF and X-ray analysis (XRD). All evidences showed that there is a close structural relationship between the host mineral and the weathering products. Microscopic observation of soils and weathered rock derived from granite showed evidence of the alteration of feldspars to sericite and chlorite under hydrothermal condition and clay minerals under soil condition. In soils and weathered rock derived from andesitic basalt, alteration of plagioclase (labradorite) and olivine to chlorite and clay minerals as well as pyroxene to amphibole were dominant process. Moreover, higher depth of soil under andesitic basalt with comparison to granite was the morphological evidence of intense weathering effects of humid condition in the study area for different minerals.