Soil-parent material relationship in forest ecosystems of western Alborz: Clay mineralogy

Lithology and soil forming conditions play a very important role in controlling the properties of residual soils. To assess the role of these factors on the composition of clay minerals, residual soils developed on three parent materials including peridotite (Pe), tonalite (To) and limestone (Li) were studied in forest ecosystems of Fuman-Masoule region. Parent rock mineralogy, clay mineralogy of saprolite and soil genetic horizons were determined. In Pe pedon, saprolite and soil genetic horizons have different clay mineralogy. Kaolinite and vermiculite are the major clay minerals present in C2 horizon, whereas smectite, illite, illitevermiculite, illite-smectite and kaolinite are present in upper horizons. Smectite and kaolinte might have formed from pyroxene, olivine, biotite and vermiculite via autogenic and transformation pathways. The presence of illite in pedogenic horizons of Pe pedon is in line with the vertical distribution of total K concentration of clay fraction and could be due to accumulation of K and formation of secondary illite in topsoil. In the saprolite of Li pedon, illite and smectite are present in clay fraction. However, illite is replaced by kaolinite in genetic horizons. Pyrite oxidation in parent material seems to enhance the pedogenic formation of smectite and kaolinite. Clay mineral composition of saprolite layers, especially in Pe and Li pedons, is highly influenced by parent materials and is different from genetic horizons. Although the mineralogical composition of genetic horizons developed on different parent materials is not substantially different, the relative content of given clay minerals might be different in various soils.