Sand Mineralogy of the Alluvium-derived Soils of Southern Bank of the River Brahmaputra in Assam

The distribution, size and shape of light and heavy minerals (in sand fraction of 50–100 microns) from eight pedons developed under different climatic and topographic positions in the southern bank of the Brahmaputra river in Assam indicate the origin of the alluvium from a variety of rocks, viz, igneous, metamorphic and sedimentary occurring in Assam Plateau and Assam Himalayas. The mixed mineralogy of the Brahmaputra alluvium, dominated by the resistant minerals, like quartz, zircon and weatherable minerals, like mica, feldspar, chlorite, etc., is an expression of weathering from parent rocks. The decrease of resistant and a corresponding increase of weatherable minerals with depth in the pedons in upland is attributed to higher rate of surface weathering.