Textural Control on the Weathering of Basic Igneous Rocks: A Micro-morphological Approach

The micro-morphological studies on the rock weathering suggest that basalt shows strong mechanical fracturing during the early stages of weathering. In basalts labradorites are slightly to moderately altered while augites are moderately to strongly altered. In their most advanced stage of weathering the labradorites and augites are highly or even completely altered thereby indicating higher to highest degree of alteration. In the initial stages of weathering of dolerite; labradorites and augites still retain their identity with labradorite showing nuclei of alteration at their centers. However during the more advanced stage of weathering the plagioclases (labradorites) and augites rarely show preservation of diminishing cores of fresh rocks. Formation of enormous clays and related minerals indicate that the dolerite has been strongly to completely altered. During an early stage of weathering of gabbro, labradorite is seen to be altered rapidly than augite. However, in the most advanced stage of weathering the augite is highly altered than labradorite. Camptonite (basic lamprophyre) in its early stages of weathering is not much altered although the minerals are seen to be intensely fractured. However in the most advanced stages of weathering of camptonite the groundmass is seen to be highly altered while labradorite and augite crystals have become highly dirty with the formation of clayey material indicating strong to complete alteration. It is seen that rocks with high amount of glass (e.g. basalt and camptonite) weather more rapidly than the rocks with low glass content and the fine- grained rocks weather more slowly than the coarse-grained rocks. Gabbro shows effects of both mechanical and chemical weathering. In case of the finegrained rocks like basalt, as they contain plenty of glass, there is a little scope of expansion and contraction of minerals due to their tiny crystals.