Rock-Soil Transition In Weathering Of A Porphyritic Biotite Granite

Abstract

Concentric layers of weathered materials around core-stones show the porphyritic biotite granite to experience sequential, but gradational, changes in visible features, textures and mineralogy as it transforms from ‘rock’ into ‘soil’. The changes start with the opening-up of grain boundaries and micro-cracks (stage 1) followed by their dark brown staining (stage 2) and the subsequent alteration (to sericite and clay minerals) of groundmass plagioclase feldspar grains (stage 3). Biotite flakes are then bleached and altered (to chlorite and clay minerals) (stage 4) before there starts alteration (to sericite and clay minerals) of groundmass alkali feldspar grains (stage 5) and finally alteration (to sericite and clay minerals) of the alkali feldspar phenocrysts (stage 6). Quartz grains are not altered during these stages of weathering, but disaggregate and reduce in size due to continual opening-up of grain boundaries and micro-cracks. Increasing stages of weathering are marked by decreasing dry unit weights, dry densities and uniaxial compressive strengths, but increasing apparent porosities. The transition between ‘rock’ and ‘soil’ occurs during stage 6 when all plagioclase, and most alkali feldspar, groundmass grains have been altered as are some alkali feldspar phenocrysts. Stage 6 is marked by large apparent porosities (>18%) but low values of dry unit weight (<20.81 kN/m3), dry density (<2,122 kg/m3) and uniaxial compressive strength (<1.8 MPa).