Rock - soil transition during weathering of rhyolite

Abstract

Concentric layers of weathered materials around core-stones show the rhyolite 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 plagioclase feldspar groundmass grains and phenocrysts (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 of 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 weathering stage 6 when all plagioclase groundmass grains and phenocrysts have been altered as have been all alkali feldspar groundmass grains and most phenocrysts. Stage 6 is marked by large apparent porosities (>14%) but low values of dry unit weight (<21.90 kN/m3) and dry density (<2,232 kg/m3).