Biotite alteration to halloysite and kaolinite in soil-saprolite profiles developed from mica schist and granite gneiss

Abstract

The chemical weathering of biotite and associated formation of secondary minerals has important implications for the genesis, mineralogy, chemical properties, and physical properties of soils and saprolites developed from biotite-rich parent rocks. In this study, we used a combination of X-ray diffraction, micromorphological, and scanning electron microscopy techniques to investigate biotite weathering in two soil-saprolite profiles (Typic Kanhapludults) developed from granite gneiss and mica schist in the Piedmont region of North Carolina, USA. In both profiles, sand-sized biotite grains appeared to be transformed directly into kaolinized pseudomorphs of biotite without going through a detectable vermiculite or interstratified biotite-vermiculite intermediate weathering stage. Minerals with biotite-vermiculite mixed layers were only detected in small amounts in the clay- and silt-sized fractions of the saprolite. Weathering sand-sized biotite grains exhibited expanded edges, exfoliation, and cleavage along (001) planes. In the saprolite developed from granite gneiss, kaolin intergrowths within weathering biotite grains were observed. The edges of weathering biotite grains were densely covered with tubular halloysite, suggesting a complex transformation of biotite to halloysite. Halloysite was the dominant clay mineral in the saprolite, but the halloysite content in the clay fractions diminished towards the soil surface.