BIOTITE ALTERATION TO HALLOYSITE AND KAOLINITE IN SOIL-SAPROLITE PROFILES DEVELOPED FROM MICA SCHIST AND GRANITE GNEISS

The chemical weathering of biotite and associated formation of seconda ry minerals has important implications for the genesis, mineralogy, ch emical properties, and physical properties of soils and saprolites dev eloped from biotite-rich parent rocks. In this study, we used a combin ation of X-ray diffraction, micromorphological, and scanning electron microscopy techniques to investigate biotite weathering in two soil-sa prolite profiles (Typic Kanhapludults) developed from granite gneiss a nd mica schist in the Piedmont region of North Carolina, USA. In both profiles, sand-sized biotite grains appeared to be transformed directl y into kaolinized pseudomorphs of biotite without going through a dete ctable 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 expan ded edges, exfoliation, and cleavage along (001) planes. In the saprol ite developed from granite gneiss, kaolin intergrowths within weatheri ng biotite grains were observed. The edges of weathering biotite grain s were densely covered with tubular halloysite, suggesting a complex t ransformation of biotite to halloysite. Halloysite was the dominant cl ay mineral in the saprolite, but the halloysite content in the clay fr actions diminished towards the soil surface.