New insight into biotite weathering in the subtropic Tongcheng granite regolith, Hubei Province, South China

Alteration of biotite during igneous rock weathering plays an important role in global geochemical cycle and formation of ion-adsorption type rare earth element ores. In order to better understanding the transformation and geochemical processes of biotite weathering, investigation of alteration of biotite was undertaken in the granite regolith in subtropical Tongcheng, Hubei Province, South China. Our results show that weathering of biotite synchronously produced randomly interstratified biotite/vermiculite (B/V) and halloysite, and the B/V clay partially decomposed into discrete vermiculite in the topsoil due to increasing weathering. Vermiculitization of biotite involved leaching of K+ and accumulation of hydrated Mg2+ and Fe3+ in the interlayer space of biotite precursor and thus occurred in a solid-solid state transformation. The vermiculite components in B/V phase are hydroxyl-interlayered species, and the neoformed halloysite could not be expanded by formamide treatment, indicative of dehydrated halloysite. Halloysite occurs in tubular morphology at the edge of biotite or in well-oriented lath-like clusters on the basal surface of biotite crystals. The former crystallized from solutions, whereas the latter formed from local leaching of SiO sheets of biotite and subsequently in situ recrystallization. The parallel lath-like halloysite clusters occur in certain extending directions with angles of ~30° and ~ 60° respectively, resulting from the size misfit between the tetrahedron and octahedron of the crystal structure of biotite precursor. Thus, the formation of halloysite can be considered as a dissolution–precipitation process.