Summary: The weathering of biotite in some freely drained soils developed from a biotite-rich quartz gabbro near Strathdon, Aberdeenshire, has been studied. Using a combination of X-ray, chemical, differential thermal, infra-red, and optical techniques, it was found that the biotite weathers to an intergrade aluminous vermiculite-chlorite together with zones of kaolinite and gibbsite. The crystallographic axes of biotite and included kaolinite are common and parallel and it is proposed that both kaolinite and gibbsite zones crystallized epitaxially within opened out cleavage spaces of the host biotite. The development of kaolinite and gibbsite in this manner could have occurred relatively quickly and the significance of this with regard to generalized pedogenic weathering reactions of biotite is discussed.