The dependence of localized crystallization of halloysite and kaolinite onprimary minerals in the weathering profile of granite

The formation of kaolin-group minerals in the weathering profile of granite , under the humid, temperate climate as found in Korea, was studied by X-ra y diffraction (XRD), scanning electron microscopy (SEM), and electron micro probe analysis (EMA). The granite was gradually weathered to saprolite. K-r ich feldspar was not weathered in the profile, but plagioclase partially we athered to halloysite septa (i.e., wall-like masses). At the bottom of the profile, biotite had weathered to regularly interstratified biotite-vermicu lite (B-V), and subsequently to kaolinite, with a considerable increase in grain volume. In the upper part of the profile, loose aggregates of transpo rted clays, including halloysite and kaolinite, coated the preformed halloy site septa in the weathered plagioclase. Halloysite had precipitated as a m etastable phase in the microfissures of partially weathered plagioclase. Ka olinite had precipitated heavily in the weathered biotite, where surfaces s upply abundant templates facilitating the nucleation of kaolinite. The loca lized crystallization of halloysite and kaolinite, depending on the distrib ution of primary minerals, strongly influenced the kaolin mineralogy of the granite weathering profile.