The weathering of biotite in soils formed in saprolites derived from granite and gneiss was studied. The soils occurred in the Southwestern Upland of Nigeria which is characterized by a humid tropical climate. Vermiculite and montmorillonite are the usual products of biotite weathering in well‐drained temperature soils, whereas kaolinite appears to be the dominant product in the well‐drained, moderately acidic, tropical soils studied. The formation of kaolinitic pseudomorphs represented a stage in the kaolinization of biotite in the soils. A mechanism for the kaolinization of biotite has been suggested.
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A. G. Ojanuga (1973) studied this question.