The alteration of detrital biotites has been studied in two clastic Proterozoic sedimentary sequences from southern and central Sweden, namely the Visingsö Group and the Dala Sandstone, respectively. The results showed that the biotites are altered through replacement by clay minerals (illite, chlorite and less commonly kaolinite). Biotite may either be replaced pseudomorphically (one biotite crystal by one clay mineral crystal) or by fine-crystalline clay minerals which merge into the interstitial clay matrix. In reddish rocks, biotite may be associated with pigmentary hematite and in greenish rocks with pyrite. The variation in the two textural habits of biotite alteration, i.e., pseudomorphic or fine crystalline, is related to physico-chemical properties of pore solution and nature of biotite particles.
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