Phengitization, sericitization and potassium-beidellite in a hydrothermally-altered granite

X-ray diffraction, optical and microprobe studies of a kaolinized oligoclase—orthoclase—two-mica—granite (Southern Armorican Massif) indicated that low-temperature metasomatism had produced three series of minerals. 1. Partial alteration of muscovite resulted in the formation of ferrous-magnesium-phengite. Biotite was destabilized to a kaolinite-titanium oxide assemblage. 2. Phengite, unreacted muscovite and feldspars recrystallized to an aluminous, magnesian 1 M sericite mineral ranging in composition from mica to illite. 3. Feldspars were entirely destabilized to produce the assemblage potassic-beidellite-kaolinite—Al-illite. Kaolinite was the predominant mineral in the most altered rock. A fourth stage of mineral equilibrium was also observed; this occurred after material from stage 3 had been reworked. In this environment, the potassic-beidellite was replaced by a calcic-montmorillonite. Potassic-beidellite was not stable under weathering conditions.