POTASSIUM-BE IDELL ITE IN A HYDROTHERMALLY-ALTERED GRANITE

metasomatism had produced three series of minerals. 1. Partial alteration of muscovite resulted in the formation of ferrous-magnesium-phengite. Bio ite was destabilized toa kaolinite-titanium oxide assemblage. 2. Phengite, unreacted muscovite and feldspars recrystallized to an aluminous, magnesian 1M sericite mineral ranging in composition from mica to illite. 3. Feldspars were entirely destabilized toproduce the assemblage potassic-beideUite-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-montrnorillonite. Potassic-beidellite was not stable under weathering conditions