The Mineralogy and Petrology of the Transition from the Lower to Upper Sillimanite Zone in the Oquossoc Area, Maine

Systematic changes in the assemblages and compositions of minerals suggest that the transition from lower to upper sillimanite zone in the Oquossoc area, Maine, is marked by the reaction Staurolite+Sodic Muscovite+Quartz⇌Sillimanite+Biotite+K-richer Muscovite+Albite+Garnet+H2O. Moreover, the mineralogic data for the silicates suggest that H2 O is buffered by some of the assemblages present but PH2O