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
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C. V. Guidotti (1970) studied this question.