A study of mica composition in a metamorphic terrain which displays a variation in metamorphic grade from the garnet to the sillimanite–garnet–biotite zone, has revealed that muscovite and phengite occur over the whole range of metamorphic conditions that were encountered. The distribution of Mn, Li, Ti, Na, Fe, and Mg between these minerals and coexisting biotite shows many regularities which are interpreted in relation to exchange equilibrium. Two trends of element distribution are suggested, corresponding to the association phengite–biotite and muscovite–biotite. The chemical composition of the mica minerals is not correlative with metamorphic grade.A compositional study of phengite has revealed that the Fe2+/Fe3+ ratio in this mineral is higher in rocks that contain epidote than in rocks that are free of epidote.The distribution of Na and K between muscovite and plagioclase is affected by the Fe content of muscovite and the An content of plagioclase.Mineralogical and chemical data from the study area provide an indication that a generalized chemical equilibrium was established in the rocks during metamorphism.

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