Mg/(Mg + Fe) = 0.35 to 0.95. Strong correlations among Mg, Fe, Ti, VI Al, and IV Al. Principal components analysis shows that 98% of the total variance in compositions of biotite from the muscovite-bearing assemblage can be ascribed to two substitutions with phlogopite. Biotite compositions from a particular assemblage might be sensitive indicators of the thermal gradient.--Modified journal abstract.

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