Distribution of iron between coexisting biotite and hornblende supports some earlier conclusions regarding the complex nature of mixing among octahedrally coordinated ions in both the minerals. Distribution of iron and magnesium between garnet and coexisting orthopyroxene, hornblende or biotite is orderly and the distribution coefficients for such pairs of minerals are a function of p and T and of the concentration of Mn and Ca in garnet. The nature of the garnet solid solution formed by the mixing of Fe2+, Mg, Mn and Ca depends on the p and T of formation: (1) In garnets of high grade rocks of the granulite facies, Fe2+, Mg and Mn form a nearly ideal mixture and the role of Ca is mainly that of dilution; (2) In garnets of low grade rocks of the epidote-amphibolite facies Fe2+, Mn and Ca tend to mix ideally but the role of Mg in such garnets could not be ascertained. Similarity of the size of ions is important at lower p and T, whereas, the similarity of the ionization potentials of the ions assumes importance at higher p and T in deciding the nature of the final solid solution. Because of the complex nature of solid solution, the composition of garnets alone does not indicate unambiguously the metamorphic grade of the host rocks of variable composition; but in an isochemical rock series, the ratio Mg/Fe2+, along with the concentration of Ca in garnets containing little Mn, may roughly indicate the metamorphic grade.

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