Analysis of relations among Mg, Fe and Mn in certain metamorphic minerals

Abstract

Existing data on the distribution of Mg, Fe and Mn among coexisting metamorphic aotinolites, cummingtonites, Ca-pyroxenes, and orthopyroxenes, are examined in terms of theoretical solution models applicable to binary and ternary solid solutions. Analysis indicates that the data on the distribution of the major components Mg and Fe may be satisfactorily approximated by functions derived on the assumption that actinolite, Ca-pyroxene and orthopyroxene are ideal binary solutions of their Mg and Fe end members and that cummingtonite is of the general type of regular solution, at least with respect to composition. Although the distribution of Mg and Fe may be regarded as governed by the laws of binary solutions, thus disregarding the minor element Mn, the distribution of Mn can be explained only with reference to the ternary system. One predictable ternary effect is the increase in the atomic fraction Mn/(Mg + Fe 2+ + Mn) of cummingtonite and orthopyroxene relative to the same fraction of actinolite and Capyroxene as these minerals increase in Mg relative to Fe content. Certain observed invariant relations of the distribution functions are also interpreted in terms of the multicomponent systems. The data are also examined with a view of detecting the elemental source of certain deviations in the atomic fractions which might be attributed to errors in the chemical analyses or to failure of a given element to attain equilibrium. In addition, the structural-energetic basis for the thermodynamic functions is discussed in terms of the crystal structure of cummingtonite.