COLLABORATIVE RESEARCH: Crystal Chemistry of Metapelite Minerals in a Petrologic Context

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This proposal requests continuing support for a research program aimed at achieving an understanding of the relationship between mineral crystal chemistry and metamorphic grade and assemblage. Previously-supported work has demonstrated that the ferric iron content and octahedral ordering in biotite vary as a function of both metamorphic grade and co-existing minerals in a given assemblage. Proposed work will expand on these results in two main directions: by looking more closely at the other minerals which coexist with the previously-studied biotites, and by examining other new suites of rocks over a range of different oxidation conditions. Samples will be studied using transmitted and reflected light petrography of phase determination, electron microprobe for major element analysis, Mossbauer spectroscopy for iron redox and site occupancies, and fast neutron activation analysis of oxygen. Our goals are to provide a more comprehen- sive data base for several common solution phases by modern approaches, to illustrate the effects of poor phase characteri- zation on petrologic models (and suggest possible improvements to such models), and finally, to derive "unique" cation substitution models for several of the solution phases we will study.