Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from Western Maine

A crystal chemical study of thirteen biotite (twelve of 1M polytype and one of 2M1 polytype) and four muscovite samples were made. The biotite, coexists with the muscovite. Samples are from metamorphic terrains and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, for increasing metamorphic grade, the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ) and the K-feldspar + Sillimanite Zone (K+SZ). The muscovite composition clusters near ideal muscovite and displays a small celadonite substitution and a small, but variable paragonite substitution. The biotite composition displays a ratio [vi]Mg2+ / [vi](Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content from 0.48 to 0.72 apfu in metamorphic biotite samples and from 0.51 to 0.67 in those from granites).In trioctahedral micas from western Maine and especially in those with graphite, there is a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation – basal oxygen atom distances and a decrease in tetrahedral flattening angle tau, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle alpha is related to crystallization temperature. In particular alpha decreases with a temperature increase, and alpha is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters (T and fO2) on mica crystal structure, and not just chemical composition.