Crystal chemistry of Al-rich biotites coexisting with muscovites in peraluminous granites

A comparison was made between single-crystal structure refinements, electro n microprobe analy ses and octahedral site populations of seven biotite cry stals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods. Both micas, From several plutons of North ern Victoria Land (Antarctica) and Sardinia (Italy), show significant octah edral substitutions: biotite has a composition characterized by relatively high Al-[6] content (0.32 less than or equal to Al-[6] less than or equal t o 0.59 apfu), whereas muscovite is characterized by phengite-like substitut ions [0.12 less than or equal to ([6])(Mg + Fe + Ti + Mn) less than or equa l to 0.35]. Mean bond-lengths and electron count data for six biotites-1M ( space group C2/m. agreement factor 2.7% less than or equal to R-obs less th an or equal to 3.6%) and a biotite-2M(1) (space group C2/c, R-obs = 2.8%) s how that Al substitutes for divalent cations in the octahedral M2 site and that the Fe and Mg distribution is disordered. The mean tetrahedral bond le ngths determined for biotite-2M(1) reflect Al-Si disorder. In coexisting mu scovite-2M(1) crystals, small positive electron density residuals close to M1 site position as well as the increase in M2 mean atomic number is in agr eement with the presence of a significant phengitic component. Reduction in biotite unit-cell dimensions with the increase of Al follows a pattern sim ilar to that of associated muscovites. and the octahedral site volumes of b oth micas are influenced by the Al saturation index (ASI) of the rock. Thes e results, and the calculated partition coefficients between biotite and mu scovite for elements in M sites, are consistent with continuous reaction an d re-equilibration of biotite and muscovite during crystallization of peral uminous granitic melts.