Crystal chemistry of trioctahedral micas-1M from Alto Paranaíba Igneous Province, South-Eastern Brazil.

The crystal structures of nine trioctahedral mica-1M crystals (phlogopite and ferroan phlogopite containing Fe3+ in tetrahedral position, ferroan tetra-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silicate lavas and alkaline-silicate, silicate-carbonatite and carbonatite plutonic rocks occurring in the Alto Paranaíba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C2/m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low Al content; therefore, Fe3+ may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe2+, Fe3+ and Ti4+ in substitution for Mg. The two independent octahedral sites, M1 and M2, show equivalent mean electron-density and mean bond-distances in most cases. Order at the octahedral sites is enhanced in the Ti-rich crystal from Presidente Olegario owing to the preference of Ti for the M2 site. The lateral parameters a and b reflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of these micas are controlled by the peculiar composition of the rocks, by the high f(O2), a(H2O), and f(CO2), and by metasomatic events that have affected the Alto Paranaíba Igneous Province.