Portal de Periódicos da CAPES

CRYSTAL CHEMISTRY OF TRIOCTAHEDRAL MICAS-1M FROM THE ALTO PARANAIBA IGNEOUS PROVINCE, SOUTHEASTERN BRAZIL

2001; Mineralogical Association of Canada; Volume: 39; Issue: 5 Linguagem: Inglês

10.2113/gscanmin.39.5.1333

1499-1276

Maria Franca Brigatti, L. Medici, Luciano Poppi, Carmela Vaccaro,

Clay minerals and soil interactions

The crystal structures of nine trioctahedral mica-1 M crystals (phlogopite and ferroan phlogopite containing Fe 3+ 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 Paranaiba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C 2/ m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low Al content; therefore, Fe 3+ may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe 2+ , Fe 3+ and Ti 4+ in substitution for Mg. The two independent octahedral sites, M 1 and M 2, 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 M 2 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 (O 2 ), a (H 2 O), and f (CO 2 ), and by metasomatic events that have affected the Alto Paranaiba Igneous Province.