Crystal chemistry of trioctahedral micas-1M from the Alto Paranaiba Igneous Province, Southeastern Brazil

The crystal structures of nine trioctahedral mica-1M crystals (phlogopite a nd ferroan phlogopite containing Fe3+ in tetrahedral position, ferroan tetr a-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silic ate lavas and alkaline-silicate, silicate-carbonatite and carbonatite pluto nic rocks occurring in the Alto Paranaiba Igneous Province (southeastern Br azil), 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 At content; therefore, Fell may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is c haracterized 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 octahe dral sites is enhanced in the Ti-rich crystal from Presidente Olegario owin g to the preference of Ti for the M2 site. The lateral parameters a and b r eflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of thes e micas are controlled by the peculiar composition of the rocks, by the hig h f(O-2), a(H2O), and f(CO2), and by metasomatic events that have affected the Alto Paranaiba Igneous Province.