CRYSTAL-CHEMISTRY AND PETROLOGIC SIGNIFICANCE OF FE3-RICH PHLOGOPITE FROM THE TAPIRA CARBONATITE COMPLEX, BRAZIL()

This contribution deals with the crystal chemistry of phlogopite and F e3+-rich phlogopite from the Tapira alkaline-carbonatite complex (Braz il) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion compos ed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebe dourite, garnet-magnetitite, perovskite-magnetitite, and glimmerite) w ith subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of phlogopite is related to the varia tion of f(O2), a(H2O), and a(CO2) as well as magma bulk-chemical compo sition during fractional crystallization. Phlogopite from alkaline-sil icate rocks (ranging from dunite to bebedourite) is characterized by f airly constant Al content, moderate Fe-[4](3+) substitution, and varia ble amounts of Ti. The Fe-[4](3+) substitution, accompanied by crystal s showing reverse pleochroism, increases during fractional crystalliza tion. These features correspond to crystallization at low pressure and high f(O2) and a(H2O) in the presence of moderate saturation in Ti-be aring phases, Al2O3 in the magma, or both. Phlogopite from silicate-ca rbonatite rocks, classified as ferriphlogopite on the basis of strong reverse pleochroism related to Fe-[4](3+) tetrahedral substitution, al so presents low to very low Al, Fe2+, and Ti contents. These features suggest very high f(O2), H2O, and CO2 conditions in the presence of st rong saturation in Ti-bearing phases as well as very low Al2O3 content in the liquid. The crystal-structure refinements of Tapira phlogopite show that Fe3+ substitutes for Si in tetrahedral sites; Fe distributi on is completely disordered, so the resulting space group is C2/m. The octahedral-site composition is similar to that of phlogopite, the oct ahedral sites being preferentially occupied by Mg. The presence of Fe3 + in the tetrahedral sheet enlarges the whole structure. This enlargem ent is reflected by an increase in cell-edge lengths and a decrease in beta-angle values. The increase in distortion of the tetrahedral ring (alpha angle up to similar to 11 degrees) is necessary for the tetrah edral and octahedral sheets to fit together.