Crystal chemistry and petrologic significance of Fe (super 3+) -rich phlogopite from the Tapira carbonatite complex, Brazil

Abstract

This contribution deals with the crystal chemistry of phlogopite and Fe3+-rich phlogopite from the Tapira alkaline-carbonatite complex (Brazil) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion composed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebedourite, garnet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of ph logopite is related to the variation of 102' aH20,and aC02as well as magma bulk-chemical composition during fractional crystallization. Phlogopite from alkaline-silicate rocks (ranging from dunite to bebedourite) is characterized by fairly constant Al content, moderate [4]Fe3+substitution, and variable amounts of Ti. The [4]Fe3+ substitution, accompanied by crystals showing reverse pleochroism, increases during fractional crystallization. These features correspond to crystallization at low pressure and high 102 and aH20 in the presence of moderate saturation in Ti-bearing phases, Al203 in the magma, or both. Phlogopite from silicate-carbonatite rocks, classified as ferriphlogopite on the basis of strong reverse pleochroism related to [4]Fe3+tetrahedral substitution, also presents low to very low AI, Fe2+, and Ti contents. These features suggest very high 102' H20' and C02 conditions in the presence of strong saturation in Ti-bearing phases as well as very low Al203 content in the liquid. The crystal-structure refinements of Tapira phlogopite show that Fe3+ substitutes for Si in tetrahedral sites; Fe distribution is completely disordered, so the resulting space group is C21m. 'The octahedral-site composition is similar to that of phlogopite, the octahedral sites being preferentially occupied by Mg. The presence of Fe3+ in the tetrahedral sheet enlarges the whole structure. This enlargement is reflected by an increase in cell-edge lengths and a decrease in ,B-angle values. The increase in distortion of the tetrahedral ring (a angle up to 11°)is necessary for the tetrahedral and octahedral sheets to fit together.