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

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, bebe-dourite, gamet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of phlogopite is related to the variation of f_O2, a_H2O, and a_CO2 as 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]MFe³⁺ 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 f_O2 and a_H2O in the presence of moderate saturation in Ti-bearing phases, Al₂O₃ 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 Al, Fe²⁺, and Ti contents. These features suggest very high f_O2, H₂O, and CO₂ conditions in the presence of strong 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 distribution is completely disordered, so the resulting space group is C2/m. 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 β-angle values. The increase in distortion of the tetrahedral ring (α angle up to ∼ 11°) is necessary for the tetrahedral and octahedral sheets to fit together.