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Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil

By BRIGATTI MF, MEDICI L, SACCANI E and VACCARO C.

Abstract

This research deals with the crystal chemistry of phlogopite and Fe3+-rich phlogopite from the Tapira Alkaline-Carbonatite Complex (Brazil), in order to assess their petrological significance and genetic conditions. The Tapira Complex consists of a layered intrusion mainly composed of ultramafic rocks (dunites, wehrlites, clinopyroxenites, bebedourites, perovskite-magnetitites, and glimmerites) and, subordinately, carbonatites. The wide range of textural, optical and crystal chemical characteristics of phlogopites are related to the variation of fO2, aH2O, aCO2 and magma bulk chemical composition during fractional crystallization. Phlogopites from alkali-silicate rocks (from dunites to bebedourites) are characterized by a fairly constant Al content, moderate [4]Fe3+ substitution, and variable amounts of [6]Ti and Mg. [4]Fe3+ substitution, accompanied by crystals showing reverse pleochroism, increases during fractional crystallization; these features indicate a crystallization at low pressure, high fO2 and H2O in the presence of moderate amounts of Ti-bearing phases and Al2O3 in the magma. Phlogopites from silico-carbonatite rocks (from perovskite-magnetitites to carbonatites) are classified as ferriphlogopites with strong reverse pleochroism related to [4]Fe3+ tetrahedral substitution and low to very low Al, [6]Fe2+ and [6]Ti contents, indicating very high fO2, H2O and CO2 conditions in the presence of abundant Ti-bearing phases and very low Al2O3 content in the liquid.\ud The crystal structure refinements of Tapira phlogopites confirm that Fe3+ substitutes Si in tetrahedral sites, and that iron distribution is completely disordered, and thus the resulting space group is c2/m. The octahedral composition is similar to that of phlogopites as octahdral sites are preferentially occupied by Mg. The presence of Fe3+ in the tetrahedral sheet produces an enlargement in the whole structure which is reflected by an increase in cell edge lengths and by a decrease in monoclinic ≤ angle values. The increase in distortion of the tetrahedral ring (± angle up to H 11°) is necessary in order for the tetrahedral and octahdral sheets to fit together

Year: 1996
OAI identifier: oai:iris.unife.it:11392/1210422
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