Evaluating micas in petrologic and metallogenic aspect: I–definitions and structure of the computer program MICA +

Abstract

Micas are significant ferromagnesian minerals in felsic to mafic igneous, metamorphic, and hydrothermal rocks. Because of their considerable potential to reveal the physicochemical conditions of magmas in terms of petrologic and metallogenic aspects, mica chemistry is used extensively in the earth sciences. For example, the composition of phlogopite and biotite can be used to evaluate the intensive thermodynamic parameters of temperature ( T, °C), oxygen fugacity ( fO 2), and water fugacity ( fH 2O) of magmatic rocks. The halogen contents of micas permit the estimation of the fluorine and chlorine fugacities that may be used in understanding the metal transportation and deposition processes in hydrothermal ore deposits. The Mica + computer program has been written to edit and store electron-microprobe or wet-chemical mica analyses. This software calculates structural formulae and shares out the calculated anions into the I, M, T, and A sites. Mica + classifies micas in terms of composition and octahedral site-occupancy. It also calculates the intensive parameters such as fO 2, T, and fH 2O from the composition of biotite in equilibrium with K-feldspar and magnetite. Using the calculated F–OH and Cl–OH exchange systematics and various log ratios ( fH 2O/ fHF, fH 2O/ fHCl, fHCl/ fHF, X Cl/ X OH, X F/ X OH, X F/ X Cl) of mica analyses. Mica + gives valuable determinations about the characteristics of hydrothermal fluids associated with alteration and mineralization processes. The program output is generally in the form of screen outputs. However, by using the “Grf” files that come up with this program they can be visualized under the Grapher software both as binary and ternary diagrams. Mica analyses subjected to the Mica + program were calculated on the basis of 22+ z positive charges taking into account the procedure by the Commission on New Mineral Names Mica Subcommittee of 1998.