Hydrothermal Studies of Garnet — Mica Equilibria in the System 3(FeO, MnO)— 2 Al₂O₃—12 SiO₂—K₂O—H₂O

Abstract

The system has been studied under a constant pressure of 3600 atm. and within a temperature range between 450° and 720°. Oxygen fugacity has been controlled with quartz-fayalite-magnetite buffer. The proportion Fe²+ and Mn²+ has been varied while other components have been kept at a constant ratio. The experimental results are visualized in a phase diagram (Fig. 2). The most important equilibrium, annite+muscovite+3 quartz=garnet+2 sanidine+2 H₂O has been treated thermdynamically. The heat of reaction is about 49 kcal. Based upon this value, the heat of formation for almandite garnet is — 84 kcal/mole.

Curves have been constructed for in garnet. The first one shows the variation in composition of garnet with the fugacity of water when total pressure is kept constant at 3000 atm. The latter curve shows the corresponding variation with total pressure, where P_t=PH₂O+PH₂.