THE STABILITY OF HERCYNITE AND HERCYNITE-GAHNITE SPINELS IN CORUNDUM- OR QUARTZ-BEARING ASSEMBLAGES

Abstract

The equilibrium. 3FeAl2O4 + 3Al2SiO3 = 5 Al2O3 (1)has been calibrated in the piston-cylinder apparatus. Experiments were carried out using well-calibrated NaCl furnace assemblies and Ag80Pd20 capsules with oxygen fugacity buffered at or near iron-wüstite. The equilibrium is located at less than 7·2 kb at 85O°C and between 8·0 and 8·2, 10·0 and 10·5, and 12·0 and 12·2 kb at 900, 1000, and 1100°C, respectively. Experiments have also been conducted to determine the effect of gahnite (ZnAl2O4) component in spinel on equilibrium (1). Graphite capsules were used with oxygen fugacity buffered at or near graphite-CO-CO2. The addition of zinc displaces the reaction to higher pressures. For hercyniteg86-gahnite14, the equilibrium is located between 9·4 and 9·6 and 12·7 and 13·0kb at 900 and 1050°C, respectively. For hercynite70-gahnite30, the equilibrium is located between 10·8 and 11·0 and 15·4 and 15·6 kb at 900 and 1050°C, respectively. The results indicate that hercynite-gahnite solutions are somewhat non-ideal (WG = +6·54 kJ/mol at 900°Q assuming a symmetric regular solution model.A thermobarometer based on equilibrium (1) is applicable in high-grade metapelitic rocks. Pressure/temperature estimates using this equilibrium agree with other well-calibrated thermometers and barometers.Failure of equilibrium (1) to intersect the equilibrium. 3 FeAl2O4 + 5 SiO2 = Fe3Al2Si3O12 + 2 Al2SiO5 (2)indicates that the equilibrium corundum + quartz = sillimanite is metastable at all pressures and temperatures. This implies that co-occurrences of corundum and quartz in granulites are metastable.