The Stability of Hercynite and Hercynite-Gahnite Spinels in Corundum- or Quartz-Bearing Assemblages

• Published in: Journal of petrology 1989-08, Vol.30 (4), p.1017-1031
• Main Authors: SHULTERS, JACQUELINE C., BOHLEN, STEVEN R.
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/30.4.1017

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.