FeAlO3 under ultrahigh-temperature metamorphic conditions: Experimental evidence from the sillimanite-Fe2O3 and sillimanite-Fe3O4 systems

FeAlO3a) was synthesized under ultrahigh-temperature metamorphic conditions. The assemblage of FeAlO3, SiO2 rich melt and vapor was obtained from a mixture of Rundvagshetta sillimanite and reagent-grade Fe2O3 (weight ratio of 95:5) in Pt capsules at pressures of 5 and 9 kbar and a temperature of 1050 degrees C under hydrous conditions. No hematite was found in these runs. Corundum was a rare crystallization product in the 9 kbar experiment. We also produced FeAlO3, SiO2 -rich melt and vapor in the sillimanite-Fe3O4 (weight ratio of 86:14) system in a AuPd capsule at 9 kbar and 1050 degrees C under hydrous condition. A crystal domain comprising FeAlO3, corundum, magnetite-hercynite spinel and ulvospinel formed at the bottom part of the charge due to contamination by Ti from titanium oxide, probably rutile, included within the sillimanite. This domain was in contact with melt among sillimanite crystals. Our results suggest that FeAlO3 would be an index mineral for ultrahigh-temperature metamorphism of partially melted Fe- and Al-rich granulites generated under hydrous and oxidizing conditions.