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

FeAlO₃ a) was synthesized under ultrahigh–temperature metamorphic conditions. The assemblage of FeAlO₃, SiO2– rich melt and vapor was obtained from a mixture of Rundvågshetta 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 °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°C under hydrous condition. A crystal domain comprising FeAlO₃, corundum, magnetite– hercynite spinel and ulvöspinel 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.