The stability of hercynite at high pressures and temperatures

The stability of hercynite (FeAl₂O₄) has been investigated experimentally between 7 and 24 GPa and 900 and 1,700°C. Hercynite breaks down to its constituent oxides at 7–8.5 GPa and temperatures >1,000°C. The incorporation of a small magnetite component in the hercynite necessitated a small correction to fix the location of the endmember reaction: FeAl₂O₄  = Al₂O₃ + FeO in P–T space. After making this correction, the position of the phase boundary was used to evaluate thermodynamic data for hercynite. Our results support a relatively large S ₂₉₈^° for hercynite, on the order of 115 J mol⁻¹ K⁻¹. Experiments up to 24 GPa and 1,400°C failed to detect any high-pressure polymorph of FeAl₂O₄; only corundum + wüstite were detected. This behaviour contrasts with that observed for the analogous MgAl₂O₄ system where the constituent oxides recombine at high pressure to produce “post-spinel” phases with CaFe₂O₄-type and CaTi₂O₄-type structures.