Equation of state of hercynite, FeAl2O4, and high-pressure systematics of Mg-Fe-Cr-Al spinels

In this work a single crystal of synthetic hercynite, FeAl2O4, was investigated by X-ray diffraction up to 7.5 GPa and at room temperature, in order to determine its pressurevolume equation of state. The unit-cell volume decreases non-linearly with a reduction of 3.4% (i.e. 18.43 A ˚ 3). The pressure-volume data were fitted to a third-order Birch-Murnaghan equation of state providing the following coefficients: V0 = 542.58(3) A ˚ 3, KT0 = 193.9(1.7) GPa, K’ = 6.0(5). These results are consistent with previous investigations of Cr and Al spinels measured with the same experimental approach but the KT0 differs significantly from the experimental determination carried out more than 40 years ago by Wang and Simmons (1972) by the pulse echo overlap method. Our new results were used to redetermine the FeAl2O4 (hercynite) = FeO (wustite) + Al2O3 (corundum) equilibrium in PT space and obtain geobarometric information for Cr-Al spinels found as inclusions in diamond.