High-pressure phase transitions in FeCr₂O₄and structure analysis of new post-spinel FeCr₂O₄and Fe₂Cr₂O₅phases with meteoritical and petrological implications

We determined phase relations in FeCr₂O₄at 12-28 GPa and 800-1600 °C using a multi-anvil apparatus. At 12-16 GPa, FeCr₂O₄spinel (chromite) first dissociates into two phases: a new Fe₂Cr₂O₅phase + Cr₂O₃with the corundum structure. At 17-18 GPa, the two phases combine into CaFe₂O₄-type and CaTi₂O₄-type FeCr₂O₄below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi₂O₄-structured FeCr₂O₄(Cmcm), and indicated that the Fe₂Cr₂O₅phase is isostructural to a modified ludwigite-type Mg₂Al₂O₅(Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe₂O₄-type FeCr₂O₄is unquenchable and is converted into another FeCr₂O₄phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr₂O₄phase has a new structure related to CaFe₂O₄-type. The high-pressure phase relations in FeCr₂O₄reveal that natural FeCr₂O₄-rich phases of CaFe₂O₄-and CaTi₂O₄-type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.