High-pressure phase transitions in FeCr₂O₄ and structure analysis of new post-spinel FeCr₂O₄ and Fe₂Cr₂O₅ phases with meteoritical and petrological implications

Abstract

We determined phase relations in FeCr₂O₄ at 12-28 GPa and 800-1600 °C using a multi-anvil apparatus. At 12-16 GPa, FeCr₂O₄ spinel (chromite) first dissociates into two phases: a new Fe₂Cr₂O₅ phase + Cr₂O₃ with the corundum structure. At 17-18 GPa, the two phases combine into CaFe₂O₄- type and CaTi₂O₄-type FeCr₂O₄ below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi₂O₄-structured FeCr₂O₄ (Cmcm), and indicated that the Fe₂Cr₂O₅ phase is isostructural to a modified ludwigite-type Mg₂Al₂O₅ (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe₂O₄-type FeCr₂O₄ is unquenchable and is converted into another FeCr₂O₄ phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr₂O₄ phase has a new structure related to CaFe₂O₄-type. The high-pressure phase relations in FeCr₂O₄ reveal 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.