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 FeCr 2 O 4 at 12–28 GPa and 800–1600 °C using a multi-anvil apparatus. At 12–16 GPa, FeCr 2 O 4 spinel (chromite) first dissociates into two phases: a new Fe 2 Cr 2 O 5 phase + Cr 2 O 3 with the corundum structure. At 17–18 GPa, the two phases combine into CaFe 2 O 4 -type and CaTi 2 O 4 -type FeCr 2 O 4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi 2 O 4 -structured FeCr 2 O 4 ( Cmcm ), and indicated that the Fe 2 Cr 2 O 5 phase is isostructural to a modified ludwigite-type Mg 2 Al 2 O 5 ( Pbam ). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe 2 O 4 -type FeCr 2 O 4 is unquenchable and is converted into another FeCr 2 O 4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr 2 O 4 phase has a new structure related to CaFe 2 O 4 -type. The high-pressure phase relations in FeCr 2 O 4 reveal that natural FeCr 2 O 4 -rich phases of CaFe 2 O 4 - and CaTi 2 O 4 -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.