High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites

We determined phase relations in MgCr 2 O 4 at 12-28 GPa and 1000-1600 °C using a multi-anvil apparatus. At 12-15 GPa, spinel-type MgCr 2 O 4 (magnesiochromite) first decomposes into a mixture of new Mg 2 Cr 2 O 5 phase + corundum-type Cr 2 O 3 at 1100-1600 °C, but it dissociates first into MgO periclase + corundum-type Cr 2 O 3 at l000 °C. At about 17-19 GPa, the mixture of Mg 2 Cr 2 O 5 phase + corundum-type Cr 2 O 3 transforms to a single MgCr 2 O 4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr 2 O 4 phase has a CaTi 2 O 4 -type structure (Cmcm), and that the basic structure of the Mg 2 Cr 2 O 5 phase is the same as that of recently found modified ludwigite-type Mg 2 Al 2 O 5 and Fe 2 Cr 2 O 5 (Pbam). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12-15 GPa because of absence of the assemblage of (Mg,Fe) 2 Cr 2 O 5 + Cr 2 O 3 in the chromitites.