Elasticity and high-pressure behavior of Mg2Cr2O5and CaTi2O4-type phases of magnesiochromite and chromite

Abstract

In situ high-pressure and high-temperature X-ray diffraction studies on magnesiochromite, MgCr2O4, and a natural chromite, (Mg,Fe)(Al,Cr)2O4, using a laser-heated diamond-anvil cell technique were performed at pressures to ~45 GPa. Our results on MgCr2O4 at ~15 GPa showed temperature-induced dissociation of MgCr2O4 to Cr2O3+MgO below ~1500 K and formation of modified ludwigite (mLd)-type Mg2Cr2O5+Cr2O3 above ~1500 K. Above 20 GPa, only a single phase with the CaTi2O4-type structure of MgCr2O4 was observed at 1400-2000 K. A second-order Birch-Murnaghan fit to pressure-volume data for the CaTi2O4-type phase of MgCr2O4 yields zero-pressure volume (V0) = 264.4(8) Å3 and bulk modulus (K0) = 185.4(4) GPa, and for the CaTi2O4-type structure of natural (Mg,Fe)(Al,Cr)2O4 yields V0 = 261(1) Å3 and K0 = 175.4(2) GPa. A second-order Birch-Murnaghan fit to pressure-volume data of mLd-type Mg2Cr2O5 yields V0 = 338.9(8) Å3 and K0 = 186.5(6) GPa. The obtained high-pressure phase relations of chromite spinels can be used as an indicator for shock pressure in impact rocks and meteorites. The bulk moduli of the high-pressure phases of MgCr2O4 and FeCr2O4 can help develop a thermodynamic model for Mg and Fe end-member spinels in the upper mantle and transition zone.