First principles study on phase relations of MgAl2O4 spinel polymorphs at mantle conditions

MgAl2O4 is a common accessory mineral in igneous rocks and meteorites. Natural high-pressure polymorph of MgAl2O4 was found to be stable up to 100 GPa in the mid-oceanic ridge basalt (MORB). The high-pressure experiments have revealed that MgAl2O4 spinel will decomposes into an assemblage of periclase (MgO) and corundum (Al2O3) at about 15 GPa. As the pressure increases, the assemblage will react to form two high-pressure post-spinel polymorphs with CaFe2O4 (CF) structure and CaTi2O4 (CT) structure respectively. However, the prediction of the phase transition pressure between CF phase and CT phase using zero-temperature ab-initio calculations in the previous studies is not very clear because the two phases have very similar structures and hence very small difference in free energy (enthalpy) . In this study, the phase relations in MgAl2O4 polymorphs have been investigated by first principles lattice dynamic calculations within quasi-harmonic approximation. The P-T phase diagram of MgAl2O4 is obtained computationally for the first time. The calculated cell parameters, bulk modulus, equation of states and thermodynamic properties of spinel at ambient pressure are in good agreement with experimental data. The derived phase stability area and phase transition boundaries of different phases are all consistent with experimental results in a wide range of P and T. The calculated results show that CF phase does transform to CT phase at around 50 GPa at lower mantle temperatures, which suggests CaTi2O4 structure spinel is the most stable phase throughout the lower mantle and may be the major host mineral of aluminium in the interior of the Earth. The phase diagram at finite temperatures will reflect more information of phase transitions than the zero-temperature case. This is expected to provide more accurate predictions for application to realistic mantle temperature-pressure conditions.