Pressure dependence of Fe-Ti order in the ilmenite-hematite solid solution: Implications for the origin of lower crustal magnetization

The R 3̄ to R 3̄ c cation-ordering phase transition in the ilmenite (FeTiO₃)-hematite (Fe₂O₃) solid solution has been studied at high temperature and pressure using in-situ time-of-flight powder neutron diffraction. Experiments were performed on a synthetic sample of (FeTiO₃)_0.65 (Fe₂O₃)_0.35 at T ≤ 933 °C and P ≤ 1.1 GPa using an internally heated Paris-Edinburgh cell. The degree of cation order is observed to increase significantly with pressure. Thermodynamic analysis reveals an increase in the transition temperature from 911 °C at atmospheric pressure to 1010 °C at P = 0.64 GPa. This increase is much larger than that predicted on the basis of the volume change for the transition, indicating that the pressure dependence of cation ordering is dominated by enhanced cation-cation interaction, and not by volume strain. We predict an increase in the eutectoid temperature of up to 150 °C at 1 GPa. This implies that intergrowths of antiferromagnetic hematite and paramagnetic ilmenite could be stable at temperatures up to 540 °C at 1 GPa and cannot be ruled out as a potential source of lower crustal magnetization.