The MgTiO3-FeTiO3 join at high pressure and temperature

The phase relations at high pressure and high temperature for the FeTiO3-MgTiO3 join were determined using several different experimental methods. Through a series of multi-anvil experiments, a phase boundary with a negative slope was observed between MgTiO3 I (ilmenite structure) and a high pressure phase with the MgTiO3 II (lithium niobate structure) after quenching. The enthalpy of transformation of MgTiO3 I to MgTiO3 II was determined through transposed-temperature-drop calorimetry to be 28.78 +/- 1.45 kJ/mol. The enthalpy of transformation from ilmenite to lithium niobate structure was also determined for three intermediate compositions on the FeTiO3-MgTiO3 join, Fe0.2Mg0.8TiO3, Fe0.5Mg0.5TiO3 and Fe0.8Mg0.2TiO3, and confirmed for FeTiO3, and was found to be a linear function of composition. These experiments represent one of the first successful calorimetric measurements on small samples (1 to 3 mg) synthesized at high pressures (15 to 21 GPa). X-ray analysis during compression of Fe0.5Mg0.5TiO3 II in a diamond cell confirmed a room temperature transition at 28 GPa to Fe0.5Mg0.5TiO3 III (a GdFeO3-type perovskite structure), similar to the transitions previously observed in FeTiO3 and MnTiO3. The Fe0.5Mg0.5TiO3 sample was heated to 802 degrees C at 21 GPa, and it was observed that the stable high temperature, high pressure phase is perovskite, Fe0.5Mg0.5TiO3 III. The above data combined confirm the stability of a continuous perovskite solid solution at high pressure and temperature for the FeTiO3-MgTiO3 join.