The phase relations at high pressure and high temperature for the FeTiO 3 -MgTiO 3 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 MgTiO 3 I (ilmenite structure) and a high pressure phase with the MgTiO 3 II (lithium niobate structure) after quenching. The enthalpy of transformation of MgTiO 3 I to MgTiO 3 II was determined through transposed-temperature-drop calorimetry to be 28.78+ or -1.45 kJ/mol. The enthalpy of transformation from ilmenite to lithium niobate structure was also determined for three intermediate compositions on the FeTiO 3 -MgTiO 3 join, Fe (sub 0.2) Mg (sub 0.8) TiO 3 , Fe (sub 0.5) Mg (sub 0.5) TiO 3 and Fe (sub 0.8) Mg (sub 0.2) TiO 3 , and confirmed for FeTiO 3 , 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 Fe (sub 0.5) Mg (sub 0.5) TiO 3 II in a diamond cell confirmed a room temperature transition at 28 GPa to Fe (sub 0.5) Mg (sub 0.5) TiO 3 III (a GdFeO 3 -type perovskite structure), similar to the transitions previously observed in FeTiO 3 and MnTiO 3 . The Fe (sub 0.5) Mg (sub 0.5) TiO 3 sample was heated to 802 degrees C at 21 GPa, and it was observed that the stable high temperature, high pressure phase is perovskite, Fe (sub 0.5) Mg (sub 0.5) TiO 3 III. The above data combined confirm the stability of a continuous perovskite solid solution at high pressure and temperature for the FeTiO 3 -MgTiO 3 join.

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