The thermodynamics of ordered perovskites on the CaTiO₃-FeTiO₃ join

The enthalpies of formation from ilmenite, FeTiO₃, and perovskite, CaTiO₃, of two intermediate ordered perovskite phases, CaFeTi₂O₆ and CaFe₃Ti₄O₁₂, have been measured at 801°C using oxide melt solution calorimetry. These data, in combination with experiments at high pressure and temperature, indicate that below 1518±50°C CaFe₃Ti₄O₁₂ is stable at lower pressures (~7 GPa at 1200°C) than CaFeTi₂O₆ (~13 GPa at 1200°C). This relationship should be reversed, and CaFeTi₂O₆ should become stable at lower pressures than CaFe₃Ti₄O₁₂, at temperatures above 1518±50°C. These intermediate phases are of petrological interest because they forms as a reaction between two minerals, ilmenite and perovskite, which are commonly associated in kimberlites, and because their pressure-temperature range of formation overlaps that of origin of kimberlites.