THE THERMODYNAMICS OF ORDERED PEROVSKITES ON THE CATIO3-FETIO3 JOIN

The enthalpies of formation from ilmenite, FeTiO3, and perovskite, CaT iO3, of two intermediate ordered perovskite phases, CaFeTi2O6 and CaFe 3Ti4O12, have been measured at 801 degrees C using oxide melt solution calorimetry. These data, in combination with experiments at high pres sure and temperature, indicate that below 1518+/-50 degrees C CaFe3Ti4 O12 is stable at lower pressures (similar to 7 GPa at 1200 degrees C) than CaFeTi2O6 (similar to 13 GPa at 1200 degrees C). This relationshi p should be reversed, and CaFeTi2O6 should become stable at lower pres sures than CaFe3Ti4O12, at temperatures above 1518+/-50 degrees C. The se intermediate phases are of petrological interest because they form as a reaction between two minerals, ilmenite and perovskite, which are commonly associated in kimberlites, and because their pressure-temper ature range of formation overlaps that of origin of kimberlites.