Abstract

The phase transformation from ilmenite to perovskite in FeTiO3 was directly observed using synchrotron-based X-ray diffraction and a large-volume press. The perovskite phase is temperature quenchable at 20 GPa and converts into the LiNbO3 phase at pressures below 15 GPa at room temperature. The LiNbO3 phase transforms into the ilmenite phase at 10 GPa and 673 K. However, the back-transformation from the ilmenite to the LiNbO3 phase was not observed, thus strongly suggesting that the LiNbO3 phase is not thermodynamically stable but rather a retrogressive phase formed from perovskite during decompression at room temperature.

By cycling the pressure up and down at temperatures between 773 and 1023 K, the perovskite-ilmenite transformation could be observed in both directions, thus confirming that perovskite is the true high-pressure phase with respect to the ilmenite phase at lower pressures. The phase boundary of the perovskite-ilmenite transformation thus determined in this study is represented by P (GPa) = 16.0 (± 1.4) – 0.0012 (± 0.0014) T (K), which is inconsistent with P = 25.2 – 0.01 T (K) reported previously (Syono et al. 1980). The discrepancy could be attributed to the different experimental methods (i.e., in situ vs. quench) used in the two studies. The ilmenite-perovskite phase boundary with such a small slope would potentially serve as a useful geobarometer for ilmenite-bearing rocks derived from the deep mantle or for those shocked in meteor craters.

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