Ilmenite-perovskite transformation in germanates at high pressure

Abstract

Some germanates transform from ilmenite to perovskite at high pressures in a similar manner to that of magnesium silicate. However, the ilmenite-perovskite phase relations in germanates have not yet been examined in detail. In this study, we have examined the phase equilibrium relations of MgGeO3, ZnGeO3 and MnGeO3 at high pressures and high temperatures. A Kawai-type multianvil apparatus was used for high pressure experiments. Starting materials were MgGeO3 pyroxene, ZnGeO3 ilmenite, and MnGeO3 ilmenite. The sample was kept at 14-27 GPa and 1200-1800C for 1 hour. The quenched samples were examined by a microfocus X-ray diffractometer. MgGeO3 ilmenite transformed to a high-pressure phase which was recovered as the LiNbO3-type phase at ambient conditions. Based on Leinenweber et al. (1994) study, we interpreted that the recovered LiNbO3-type phase was originally perovskite at high pressure. The MgGeO3 ilmenite-perovskite transition boundary was determined as P(GPa) = 36.2-0.008T(C). The slope is much larger than that of MgSiO3. At 23 and 16 GPa, ilmenite phases of ZnGeO3 and MnGeO3, respectively, transformed to high pressure phases both of which were recovered as the LiNbO3-type phases. We assumed that recovered LiNbO3-type phase was originally perovskite at high pressure. The ZnGeO3 ilmenite-perovskite transition boundary was determined as P(GPa) = 26.4-0.003T(C). We interpret that the high entropies of MgGeO3 and ZnGeO3 perovskites result from high vibrational entropies which occur due to incorporation of relatively small Mg2+ and Zn2+ in large sites of perovskite structure. The pressure of ilmenite-perovskite transition of MnGeO3 is the lowest among the three germinates. This suggests that MnGeO3 perovskite is energetically more stable than MgGeO3 and ZnGeO3 perovskites due to relatively large divalent cation in perovskite structure.