High-Pressure Decomposition of Synthetic Garnets

The stability of synthetic garnets {Y3}[Fe2](Fe3)O12, {Y3}[Al2](Al3)O12, and {Y3}[Ga2](Ga3)O12 has been investigated at high pressures and high temperatures. Decomposition to the YXO3 compound (X=Fe or Al) with the perovskitelike structure and to the sesquioxide with the corundum structure has been found to occur in the former two garnets: {Y3}[X2](X3)O12→3 YXO3+X2O3, while in the case of yttrium—gallium garnet no breakdown was detected at pressures up to 44 kbar and 1000°C. The three garnets seem to behave differently at high pressure and high temperature. This appears to be related to the differences in the preference of the three cations for the tetrahedral site. Results of preliminary studies of the effect of high pressure and high temperature on Li0.5Fe2.5O4 are presented. The geophysical implications of the decomposition under high pressure and high temperature of the garnet structure are discussed.