Multi-mbar phase transitions in minerals

Two post-post-perovskite transitions have been identified by first principles calculations: the dissociation of MgSiO₃ into CsCl-type MgO and cotunnite-type SiO₂ at ∼1 TPa and the transition Of Al₂O₃ to the U₂S₃-type phase at -370 GPa. These transition pressures are currently very challenging for static compression experiments. NaMgF₃ may be a good low-pressure analog of MgSiO₃. We found a close relationship between the multi-Mbar crystal chemistry of planet-forming minerals and that of rare-earth sesquisulfides. Some of them may be used as low-pressure analogs of planet-forming minerals in the multi-Mbar range. In transitionmetal sesquioxides, the Th₂S ₃-type phase (very similar to U₂S_3-type) was discovered in Ti₂O₃, suggesting that some transition-metal sesquioxides could serve as low-pressure analog(s) of Al₂O ₃. Finally, a new type of dissociation, FeTiO₃ perovskite into (Fe_1,δTi_δ)O and Fe¹⁺ 7 _delta;Ti_2-delta;O₅, has recently been reported (Wu et al. 2009). This finding suggests that solid solutions (probably with transition-metals) among dissociation products (and in parent phases) could complicate the transition mechanisms we have discussed in this chapter.