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 U2S3-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 (Fe1,δTiδ)O and Fe1+ 7 delta;Ti2-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.