Post-perovskite's a high-pressure phase of magnesium silicate (MgSiO3). It's composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO2), and its pressure and temperature for stability imply that it's likely to occur in portions of the lowermost few hundred km of Earth's mantle. Post-perovskite has the same crystal structure as the synthetic solid compound CaIrO3, and's often referred to as the "CaIrO3-type phase of MgSiO3" in the literature. The crystal system of post-pe… (More on Post-perovskite)