Wikipedia
Post-perovskite (pPv) is a high-pressure phase of magnesium silicate (MgSiO). It is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO), and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.
The post-perovskite phase has implications for the D′′-layer that influences the convective mixing in the mantle responsible for plate tectonics.
Post-perovskite has the same crystal structure as the synthetic solid compound CaIrO, and is often referred to as the "CaIrO-type phase of MgSiO" in the literature. The crystal system of post-perovskite is orthorhombic, its space group is Cmcm, and its structure is a stacked SiO-octahedral sheet along the b axis. The name "post-perovskite" derives from silicate perovskite, the stable phase of MgSiO throughout most of Earth's mantle, which has the perovskite structure. The prefix "post-" refers to the fact that it occurs after perovskite structured MgSiO as pressure increases (and historically, the progression of high pressure mineral physics). At upper mantle pressures, nearest Earth's surface, MgSiO persists as the silicate mineral enstatite, a pyroxene rock forming mineral found in igneous and metamorphic rocks of the crust.