Post-perovskite has the same crystal structure as the synthetic solid compound CaIrO3, and is often referred to as the "CaIrO3-type phase of MgSiO3" in the literature. The crystal system of post-perovskite is orthorhombic, its space group is "Cmcm", and its structure is a stacked SiO6-octahedral sheet along the "b" axis. The name "post-perovskite" derives from silicate perovskite, the stable phase of MgSiO3 throughout most of Earth's mantle, which has the perovskite structure. The prefix "post-" refers to the fact that it occurs after perovskite structured MgSiO3 as pressure increases (and historically, the progression of high pressure mineral physics). At upper mantle pressures, nearest Earth's surface, MgSiO3 persists as the silicate mineral enstatite, a pyroxene rock forming mineral found in igneous and metamorphic rocks of the crust.