Perovskite (structure) - Common Occurrence

Common Occurrence

As pressure increases, the O2− ions compress so olivine transforms to the spinel structure, which at still higher pressure converts to a perovskite structure and a coexisting periclase structure.

At the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSiO₃, transforms into a denser perovskite-structured polymorph; this phase may be the most common mineral in the Earth. This phase has the orthorhombically distorted perovskite structure (GdFeO₃-type structure) that is stable at pressures from ~24 GPa to ~110 GPa. However, it cannot be transported from depths of several hundred km to the Earth's surface without transforming back into less dense materials. At higher pressures, MgSiO₃ perovskite transforms to post-perovskite.

Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Fluoride perovskites such as NaMgF₃ are well known. A large family of metallic perovskite compounds can be represented by RT₃M (R: rare-earth or other relatively large ion, T: transition metal ion and M: light metalloids). The metalloids occupy the octahedrally coordinated "B" sites in these compounds. RPd₃B, RRh₃B and CeRu₃C are examples. MgCNi₃ is a metallic perovskite compound and has received lot of attention because of its superconducting properties. An even more exotic type of perovskite is represented by the mixed oxide-aurides of Cs and Rb, such as Cs₃AuO, which contain large alkali cations in the traditional "anion" sites, bonded to O2− and Au− anions.