Perovskite

Lore & Background

Perovskite is an orthorhombic calcium titanium oxide mineral with the composition of calcium titanate. Its defining characteristic is its crystal structure, which serves as the namesake for a broader class of compounds known as the perovskite structure. This structure is nearly cubic, with a general formula of ABO₃ for chalcogen perovskites or ABX₃ for halogen perovskites, the latter being common in modern photovoltaics. In this arrangement, the A-site ion, typically an alkaline earth or rare-earth element, sits at the center of the lattice in twelve-fold coordination with anions, while the B-site ion, a transition metal, occupies the corners in six-fold coordination. The mineral itself crystallizes in the orthorhombic system with space group Pbnm, though crystals often appear pseudocubic. Its appearance varies widely: colors include black, brown, gray, and orange to yellow, with a sub-metallic to metallic luster, a colorless streak, and imperfect cleavage with brittle tenacity. Perovskite can be mistaken for galena, but galena has a better metallic luster, greater density, perfect cleavage, and true cubic symmetry. The mineral occurs in the Earth’s mantle and is found in silica-undersaturated ultramafic rocks and foidolites, such as at the Khibina Massif, where it forms small anhedral to subhedral crystals between rock-forming silicates. It also appears in contact carbonate skarns, altered limestone blocks from Mount Vesuvius, chlorite and talc schist, and as an accessory mineral in alkaline and mafic igneous rocks, nepheline syenite, melilitite, kimberlites, and rare carbonatites. Perovskite is common in Ca-Al-rich inclusions in chondritic meteorites. Its stability in igneous rocks is limited by a reaction with sphene; the two are not found together in volcanic rocks except in an etindite from Cameroon. Rare-earth-bearing varieties include knopite, found in alkali intrusive rocks of the Kola Peninsula and Sweden, and niobium-bearing dysanalyte, occurring in carbonatite near Schelingen, Germany. In stars and brown dwarfs, perovskite grains form at low temperatures, depleting titanium oxide in the photosphere; the absence of TiO bands defines the transition from cool M-dwarf stars to colder L-dwarfs.

Reader's Guide

Perovskite is significant as the namesake for a broad class of compounds with the same crystal structure, which can accommodate a wide variety of cations. This structural flexibility has enabled the development of engineered materials for applications including photovoltaics, where halogen perovskites are used in photodiodes and solar panels. The mineral itself occurs in diverse geological settings, from the Earth's mantle to chondritic meteorites, and its formation in stars and brown dwarfs depletes titanium oxide in their photospheres, defining the transition between cool M-dwarf stars and colder L-dwarfs. The perovskite structure's stability is characterized by the Goldschmidt tolerance factor, and its derivatives, such as double perovskites, exhibit ordering that modifies electronic, magnetic, and transport properties, leading to functional materials for magnetism, ferroelectricity, catalysis, and energy conversion.

Did You Know?

• Perovskite formation in stars depletes titanium oxide, defining the transition between M-dwarf and L-dwarf stars.
• A rare-earth-bearing variety of perovskite, knopite, is found in alkali intrusive rocks in the Kola Peninsula.