Perovskite


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perovskite

[pə′rävz‚kīt]
(mineralogy)
Ca[TiO3] A natural, yellow, brownish-yellow, reddish, brown, or black mineral and a structure type which includes no less than 150 synthetic compounds; the crystal structure is ideally cubic, it occurs as rounded cubes modified by the octahedral and dodecahedral forms, luster is subadamantine to submetallic, hardness is 5.5 on Mohs scale, and specific gravity is 4.0.

Perovskite

 

(named for L. A. Perovskii). (1) A mineral of the oxide class. The chemical formula of perovskite is CaTiO3, with CaO accounting for 41.24 percent of the composition, and TiO2 for 58.76 percent. Perovskite that contains admixtures of Ce is called knopite; of Ni, Ce, and Fe, dysanalite; and of Na, Ce, Ti, and Nb, loparite.

Perovskite crystallizes in the pseudoisometric system. Each calcium atom is surrounded by 12 oxygen atoms, which are located at the apexes of a cuboctahedron; the coordination number of Ti is eight. Characteristic striations appear on the cubic faces parallel to the edges. Cleavage is cubic, hardness is 5.5–6 on Mohs’ scale, and the density ranges from 3,950 to 4,040 kg/m3. The color may be black, grayish black, or brownish.

Perovskite frequently occurs in ultrabasic alkaline rocks, including olivinites, pyroxenes, and kimberlites, as well as in associated ore segregations and carbonatites. It fills the interstices at the point of contact of basic rocks with limestone; it also is interstitially distributed in chloritic shale and in nephelinic, leu-citic, and melilitic basalts.

Knopite is used as a source of rare-earth elements of the cerium group, and loparite is a source of niobium and titanium.

(2) A group of chemical compounds whose crystal structure resembles that of perovskite [in English usage “perovskite” is the adjectival noun that denotes the structure type of these compounds and not the compounds themselves]. Perovskites have the general formula ABX3, where A is a tetravalent cation, B is a heptavalent cation, and X is an anion—usually oxygen, as in NaNbO3 and BaTiO3. Many of these compounds are ferroelectric, including BaTiO3 and LiNbO3; the group also includes some superconductors (for example, SrTiO3), semiconductors, and compounds that display magnetic ordering.

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