Perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (ABX), known as the perovskite structure. Many different cations can be embedded in this structure, allowing the development of diverse engineered materials. History. The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792–1856). Perovskite's notable crystal structure was first described by Victor Goldschmidt in 1926 in his work on tolerance factors . The crystal structure was later published in 1945 from X-ray diffraction data on Barium Titanate by Helen Dick Megaw. Occurrence. Found in the Earth's mantle, perovskite's occurrence at Khibina Massif is restricted to the silica under-saturated ultramafic rocks and foidolites, due to the instability in a paragenesis with feldspar. Perovskite occurs as small anhedral to subhedral crystals filling interstices between the rock-forming silicates. Perovskite is found in contact carbonate skarns at Magnet Cove, Arkansas, in altered blocks of limestone ejected from Mount Vesuvius, in chlorite and talc schist in the Urals and Switzerland, and as  an accessory mineral in alkaline and mafic igneous rocks, nepheline syenite, melilitite, kimberlites and rare carbonatites. Perovskite is a common mineral in the Ca-Al-rich inclusions found in some chondritic meteorites. A rare-earth-bearing variety "knopite" with the chemical formula is found in alkali intrusive rocks in the Kola Peninsula and near Alnö, Sweden. A niobium-bearing variety "dysanalyte" occurs in carbonatite near Schelingen, Kaiserstuhl, Germany. In stars and brown dwarfs. In stars and brown dwarfs the formation of perovskite grains is responsible for the depletion of titanium oxide in the photosphere. Stars with a low temperature have  dominant bands of TiO in their spectrum; as the temperature gets lower for stars and brown dwarfs with an even lower mass, CaTiO forms and at temperatures below 2000 K TiO is undetectable. The presence of TiO is used to define the transition between cool M-dwarf stars and the colder L-dwarfs. Special characteristics. The stability of perovskite in igneous rocks is limited by its reaction relation with sphene. In volcanic rocks perovskite and sphene are not found together, the only exception being an etindite from Cameroon. Physical properties. Perovskites have a nearly cubic structure with the general formula  . In this structure the A-site ion, in the center of the lattice, is usually an alkaline earth or rare-earth element. B-site ions, on the corners of the lattice, are 3d, 4d, and 5d transition metal elements. A large number of metallic elements are stable in the perovskite structure if the Goldschmidt tolerance factor formula_1 is in the range of 0.75–1.0 where "R", "R" and "R" are the ionic radii of "A" and "B" site elements and oxygen, respectively. Perovskites have sub-metallic to metallic luster, colorless streak, and cube-like structure along with imperfect cleavage and brittle  tenacity. Colors include black, brown, gray, orange to yellow. Perovskite crystals may appear to have the cubic crystal form, but are often "pseudocubic" and actually crystallize in the orthorhombic system, as is the case for (Strontium titanate, with the larger strontium cation in the A-site, is cubic). Perovskite crystals have been mistaken for galena; however, galena has a better metallic luster, greater density, perfect cleavage and true cubic symmetry.