Perovskite

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).[3] Perovskite's notable crystal structure was first described by Victor Goldschmidt in 1926 in his work on tolerance factors.[8] The crystal structure was later published in 1945 from X-ray diffraction data on barium titanate by Helen Dick Megaw.[9]

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.[10]

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,[11] 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.[4]

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.[12]

A rare-earth-bearing variety knopite with the chemical formula (Ca,Ce,Na)(Ti,Fe)O₃ 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.[11][13]

Crystal structure of perovskite CaTiO₃; red=oxygen, grey=titanium, blue=calcium

The eponymous Perovskite CaTiO₃ crystallizes in the Pbnm space group (No. 62) with lattice constants a = 5.39 Å, b = 5.45 Å and c = 7.65 Å.[16]

Perovskites have a nearly cubic structure with the general formula ABO₃. 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. The A-site cations are in 12-fold coordination with the anions, while the B-site cations are in 6-fold coordination. A large number of metallic elements are stable in the perovskite structure if the Goldschmidt tolerance factor t is in the range of 0.75 to 1.0.[17]

${\displaystyle t={\frac {R_{\rm {A}}+R_{\rm {O}}}{{\sqrt {2}}\left(R_{\rm {B}}+R_{\rm {O}}\right)}},}$

where R_A, R_B and R_O are the ionic radii of A and B site elements and oxygen, respectively. The stability of perovskites can be characterized with the tolerance and octahedral factors. When conditions are not fulfilled, a layered geometry for edge-sharing or face-sharing octahedra or lower B-site coordination is preferred. These are good structural bounds, but not an empirical prediction.[18]

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 CaTiO₃ (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.[19]

• Post-perovskite
• Silicate perovskite
• Haber process

• "Perovskite" . Encyclopædia Britannica (11th ed.). 1911.
• Media related to Perovskite at Wikimedia Commons