Brookite

Brookite is the orthorhombic variant of titanium dioxide (TiO2), which occurs in four known natural polymorphic forms (minerals with the same composition but different structure). The other three of these forms are akaogiite (monoclinic), anatase (tetragonal) and rutile (tetragonal). Brookite is rare compared to anatase and rutile and, like these forms, it exhibits photocatalytic activity. Brookite also has a larger cell volume than either anatase or rutile, with 8 TiO2 groups per unit cell, compared with 4 for anatase and 2 for rutile. Iron (Fe), tantalum (Ta) and niobium (Nb) are common impurities in brookite. Brookite was named in 1825 by French mineralogist Armand Lévy for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader. Arkansite is a variety of brookite from Magnet Cove, Arkansas, US. It is also found in the Murun Massif on the Olyokma-Chara Plateau of Eastern Siberia, Russia, part of the Aldan Shield. At temperatures above about 750 °C, brookite will revert to the rutile structure.