Brookite

Brookite is the orthorhombic variant of titanium dioxide, TiO2, which occurs in many natural polymorphic forms (minerals with the same composition but different structure). The International Mineralogical Association (IMA) recognizes four forms; the others 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 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. Brookite is the orthorhombic variant of titanium dioxide, TiO2, which occurs in many natural polymorphic forms (minerals with the same composition but different structure). The International Mineralogical Association (IMA) recognizes four forms; the others 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 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. It 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 Arkansas, US, that is also found in the Murunskii Massif, in the Eastern Siberian region of Russia, where many other unusual minerals occur. At temperatures above about 750 °C, brookite will revert to the rutile structure. Brookite belongs to the orthorhombic dipyramidal crystal class 2/m 2/m 2/m (also designated mmm). The space group is Pcab and the unit cell parameters are a = 5.4558 Å, b = 9.1819 Å and c = 5.1429 Å. The formula is TiO2, with 8 formula units per unit cell ). The brookite structure is built up of distorted octahedra with a titanium ion at the center and oxygen ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic structure. Crystals typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo-hexagonal or prismatic. Brookite and rutile may grow together in an epitaxial relationship. Brookite is usually brown in color, or sometimes yellowish or reddish brown, or even black. Beautiful, deep red crystals (seen above-right) similar to pyrope and almandite garnet are also known. Brookite displays a submetallic luster. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light. Brookite is doubly refracting, as are all orthorhombic minerals, and it is biaxial (+). Refractive indices are very high, above 2.5, which is even higher than diamond at 2.42. For comparison, ordinary window glass has a refractive index of about 1.5.