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.[5] 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.[6]Iron (Fe), tantalum (Ta) and niobium (Nb) are common impurities in brookite.[3]
Brookite was named in 1825 by French mineralogist Armand Lévy[3] for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.[1]
At temperatures above about 750 °C, brookite will revert to the rutile structure.[8]
Unit cell
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.[1][3][4]
Structure
Crystal structure of brookite
The brookite structure is built up of distorted octahedra with a titaniumion at the center and oxygen ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic structure.[9]
Brookite is usually brown in color, 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.[1][3][4]
Optical properties
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.
Brookite is a brittle mineral, with a subconchoidal to irregular fracture and poor cleavage in one direction parallel to the ccrystal axis and traces of cleavage in a direction perpendicular to both the a and the b crystal axes.[1][3][4]Twinning is uncertain.[3][4] The mineral has a Mohs hardness of 5+1⁄2 to 6, between apatite and feldspar. This is the same hardness as anatase and a little less than that of rutile (6 to 6+1⁄2). The specific gravity is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.[3][4]
