Brookite is the orthorhombic variant of titanium dioxide (TiO<sub>2</sub>), 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 TiO<sub>2</sub> 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.
At temperatures above about 750 °C, brookite will revert to the rutile structure.
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 TiO<sub>2</sub>, with 8 formula units per unit cell.
Appearance
thumb|left|Brookite from [[Pakistan]]
Brookite crystals are typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo-hexagonal or prismatic.