Brookite is orthorhombic, and one of the four naturally occurring polymorphs (minerals with the same composition but different structure) of titanium dioxide, TiO₂, approved by the International Mineralogical Association (IMA). 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.