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.