Brookite is the orthorhombic variant of titanium dioxide, TiO₂, which occurs in four natural polymorphic forms (minerals with the same composition but different structure). The International Mineralogical Association (IMA) recognizes these 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.