brookite

{{Infobox mineral | name = Brookite | image = Brookite-gem7-07a.jpg | imagesize = 260px | alt = | caption = Brookite from Balochistan | category = Oxide minerals | formula = TiO2 | molweight = 79.88 g/mol | strunz = 4.DD.10 (10 ed) 4/D.15-10 (8 ed) | dana = 4.4.5.1 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Pbca | unit cell = a = 5.4558 Å, b = 9.1819 Å, c = 5.1429 Å; Z = 8 | color = Deep red, reddish brown, yellowish brown, brown, or black | habit = Tabular and striated, pyramidal or pseudohexagonal | twinning = On {120}, uncertain | cleavage = Poor on {120}, in traces on {001} | fracture = Subconchoidal to irregular | tenacity = Brittle | mohs = to 6 | luster = Submetallic | streak = White, greyish or yellowish | diaphaneity = Opaque to translucent | gravity = 4.08 to 4.18 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 2.583 nβ = 2.584 nγ = 2.700 | birefringence = δ = 0.117 | pleochroism = Very weak, yellowish, reddish, orange to brown | 2V = Calculated: 12° to 20° | dispersion = 0.131 (compare to diamond at 0.044) | extinction = | length fast/slow = | fluorescence = Non-fluorescent | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }}

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. 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. Iron (Fe), tantalum (Ta) and niobium (Nb) are common impurities in brookite.