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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.〔(Anatase and Brookite ). Wikis.lib.ncsu.edu (2007-05-08). Retrieved on 2011-10-14.〕 Iron Fe, tantalum Ta and niobium Nb are common impurities.〔 It was named in 1825 by French mineralogist Armand Lévy〔 for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.〔 Arkansite is a variety of brookite from Arkansas, USA, that is also found in the Murunskii Massif, in the Eastern Siberian region of Russia, where many other unusual minerals occur.〔 At temperatures above about 750 °C, brookite will revert to the rutile structure.〔(Brookite (Titanium Oxide) ). Galleries.com. Retrieved on 2011-10-14.〕 == 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 TiO2, with 8 formula units per unit cell (Z = 8).〔〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Brookite」の詳細全文を読む スポンサード リンク
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