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 Photocatalysis activity.[ 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 in brookite.[
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Brookite was named in 1825 by French mineralogist Armand Lévy[ for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.][
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Arkansite is a variety of brookite from Magnet Cove, Arkansas, US. It is also found in the Murun Massif on the Olyokma-Chara Plateau of Eastern Siberia, Russia, part of the Aldan Shield.[ Arkansite on Mindat]
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 Chemical formula is TiO2, with 8 formula units per unit cell.[
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Structure
The brookite structure is built up of distorted Octahedron with a titanium ion at the center and oxygen ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic structure.[ The Crystal Structure of Brookite. paulingblog.wordpress.com. 12 January 2010]
Appearance
Brookite crystals are typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo- or prismatic.[ Brookite and rutile may grow together in an epitaxial relationship.][
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Brookite is usually brown in color, sometimes yellowish or reddish brown, or even black. Beautiful, deep red crystals (seen above-right) similar to pyrope and Almandine garnet are also known. Brookite displays a submetallic luster. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light.[
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Optical properties
Brookite is Birefringence, as are all orthorhombic minerals, and it is Birefringence. Refractive Index are very high, above 2.5, which is even higher than diamond at 2.42. For comparison, ordinary window glass has a refractive index of about 1.5.
Brookite exhibits very weak pleochroism, yellowish, reddish and orange to brown.[ It is neither Fluorescence nor radioactive.][
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Physical properties
Brookite is a Brittleness mineral, with a subconchoidal to irregular fracture and poor cleavage in one direction parallel to the c crystal axis and traces of cleavage in a direction perpendicular to both the a and the b crystal axes.[ Crystal twinning is uncertain.][ The mineral has a Mohs hardness of to 6, between apatite and feldspar. This is the same hardness as anatase and a little less than that of rutile (6 to ). The specific gravity is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.][
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Occurrence and associations
Brookite is an accessory mineral in alpine veins in gneiss and schist; it is also a common detrital mineral.[
Associated minerals include its polymorphs anatase and rutile, and also titanite, orthoclase, quartz, hematite, calcite, Chlorite group and muscovite.][
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The type locality is Twll Maen Grisial, Fron Olau, Prenteg, Gwynedd, Wales.[ In 2004, brookite crystals were found in the Kharan District, in Balochistan, Pakistan.][
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See also
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List of minerals
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List of minerals recognized by the International Mineralogical Association
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List of minerals named after people
External links
