Brookite
Template:Short description Template:Infobox mineral
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.<ref name="Brookite Photocatalysis">Template:Cite journal</ref> 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.<ref name=ncsu>Anatase and Brookite Template:Webarchive. Wikis.lib.ncsu.edu (2007-05-08). Retrieved on 2011-10-14.</ref> Iron (Fe), tantalum (Ta) and niobium (Nb) are common impurities in brookite.<ref name=Mindat/>
Brookite was named in 1825 by French mineralogist Armand Lévy<ref name=Mindat/> for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.<ref name=Webmin/>
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.<ref>Arkansite on Mindat</ref>
At temperatures above about 750 °C, brookite will revert to the rutile structure.<ref name=amethyst>Brookite (Titanium Oxide). Galleries.com. Retrieved on 2011-10-14.</ref>
Unit cellEdit
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.<ref name=Webmin/><ref name=Mindat/><ref name=HOM/>
StructureEdit
The brookite structure is built up of distorted octahedra 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.<ref name=pauling>The Crystal Structure of Brookite. paulingblog.wordpress.com. 12 January 2010</ref>
AppearanceEdit
Brookite crystals are typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo-hexagonal or prismatic.<ref name=Mindat/> Brookite and rutile may grow together in an epitaxial relationship.<ref name=Mindat/>
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 almandite 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.<ref name=Webmin/><ref name=Mindat/><ref name=HOM/>
Optical propertiesEdit
Brookite is doubly refracting, as are all orthorhombic minerals, and it is biaxial (+). Refractive indices 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.<ref name=Mindat/><ref name=HOM/> It is neither fluorescent nor radioactive.<ref name=Webmin/>
Physical propertiesEdit
Brookite is a brittle 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.<ref name=Webmin/><ref name=Mindat/><ref name=HOM/> Twinning is uncertain.<ref name=Mindat/><ref name=HOM/> The mineral has a Mohs hardness of Template:Frac to 6, between apatite and feldspar. This is the same hardness as anatase and a little less than that of rutile (6 to Template:Frac). The specific gravity is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.<ref name=Mindat/><ref name=HOM/>
Occurrence and associationsEdit
Brookite is an accessory mineral in alpine veins in gneiss and schist; it is also a common detrital mineral.<ref name=Mindat/><ref name=HOM/> Associated minerals include its polymorphs anatase and rutile, and also titanite, orthoclase, quartz, hematite, calcite, chlorite and muscovite.<ref name=HOM/>
The type locality is Twll Maen Grisial, Fron Olau, Prenteg, Gwynedd, Wales.<ref name=Mindat/> In 2004, brookite crystals were found in the Kharan, in Balochistan, Pakistan.<ref name=Mindat/>
See alsoEdit
- List of minerals
- List of minerals recognized by the International Mineralogical Association
- List of minerals named after people