View source for Brookite

{{Short description|Titanium dioxide mineral}}
{{Infobox mineral
| name        = Brookite
| image       = Brookite-gem7-07a.jpg
| imagesize   = 260px
| alt         = 
| caption     = Brookite from [[Balochistan]]
| category    = [[Oxide minerals]]
| formula     = TiO<sub>2</sub>
| molweight   = 79.88 g/mol<ref name=Webmin/>
| strunz      = 4.DD.10 (10 ed) <br/><small>4/D.15-10 (8 ed)</small>
| dana        = 4.4.5.1
| system      = [[Orthorhombic]]
| class       = Dipyramidal (mmm) <br/>[[H-M symbol]]: (2/m 2/m 2/m)
| symmetry    = ''Pbca''
| unit cell   = a = 5.4558&nbsp;Å, <br/>b = 9.1819&nbsp;Å, <br/>c = 5.1429&nbsp;Å; Z&nbsp;=&nbsp;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        = {{frac|5|1|2}} to 6 
| luster      = Submetallic
| streak      = White, greyish or yellowish
| diaphaneity = Opaque to translucent
| gravity     = 4.08 to 4.18
| density     = 
| polish      = 
| opticalprop = Biaxial (+)
| refractive  = n<sub>α</sub> = 2.583 n<sub>β</sub> = 2.584 n<sub>γ</sub> = 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  = <ref name=Webmin>[https://www.webmineral.com/data/Brookite.shtml Brookite]. Webmineral.com. Retrieved on 2011-10-14.</ref><ref name=Dana>Gaines et al (1997) Dana’s New Mineralogy Eighth Edition.  Wiley</ref><ref name=Mindat>[http://www.mindat.org/min-787.html Brookite]. Mindat.org (2011-09-17). Retrieved on 2011-10-14.</ref><ref name=HOM>[https://rruff.geo.arizona.edu/doclib/hom/brookite.pdf Brookite]. Handbook of Mineralogy. (PDF) . Retrieved on 2011-10-14.</ref>
}}

'''Brookite''' is the [[Orthorhombic crystal system|orthorhombic]] variant of [[titanium dioxide]] (TiO<sub>2</sub>), which occurs in four known natural [[Polymorphism (materials science)|polymorphic]] forms (minerals with the same composition but different structure). The other three of these forms are [[akaogiite]] ([[Monoclinic crystal system|monoclinic]]), [[anatase]] ([[Tetragonal crystal system|tetragonal]]) and [[rutile]] ([[Tetragonal crystal system|tetragonal]]).  Brookite is rare compared to anatase and rutile and, like these forms, it exhibits [[Photocatalysis|photocatalytic]] activity.<ref name="Brookite Photocatalysis">{{cite journal|last=Di Paola|first=A |author2=Addamo, M. |author3=Bellardita, M. |author4=Cazzanelli, E. |author5=Palmisano, L. |title=Preparation of photocatalytic brookite thin films|journal=Thin Solid Films|year=2007|volume=515|issue=7–8|pages=3527–3529|doi=10.1016/j.tsf.2006.10.114|bibcode = 2007TSF...515.3527D }}</ref>  Brookite also has a larger [[Crystal structure|cell]] volume than either anatase or rutile, with 8 TiO<sub>2</sub> groups per unit cell, compared with 4 for anatase and 2 for rutile.<ref name=ncsu>[http://wikis.lib.ncsu.edu/index.php/Anatase_and_Brookite Anatase and Brookite] {{Webarchive|url=https://web.archive.org/web/20120317091256/http://wikis.lib.ncsu.edu/index.php/Anatase_and_Brookite |date=2012-03-17 }}. 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 (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>[https://www.mindat.org/min-7844.html Arkansite on Mindat]</ref>
  
At temperatures above about 750&nbsp;°C, brookite will revert to the rutile structure.<ref name=amethyst>[https://www.galleries.com/minerals/oxides/brookite/brookite.htm Brookite (Titanium Oxide)]. Galleries.com. Retrieved on 2011-10-14.</ref>

== Unit cell ==
Brookite belongs to the [[Orthorhombic crystal system|orthorhombic]] dipyramidal [[Crystallographic point group|crystal class]] 2/m 2/m 2/m (also designated mmm).  The [[space group]] is Pcab and the [[Crystal structure|unit cell]] parameters are a = 5.4558 Å, b = 9.1819 Å and c = 5.1429 Å.  The [[Chemical formula|formula]] is TiO<sub>2</sub>, with 8 formula units per unit cell.<ref name=Webmin/><ref name=Mindat/><ref name=HOM/>

== Structure ==
[[Image:Brookite.png|thumb|left|Crystal structure of brookite]]
The brookite structure is built up of distorted [[Octahedron|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 crystal system|orthorhombic]] structure.<ref name=pauling>[https://paulingblog.wordpress.com/2010/01/12/the-crystal-structure-of-brookite The Crystal Structure of Brookite]. paulingblog.wordpress.com. 12 January 2010</ref>

== Appearance ==
[[Image:Brookit, Pakistan.jpg|thumb|left|Brookite from [[Pakistan]]]]

Brookite crystals are typically tabular, elongated and [[Striation (geology)|striated]] parallel to their length. They may also be [[Pyramid (geometry)|pyramidal]], pseudo-[[hexagon]]al or [[Prism (geometry)|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 [[Almandine|almandite]] garnet are also known.  Brookite displays a submetallic [[Lustre (mineralogy)|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 properties ==
Brookite is [[Birefringence|doubly refracting]], as are all orthorhombic minerals, and it is [[Birefringence|biaxial (+)]].  [[Refractive Index|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 [[Fluorescence|fluorescent]] nor [[Radioactive decay|radioactive]].<ref name=Webmin/>

== Physical properties ==
[[File:Brookite-mun05-22b.jpg|thumb|Cherry-red Brookite from Kharan, [[Balochistan, Pakistan]] ]]
Brookite is a [[Brittleness|brittle]] mineral, with a subconchoidal to irregular [[Fracture (mineralogy)|fracture]] and poor [[Cleavage (crystal)|cleavage]] in one direction parallel to the ''c'' [[Crystal structure|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/> [[Crystal twinning|Twinning]] is uncertain.<ref name=Mindat/><ref name=HOM/> The mineral has a [[Mohs hardness]] of {{frac|5|1|2}} to 6, between [[apatite]] and [[feldspar]].  This is the same hardness as anatase and a little less than that of rutile (6 to {{frac|6|1|2}}).  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 associations ==
Brookite is an accessory mineral in [[Vein (geology)|alpine veins]] in [[gneiss]] and [[schist]]; it is also a common [[Detritus (geology)|detrital mineral]].<ref name=Mindat/><ref name=HOM/> 
Associated minerals include its [[Polymorphism (materials science)|polymorphs]] [[anatase]] and [[rutile]], and also [[titanite]], [[orthoclase]], [[quartz]], [[hematite]], [[calcite]], [[Chlorite group|chlorite]] and [[muscovite]].<ref name=HOM/>

The [[type locality (geology)|type locality]] is Twll Maen Grisial, Fron Olau, [[Prenteg]], [[Gwynedd]], [[Wales]].<ref name=Mindat/> In 2004, brookite crystals were found in the [[Kharan District|Kharan]], in [[Balochistan]], Pakistan.<ref name=Mindat/>

==See also==
* [[List of minerals]]
* [[List of minerals recognized by the International Mineralogical Association]]
* [[List of minerals named after people]]

==References==
{{Reflist}}

== External links ==
{{EB1911 poster|Brookite}}
{{Commons category|Brookite}}
* [https://web.archive.org/web/20110515111256/http://cst-www.nrl.navy.mil/lattice/struk/c21.html Brookite structure ]
* [http://ruby.colorado.edu/~smyth/min/tio2.html Crystal structures of rutile, anatase and brookite] {{Webarchive|url=https://web.archive.org/web/20090131093137/http://ruby.colorado.edu/~smyth/min/tio2.html |date=2009-01-31 }}
* [https://rruff.geo.arizona.edu/AMS/viewJmol.php?id=05208 JMol]

 
{{Titanium minerals}}

<!--- Categories--->
[[Category:Titanium minerals]]
[[Category:Oxide minerals]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 61]]
[[Category:Polymorphism (materials science)]]