Brookite

Brookite is the orthorhombic variant of titanium dioxide, TiO2, which occurs in many natural polymorphic forms (minerals with the same composition but different structure). The International Mineralogical Association (IMA) recognizes four forms; 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.[5] 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.[6] Iron Fe, tantalum Ta and niobium Nb are common impurities.[3]

Brookite
Brookite from Balochistan
General
CategoryOxide minerals
Formula
(repeating unit)		TiO2
Strunz classification4.DD.10 (10 ed)
4/D.15-10 (8 ed)
Dana classification4.4.5.1
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbca
Unit cella = 5.4558 Å,
b = 9.1819 Å,
c = 5.1429 Å; Z = 8
Identification
Formula mass79.88 g/mol[1]
ColorDeep red, reddish brown, yellowish brown, brown, or black
Crystal habitTabular and striated, pyramidal or pseudohexagonal
TwinningOn {120}, uncertain
CleavagePoor on {120}, in traces on {001}
FractureSubconchoidal to irregular
TenacityBrittle
Mohs scale hardness​5 1⁄2 to 6
LusterSubmetallic
StreakWhite, greyish or yellowish
DiaphaneityOpaque to translucent
Specific gravity4.08 to 4.18
Optical propertiesBiaxial (+)
Refractive indexnα = 2.583 nβ = 2.584 nγ = 2.700
Birefringenceδ = 0.117
PleochroismVery weak, yellowish, reddish, orange to brown
2V angleCalculated: 12° to 20°
Dispersion0.131 (compare to diamond at 0.044)
Ultraviolet fluorescenceNon-fluorescent
References[1][2][3][4]

It was named in 1825 by French mineralogist Armand Lévy[3] for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.[1]

Arkansite is a variety of brookite from Arkansas, US, that is also found in the Murunskii Massif, in the Aldan Shield of Eastern Siberia, Russia, where many other unusual minerals occur.[3]

At temperatures above about 750 °C, brookite will revert to the rutile structure.[7]

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 ).[1][3][4]

Structure
Crystal structure of brookite

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.[8]

Appearance
Brookite from Pakistan

Crystals typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo-hexagonal or prismatic.[3] Brookite and rutile may grow together in an epitaxial relationship.[3]

Brookite is usually brown in color, or 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.[1][3][4]

Optical properties

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.[3][4] It is neither fluorescent nor radioactive.[1]

Physical properties

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.[1][3][4] Twinning is uncertain.[3][4] The mineral has a Mohs hardness of ​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 ​6 1⁄2). The specific gravity is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.[3][4]

Occurrence and associations

Brookite is an accessory mineral in alpine veins in gneiss and schist; it is also a common detrital mineral.[3][4] Associated minerals include its polymorphs anatase and rutile, and also titanite, orthoclase, quartz, hematite, calcite, chlorite and muscovite.[4]

The type locality is Twll Maen Grisial, Fron Olau, Prenteg, Gwynedd, Wales, UK,[3] and in 2004 fine brookite crystals were found at Kharan, in Balochistan, Pakistan, together with brookite and rutile inclusions in quartz.[3]

See also

References
  1. Brookite. Webmineral.com. Retrieved on 2011-10-14.
  2. Gaines et al (1997) Dana’s New Mineralogy Eighth Edition. Wiley
  3. Brookite. Mindat.org (2011-09-17). Retrieved on 2011-10-14.
  4. Brookite. Handbook of Mineralogy. (PDF) . Retrieved on 2011-10-14.
  5. Di Paola, A; Addamo, M.; Bellardita, M.; Cazzanelli, E.; Palmisano, L. (2007). "Preparation of photocatalytic brookite thin films". Thin Solid Films. 515 (7–8): 3527–3529. Bibcode:2007TSF...515.3527D. doi:10.1016/j.tsf.2006.10.114.
  6. Anatase and Brookite. Wikis.lib.ncsu.edu (2007-05-08). Retrieved on 2011-10-14.
  7. Brookite (Titanium Oxide). Galleries.com. Retrieved on 2011-10-14.
  8. The Crystal Structure of Brookite. paulingblog.wordpress.com. 12 January 2010
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