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Brookite
Chemical Formula	TiO2
Species	Oxides & Hydroxides
Crystal System	Orthorhombic
Mohs Scale	5-6
Specific Gravity	4.08-4.18
Color	Brown, yellowish brown, or black
Streak	White, greyish or yellowish
Luster	Adamantine, Sub-Metallic
Refractive Index	n = 2.583 n = 2.584 n = 2.700
Diaphaneity	Translucent, Opaque
Cleavage	Poor/Indistincton , in traces on
Fracture	Irregular/Uneven, Sub-Conchoidal
Crystal Habit：Tabular and striated, pyramidal or pseudohexagonal
Geological Setting：An accessory mineral in alpine veins in gneiss and schist; in contact metamorphic zones and hydrothermal veinlets; a common detrital mineral.

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. Iron Fe, tantalum Ta and niobium Nb are common impurities.

It was named in 1825 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.

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 ?, b = 9.1819 ? and c = 5.1429 ?. The formula is TiO₂, with 8 formula units per unit cell (Z = 8).

Structure

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.

Appearance

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

Brookite is usually brown in color, sometimes yellowish or reddish brown or even black, with a submetallic luster. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light.

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. It is neither fluorescent nor radioactive.

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. Twinning is uncertain. The mineral has a Mohs hardness of 5? to 6, between apatite and feldspar. This is the same hardness as anatase and a little less than that of rutile (6 to 6?). The specific gravity is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.

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 and muscovite.

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