Brookite
| Brookite | |
|---|---|
Brookite from Balochistan
|
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| General | |
| Category | Oxide minerals |
|
Formula (repeating unit) |
TiO2 |
| Strunz classification | 4.DD.10 (10 ed) 4/D.15-10 (8 ed) |
| Dana classification | 4.4.5.1 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pbca |
| Unit cell | a = 5.4558 Å, b = 9.1819 Å, c = 5.1429 Å; Z = 8 |
| Identification | |
| Formula mass | 79.88 g/mol |
| Color | Deep red, reddish brown, yellowish brown, brown, or black |
| Crystal 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 scale hardness | 5½ to 6 |
| Luster | Submetallic |
| Streak | White, greyish or yellowish |
| Diaphaneity | Opaque to translucent |
| Specific gravity | 4.08 to 4.18 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 2.583 nβ = 2.584 nγ = 2.700 |
| Birefringence | δ = 0.117 |
| Pleochroism | Very weak, yellowish, reddish, orange to brown |
| 2V angle | Calculated: 12° to 20° |
| Dispersion | 0.131 (compare to diamond at 0.044) |
| Ultraviolet fluorescence | Non-fluorescent |
| References | |
Brookite is the orthorhombic variant of titanium dioxide, TiO2, which occurs in four natural polymorphic forms (minerals with the same composition but different structure). The International Mineralogical Association (IMA) recognizes these 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 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 by French mineralogist Armand Lévy for Henry James Brooke (1771–1857), an English crystallographer, mineralogist and wool trader.
Arkansite is a variety of brookite from Arkansas, US, 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.
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 (Z = 8).
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