Brookite
Orthorhombic titanium dioxide polymorph, rare and photocatalytic.
Brookite is the orthorhombic polymorph of titanium dioxide (TiO₂), one of four naturally occurring mineral forms with the same chemical composition but distinct crystal structures. The other three are akaogiite (monoclinic), anatase (tetragonal), and rutile (tetragonal). It is significantly rarer than anatase and rutile, though like them, it exhibits photocatalytic activity. Its unit cell is larger than those of anatase or rutile, containing eight TiO₂ formula units, compared to four for anatase and two for rutile. Common impurities include iron, tantalum, and niobium. The mineral was named in 1825 by French mineralogist Armand Lévy in honor of Henry James Brooke (1771–1857), an English crystallographer, mineralogist, and wool trader. A variety known as arkansite occurs in Magnet Cove, Arkansas, and it is also found in the Murun Massif on the Olyokma-Chara Plateau of Eastern Siberia, Russia. Above approximately 750 °C, brookite transforms into the rutile structure.
Brookite belongs to the orthorhombic dipyramidal crystal class (2/m 2/m 2/m, also designated mmm), with space group Pcab and unit cell parameters a = 5.4558 Å, b = 9.1819 Å, and c = 5.1429 Å. Its structure consists of distorted octahedra, each with a titanium ion at the center and oxygen ions at the six vertices; each octahedron shares three edges with neighbors, forming the orthorhombic framework. Crystals are typically tabular, elongated, and striated parallel to their length, and may also be pyramidal, pseudo-hexagonal, or prismatic. Brookite and rutile can grow together in an epitaxial relationship. The mineral is usually brown, sometimes yellowish or reddish brown, or even black; deep red crystals resembling pyrope and almandite garnet are also known. It has a submetallic luster, is opaque to translucent, and appears yellowish brown to dark brown in transmitted light. Brookite is doubly refracting (biaxial positive) with very high refractive indices above 2.5, exceeding that of diamond (2.42). It shows very weak pleochroism in yellow, red, orange, and brown tones, and is neither fluorescent nor radioactive. The mineral is brittle, with subconchoidal to irregular fracture and poor cleavage parallel to the c-axis, with traces perpendicular to both a and b axes. Its Mohs hardness is 5.5 to 6, similar to anatase and slightly less than rutile (6 to 6.5). Specific gravity ranges from 4.08 to 4.18,
- crystal_system
- Orthorhombic
- formula
- TiO2
- hardness
- 5+1⁄2 to 6 (Mohs)
- specific_gravity
- 4.08 to 4.18
- refractive_index
- Above 2.5 (biaxial positive)
- common_impurities
- Iron, tantalum, niobium
- type_locality
- Twll Maen Grisial, Fron Olau, Prenteg, Gwynedd, Wales
Lore & Background
Brookite is a rare orthorhombic polymorph of titanium dioxide, distinct from the more common tetragonal forms anatase and rutile, as well as the monoclinic akaogiite. It was named in 1825 by French mineralogist Armand Lévy in honor of English crystallographer and wool trader Henry James Brooke. The mineral typically forms tabular, elongated, and striated crystals, though pyramidal, pseudo-hexagonal, and prismatic habits also occur. Its color ranges from brown to yellowish or reddish brown, sometimes black, and notably includes deep red crystals resembling garnet. Brookite has a submetallic luster, is opaque to translucent, and appears yellowish brown to dark brown in thin fragments. It is brittle, with a Mohs hardness of 5.5 to 6 and a specific gravity between 4.08 and 4.18. The mineral exhibits very weak pleochroism in yellow, red, orange, and brown tones, and is neither fluorescent nor radioactive. Brookite is doubly refracting and biaxial positive, with refractive indices above 2.5—higher than diamond. Its unit cell is orthorhombic, containing eight formula units per unit cell, and its structure consists of distorted octahedra sharing three edges each. Common impurities include iron, tantalum, and niobium. A variety known as arkansite occurs in Magnet Cove, Arkansas, and notable deposits are also found in the Murun Massif of Eastern Siberia. Brookite is an accessory mineral in alpine veins within gneiss and schist, and is common as a detrital mineral. Associated minerals include anatase, rutile, titanite, orthoclase, quartz, hematite, calcite, chlorite, and muscovite. The type locality is Twll Maen Grisial in Gwynedd, Wales. At temperatures above about 750 °C, brookite reverts to the rutile structure.
Reader's Guide
Brookite is significant as a rare polymorph of titanium dioxide, exhibiting photocatalytic activity similar to anatase and rutile but with a larger unit cell volume (8 TiO2 groups per cell). Its structure consists of distorted octahedra sharing three edges, forming an orthorhombic lattice. The mineral's high refractive indices (above 2.5) and biaxial positive optical character make it distinctive among minerals. Brookite is typically brown, with weak pleochroism, and is brittle with a hardness of 5+1⁄2 to 6. Common impurities include iron, tantalum, and niobium. A variety known as arkansite occurs in Magnet Cove, Arkansas, and the Murun Massif in Eastern Siberia. Associated minerals include anatase, rutile, titanite, quartz, and hematite.
Did You Know?
- Brookite has a larger cell volume than anatase or rutile, with 8 TiO2 groups per unit cell.
- It is named after Henry James Brooke, an English crystallographer and wool trader.
- A variety called arkansite is found in Magnet Cove, Arkansas, and the Murun Massif in Siberia.
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