Brookite

Brookite is a naturally occurring polymorph of titanium dioxide (TiO₂) that crystallizes in the orthorhombic system. It is one of the three mineral forms of TiO₂, the others being rutile (tetragonal) and anatase (tetragonal). Brookite’s crystal structure consists of chains of TiO₆ octahedra linked by shared edges and corners, giving the mineral a characteristic prismatic habit with acute angles and often a brownish‑yellow to reddish‑brown color.

Physical properties include a Mohs hardness of 5.5–6, a specific gravity of 4.3–4.5, and a refractive index ranging from 2.583 to 2.605. It exhibits strong birefringence and can display pleochroism from yellow to brown. Unlike rutile, brookite is not commonly encountered as a large, well‑formed crystal; it typically occurs as microscopic aggregates or disseminated grains within host rocks.

Brookite is most frequently found in high‑temperature, silica‑rich metamorphic environments, such as granulites and skarns, and as a secondary mineral in pegmatites and hydrothermal veins. Notable localities include the Ilmen Mountains of Russia, Bundy in New South Wales, Australia, and several sites in the United States, particularly in California and Nevada. It often forms in association with rutile, anatase, hematite, and sphalerite.

Industrial interest in brookite is limited compared to rutile, which is the primary source of titanium metal and pigment-grade TiO₂. Nevertheless, synthetic brookite has attracted research for photocatalytic applications because its band‑gap and crystal surface properties differ from those of rutile and anatase, potentially offering enhanced activity in water splitting and pollutant degradation.

The mineral was first described in 1865 by German mineralogist H. von Meyer and named after the English chemist Henry James Brooke, a pioneer in titanium research.