Brookite

Formula: TiO2
Simple oxide, polymorph of anatase and rutile
Specific gravity: 4.08 to 4.18
Hardness: 5½ to 6
Streak: White to greyish white or yellowish white
Colour: Brown, yellowish brown, reddish brown; dar brown to iron-black; yellowish brown to dark brown in transmitted light
Solubility: Insoluble in acids (Dana)
Environments:

Sedimentary environments
Metamorphic environments
Hydrothermal environments

Brookite is found principally in fissures and open cavities in gneiss and schist, in veins of the Alpine cleft type. It is also found in contact metamorphic zones, and it is a common detrital mineral. Associated minerals include anatase, rutile, titanite, orthoclase, albite, quartz, hematite, calcite, chlorite and muscovite.
Rutile may be epitaxial on brookite.

At Saint-Pierre-de-Broughton, Quebec, Canada, brookite occurs in stilpnomelane schist, as a mid- to late-stage mineral in Alpine fissures, preceded by or contemporaneous with anatase. Some crystals are partially coated with chamosite.

At the type locality, Twll Maen Grisial, Gwynedd, Wales, UK, brookite is found in a hydrothermal vein associated with quartz, chamosite and albite.

At the Ellenville lead mine, Ulster county, New York, USA, brookite occurs on quartz with galena and chalcopyrite.

At Somerville, Massachusetts, USA, brookite occurs in quartz-calcite veinlets in mafic rocks with anatase, titanite and rutile.

At the Santa Nino mine, Santa Cruz county, Arizona, USA, brookite is associated with quartz and adularia.

Common impurities: Fe3+,Ta,Nb