Brookite, one of the three modifications in which titanium dioxide (TiO) occurs in nature; the other minerals with the same chemical composition, but with different physical and crystallographic characters, being rutile (q.v.) and anatase (q.v.) The two latter are tetragonal in crystallization, whilst brookite is orthorhombic. The name was given by A. Lévy in 1825 in honour of the English mineralogist H.J. Brooke (1771-1857). Two types of brookite crystals may be distinguished. The commoner type of crystals are thin and tabular, and often terminated by numerous small and brilliant faces (fig. 1); the faces of the orthopinacoid (a) and of the prisms (m, l) are vertically striated. These crystals are of a rich reddish-brown colour and are often translucent. Crystals of the second type have the appearance of six-sided bipyramids (fig. 2) owing to the equal development of the prism m {110} and the pyramid e {122}; these crystals are black and opaque, and constitute the variety known as arkansite.

The lustre of brookite is metallic-adamantine. There is no distinct cleavage (rutile and anatase have cleavages); hardness 5½-6; sp. gr. 4.0. The optical characters are interesting: the optic axes for red and for blue light lie in planes at right angles to each other, whilst for yellow-green light the crystals are uniaxial. The acute bisectrix of the optic axes is perpendicular to the orthopinacoid (a) for all colours, so that this phenomenon of the crossing of the optic axial planes may be readily observed in the thin tabular crystals of the first-mentioned type.

Brookite occurs only as crystals, never in compact masses, and is usually associated with either anatase or rutile. The crystals are found attached to the walls of cavities in decomposed igneous rocks and crystalline schists; it is also found as minute isolated crystals in many sedimentary rocks. The best-known locality is Fronolen near Tremadoc in North Wales, where crystals of the thin tabular habit occur with crystallized quartz, albite and anatase on the walls of crevices in diabase. Similar crystals of relatively large size are found attached to gneiss at several places in the Swiss and Tirolese Alps. Thicker crystals of prismatic, rather than tabular, habit and of a rich red colour combined with considerable transparency and brilliancy are found in the gold-washings of the Sanarka river in the southern Urals. The arkansite variety occurs with rutile in the elaeolite-syenite of Magnet Cove in Hot Spring county, Arkansas. Minute crystals of brookite have been detected with anatase and rutile in the iron-ore of Cleveland in Yorkshire.

Crystals of brookite, as well as of anatase and rutile, have been prepared artificially by the interaction of steam and titanium fluoride, the particular modification of titanium dioxide which results depending on the temperature at which the reaction takes place. Brookite is liable to become altered to rutile: aggregates of rutile needles with the form of brookite (arkansite) are not uncommon at Magnet Cove, Arkansas.

(L. J. S.)