BROOKITE, one of the three modifications in which titanium dioxide occurs in nature ; the other minerals with the same chemical composition, but with different physical and crystallo graphic characters, being rutile (q.v.) and anatase (q.v.). The two latter are tetragonal in crystallization whilst brookite is or thorhombic. The name was given in honour of the English miner alogist H. J. Brooke (1771-1857). Two types of brookite crys tals may be distinguished. The commoner type of crystals are thin and tabular, and often terminated by numerous small and brilliant faces. 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; these crystals are black and opaque, and constitute the variety known as arkansite.
The lustre of brookite is metallic-adamantine. There is no dis tinct cleavage (rutile and anatase have cleavages) ; hardness 51-6; 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 orthopin acoid 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 crys tals 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. (L. J. S.)