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BROOKITE

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Originally appearing in Volume V04, Page 646 of the 1911 Encyclopedia Britannica.
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BROOKITE , one of the three modifications in which See also:

titanium dioxide (TiO2) occurs in nature; the other minerals with the same chemical See also:composition, but with different See also:physical and crystallographic characters, being See also:rutile (q.v.) and See also:anatase (q.v.). The two latter are tetragonal in See also:crystallization, whilst brookite is orthorhombic. The name was given by A. See also:Levy in 1825 in See also:honour of the See also:English mineralogist H. J. See also:Brooke (1771-1857). Two types of brookite crystals may be distinguished. The commoner type of crystals are thin and See also:tabular, and often terminated by numerous small .and brilliant faces (fig. 1); the faces of theorthopinacoid (a) and of the prisms (m, l) are vertically striated. These crystals are of a See also:rich reddish-See also:brown See also:colour and are often translucent. Crystals of the second type have the See also:appearance of six-sided bipyramids (fig. 2) owing to the equal development of the See also:prism m {fro) and the See also:pyramid e {1221; these crystals are See also: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 51-6; sp. gr. 4.0. The See also:

optical characters are interesting: the optic axes for red and for See also:blue See also:light See also:lie in planes at right angles to each other, whilst for yellow-See also:green light the crystals are uniaxial. The acute See also:bisectrix of the optic axes is perpendicular to the orthopinacoid (a) for all See also:colours, so that this phenomenon of the See also: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 See also:schists; it is also found as See also:minute isolated crystals in many sedimentary rocks. The best-known locality is Fronolen near Tremadoc in See also:North See also:Wales, where crystals of the thin tabular See also:habit occur with crystallized See also:quartz, See also:albite and anatase on the walls of crevices in See also:diabase. Similar crystals of relatively large See also:size are found attached to See also:gneiss at several places in the Swiss and Tirolese See also: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 See also:gold-washings of the Sanarka See also:river in the See also:southern Urals. The arkansite variety occurs with rutile in the elaeolite-See also:syenite of Magnet See also:Cove in Hot See also:Spring See also:county, See also:Arkansas. Minute crystals of brookite have been detected with anatase and rutile in the See also:iron-ore of See also:Cleveland in See also:Yorkshire.

Crystals of brookite, as well as of anatase and rutile, have been prepared artificially by the interaction of See also:

steam and titanium fluoride, the particular modification of titanium dioxide which results depending on the temperature at which the reaction takes See also:place. Brookite is liable to become altered to rutile: aggregates of rutile needles with the See also:form of brookite (arkansite) are not uncommon at Magnet Cove, Arkansas. (L. J.

End of Article: BROOKITE

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