1911 Encyclopædia Britannica/Rutile

by Leonard James Spencer

RUTILE, the most abundant of the three native forms of titanium dioxide (TiO₂); the other forms being anatase (q.v.) and brookite (q.v.). Like anatase, it crystallizes in the tetragonal system, but with different angles and cleavages, it being crystallographically related to cassiterite, with which -it is isomorphous. The crystals resemble cassiterite in their prismatic habit and terminal pyramid planes (fig. 1) and also in the twinning: the prism planes are striated vertically, Geniculated twins, with e (101) as twin-plane, are of frequent occurrence, and the twinning is usually several times repeated, giving rise to triplets (fig. 2), sextets and octets.

Fig. 1.		Fig. 2.
Fig. 1.		Fig. 2.

Twin-lamellae are often present in the crystals. Acicular crystals are sometimes twinned together to form reticulated skeletal plates to which the name "sagenite," from Gr. σαγήνη (a net), is applied. A rarer type of twinning, on the plane (301), gives rise to heart-shaped or kite-shaped forms. There are distinct cleavages parallel to the faces of the prisms m (110). and a (100). The colour is usually reddish-brown, though yellowish in the very fine needles, and black in the ferruginous varieties (“nigrine” and “ilmenorutile”): the streak is pale brown. The name rutile, given by A. G. Werner in 1803, refers to the colour, being from the Latin rutilus (red). Crystals are transparent to opaque, and have a brilliant metallic-adamantine lustre. The hardness is 61/2 and the specific gravity 4·2, ranging, however, up to 5·2 in varieties containing 10% of ferric oxide. The refractive indices and the positive birefringence are high.

Rutile occurs as a primary constituent in eruptive rocks, but more frequently in schistose rocks. As delicate acicular crystals it is often enclosed in mica and quartz: in mica (q.v.) it gives rise to the phenomenon of asterism; and clear transparent quartz (rock-crystal) enclosing rutile is often cut las a gem under the name of “Venus’ hair stone” (Veneris crinis of Pliny). Larger crystals occur in the cavities of granite and crystalline schists; very large twinned crystals have been found at Graves Mountain in Lincoln county, Georgia, and good specimens have been obtained from several places in Norway, and the Swiss and Tirolese Alps. As a secondary mineral, rutile in the form of minute needles is of wide distribution in various sedimentary rocks, especially clays and slares. As rounded grains it is often met with in auriferous sands and gravels. The mineral has little economic value: it has been used for imparting a yellow colour to glass and porcelain, and for this purpose is mined at Risor and other places in Norway.  (L. J. S.)