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 . I) and also in the twinning: the prism planes are striated vertically . Geniculated twins, with e (ior) as twin-plane, are of frequent occurrence, and the twinning is usually several times repeated, giving rise to triplets (fig . 2), sextets and octets . 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. oayi7vn (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 (loo) . The colour is usually reddish-brown, though yellowish in the very fine needles, and black in the ferruginous varieties (" nigrine " and " ilmeno ruble "): 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 6a and the specific gravity 4.2, ranging, however, up to 5.2 in varieties containing Io% of ferric oxide . The refractive indices and the positive birefringence are high . Rutile occur's 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 as 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 Ti:olese Alps . As a secondary mineral, rutile in the form of minute needles is of wide distribution in various sedimentary rocks, especially clays and slates . 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 .