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Encyclopedia Britannica - Main :: RON-SAC |
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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
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
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
mineral
mineral
porcelain
mined
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