RUTILE, the most abundant of the three native forms of titanium dioxide, ; the others being anatase and brookite (qq.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 pyra mid planes and also in the twinning ; the prism planes are striated vertically. Acicular crystals are sometimes twinned together to form reticulated skeletal plates to which the name "sagenite" (Gr. a net), is applied. At the same time, 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 (Lat. rutilus, red). Crystals are transparent to opaque, and have a brilliant metallic adamantine lustre. The hardness is 6.5; specific gravity 4.2,
ranging up to 5.2 in varieties containing of ferric oxide. 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 as a gem under the name of "Venus' hair stone" (Pliny's Veneris crinis). As a sec ondary mineral rutile in the form of minute needles is of wide dis tribution 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.
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