ZIRCON  , a

mineral composed of zirconium silicate, some-times used as a gem-stone . It is believed that the name comes from the Arabic zargun, and is essentially the same as " jar-goon," the name given to certain varieties of zircon . The mineral crystallizes in the tetragonal system, generally in combinations of square prisms and square pyramids, as in figs. i and 2 . Zircon is isomorphous with cassiterite and rutile, and like them may occur in geniculated twins . There is no distinct cleavage, and the mineral breaks with a conchoidal fracture .. The hardness is about 7.5 . It is notable that the specific gravity has a very wide range, extending from a little below 4 to rather more than 4.7, and being thus greater than that of any other gem-stone . Rarely colourless, zircon is usually brown or red, sometimes orange, yellow or green, and occasionally parti-coloured or zoned . Whilst common zircon is opaque, the gem-varieties are transparent . The dichroism of coloured zircons is always feeble; the double refraction usually strong and of positive sign; and the optical properties of some zircons suggest a biaxial mineral . It was pointed out long ago by . Sir A .

H .

Church that many transparent zircons afford a spectrum marked by certain absorption-bands, a property perhaps due to the presence of uranium . The effect of heat on zircon is remarkable . Most coloured zircons, exposed to a high temperature, either change or lose their colour, but this loss is attended by a gain in brilliancy . The " Matura diamonds " of Ceylon are zircons which have been thus artificially decolorized . Certain zircons when heated in a Bunsen-flame glow with an orange incandescence, whilst others may emit an orange glow when ground on a copper-wheel fed with diamond-dust . Even exposure to sunlight will sometimes modify the colour and lustre of a zircon . Some zircons suffer contraction when heated, so that the specific gravity becomes raised; but the behaviour of zircons in this respect shows such anomalies that S . Stevanovic has been led to suggest the existence of three classes of zircon . One group has a specific gravity of 4.0 and another of 4.7, both remaining unchanged in density when heated . L . J .

Spencer, who has studied some remarkable crystals from Ceylon, calls the former a-zircon, and the latter 13-zircon . A third class has specific gravity between 4.0 and 4.7, and increases in density on heating . These stones consist, according to Spencer, of an inter-growth of a-zircon or 3-zircon, with a third unstable modification which he distinguishes as -y-zircon, Whilst zircon is usually regarded as a zirconium silicate (ZrSiO4) it is sometimes placed with the oxides as consisting of ZrO2•SiO2 . A small proportion of ferric oxide seems to be always present, and to this the colour of zircon, according to G . Spezia, may be ascribed . Traces of so many elements have been recorded in certain zircons that it was at one time pro-posed to call the species polycrasilite from the Greek sroXvs (many) and Kpaars (mixture) . Zircon is used as a source of zirconia in various preparations, for incandescent gas-mantles, &c . It was in this mineral that zirconia was originally discovered by M . H . Klaproth in 1789 . Zircon fit for use as a gem-stone is often known as " noble " or " precious zircon." The red and orange stones are termed hyacinth (q.v.) and jacinth, whilst those of other colours, as also the colourless transparent zircons, are called jargoon (q.v.) . The lyncurium of the ancients, described as an amber-coloured stone used for signets, is supposed by some authorities to have been zircon and by others amber .

The gem varieties of zircon are found in detrital deposits, especially in Ceylon and in New

South Wales, where they accompany sapphire, &c . They occur also in the Anakie sapphire district, near Emerald, in Queensland . A . K . Coomaraswamy has pointed out that most of the stones in the gem-gravels of Ceylon, known locally as toramalli, are zircons rather than tourmalines . Zircon is an accessory constituent of many rocks, especially granite, where it appears to have crystallized at an early stage of consolidation . In microscopic sections, viewed by transmitted light, the zircon by virtue of its high refractive power appears to stand out in relief . It forms an important constituent of the zircon-syenite of Norway . Zircon occurs also in many basic eruptive rocks, notably the basalts of the Rhine and Central France . Being but little subject to alteration, it is common in secondary deposits, as in auriferous and other sands, occurring usually in small characteristic crystals, with rounded angles . Fine crystals of zircon are found in the Ilmen Mountains in Russia, and in Renfrew co., Ontario, where it occurs in crystalline limestone . Many localities in the United States yield zircon, especially in New York state and in North Carolina: it has been largely worked in Henderson co ..

N.C . Zircon occurs also in

Tasmania . Certain varieties of zircon have received distinctive names, such as the azorite, which occurs in sanidine-trachyte in the Azores . Several other minerals seem to be altered zircon, generally hydrated, such as malacon, cyrtolite and oerstedite, the last being a Norwegian mineral containing titanium and magnesium . Auerbachite is a Russian mineral closely related to zircon . (F . W .