ZIRCON, a mineral composed of zirconium silicate, sometimes used as a gem-stone. It is believed that the name comes from the Arabic zargun, and is essentially the same as “jargoon,” 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. 1 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
|Fig. 1.||Fig. 2.|
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 α-zircon, and the latter β-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 intergrowth of α-zircon or β-zircon, with a third unstable modification which he distinguishes as γ-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 proposed to call the species polycrasilite from the Greek πολύς (many) and κρᾶσις (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. Coomáraswámy has pointed out that most of the stones in the gem-gravels of Ceylon, known locally as toramalli, are zircons rather than tourmaline's.
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 sanidinc-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. R.*)