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Encyclopedia Britannica - Main :: YAK-ZYM |
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ZIRCON , a mineral
mineral
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
Spencer
Whilst zircon is usually regarded as a zirconium silicate (ZrSiO4) it is sometimes placed with the oxides as consisting of ZrO2SiO2. A small proportion of ferric oxide
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
Renfrew
York
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