Page:EB1911 - Volume 28.djvu/1017

 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 -zircon, and the latter -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 -zircon or -zircon, with a third unstable modification which he distinguishes as -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  (many) and  (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 (q.v.) and jacinth, whilst those of other colours, as also the colourless transparent zircons, are called (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 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 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.

 ZIRCONIUM [symbol Zr, atomic weight 90·6 (O＝16)], a metallic chemical element. Klaproth in 1789 analysed the mineral zircon or hyacinth and found it to contain a new earth, which he called “zirconia.” The metal was obtained by Berzelius as an iron-grey powder by heating potassium zirconofluoride with metallic potassium. The amorphous metal also results when the chloride is heated with sodium; the oxide reduced with magnesium; or when fused potassium zirconofluoride is electrolysed (Wedekind, Zeit. Elektrochem., 1904, 10, p. 331). Troost produced crystallized zirconium by fusing the double fluoride with aluminium in a graphite crucible at the temperature of melting iron, and extracting the aluminium from the melt with hydrochloric acid. It is more conveniently prepared by heating the oxide with carbon in the electric furnace. The crystals look like antimony, and are brittle, and so hard as to scratch glass and rubies; their specific gravity is 4·25. The powdery metal burns readily in air, the crystalline metal requires to be heated in an oxyhydrogen flame before it catches fire. Mineral acids generally attack the crystallized metal very little even in the heat; aqua regia, however, dissolves it readily, and so does hydrofluoric acid. In its chemical affinities zirconium resembles titanium, cerium and thorium; it occurs in company with these elements, and is tetravalent in its more important salts.

Zirconium oxide or zirconia, ZrO2, has become important since its application to the manufacture of mantles for incandescent gas-lighting. For its extraction from zircon the mineral is heated and quenched in water to render it brittle, and then reduced to a fine powder, which is fused with three to four parts of acid potassium fluoride in a platinum crucible. When the mass is quietly fusing, the crucible is heated for two hours in a wind-furnace. The porcelain-like melt is powdered, boiled with water, and acidified with hydrofluoric acid, and the residual potassium fluosilicate is filtered off. The filtrate on cooling deposits crystals of potassium zirconofluoride, K2ZrF6, which are purified by crystallization from hot water. The double fluoride is decomposed with hot concentrated sulphuric acid; the mixed sulphate is dissolved in water; and the zirconia is precipitated with ammonia in the cold. The precipitate, being difficult to wash, is (after a preliminary washing) re-dissolved in hydrochloric acid and re-precipitated with ammonia. Zirconium hydroxide, Zr(OH)4, as thus obtained, is quite appreciably soluble in water and easily in mineral acids, with formation of zirconium salts, e.g. ZrCl4. But, if the hydroxide is precipitated in the heat, it demands concentrated acids for its solution. The hydroxide readily loses its water at a dull red heat and passes into an hydride with vivid incandescence. Zirconia can be obtained crystalline, in a form isomorphous with cassiterite and rutile, by fusing the amorphous modification with borax, and dissolving out with sulphuric acid. The anhydrous oxide is with difficulty soluble even in hydrofluoric acid; but a mixture of two parts of concentrated sulphuric acid and one of water dissolves it on continued heating as the sulphate, Zr(SO4)2. Zirconia, when heated to whiteness, remains unfused, and radiates a fine white light, which suggested its utilization for making incandescent gas mantles; and, in the form of disks, as a substitute for the lime-cylinders ordinarily employed in “limelight.” Zirconia, like stannic and titanic oxides, unites not only with acids but also with basic oxides. For instance, if it be fused with sodium carbonate, sodium zirconate, Na2ZrO3, is formed. If the carbonate be in excess, the salt Na4ZrO4 results, which when treated with water gives Na2Zr8O17·12H2O, which crystallizes in hexagonal plates. When heated in a loosely covered crucible with magnesium the nitride Zr2N3 is formed (Wedekind, Zeit. anorg. Chem., 1905, 45, p. 385).

Zirconium hydride, ZrH2, is supposed to be formed when zirconia is heated with magnesium in an atmosphere of hydrogen. Zirconium fluoride, ZrF4, is obtained as glittering monoclinic tables (with 3H2O) by heating zirconia with acid ammonium fluoride. It forms double salts, named zircono-fluorides, which are isomorphous with the stanni- and titani-fluorides. Zirconium chloride, ZrCl4, is prepared as a white sublimate by igniting a mixture of zirconia and charcoal in a current of chlorine. It has the exact vapour-density corresponding to the formula. It dissolves in water with evolution of heat; on evaporation a basic salt, ZrOCl2·8H2O, separates out in star-shaped acicular aggregates. Zirconium bromide, ZrBr4, is formed similarly to the chloride. Water gives the oxybromide ZrOBr2. Zirconium iodide, ZrI4, was obtained as a yellow, microcrystalline solid by acting with hydriodic acid on heated zirconium (Wedekind, Ber., 1904, 37, p. 1135). It fumes in air; with water it gives ZrOI2·8H2O; and with alcohol ethyl iodide and zirconium hydroxide are formed. The iodide combines with liquid ammonia to form ZrI4·8NH3; and with ether to give ZrI4·4(C2H5)2O. Zirconium combines with sulphur to form a sulphide, and with carbon to form several carbides. The sulphate, Zr(SO4)2, is a white mass obtained by dissolving the oxide or hydroxide in sulphuric acid, evaporating and heating the mass to nearly a red heat. Since it forms a series of double sulphates, Ruer (Zeit. anorg. Chem., 1904, 42, p. 87) regards it as a dibasic acid, ZrOSO4·SO4H2, and that the crystalline sulphate is ZrOSO4·SO4H2·3H2O (not Zr(SO4)2·4H2O). Zirconium also forms double sulphates of the type Zr2O3(SO4M)2·nH2O, where M＝K, Rb, Cs, and n＝8 for K, 15 for Rb, 11 for Cs (Rosenheim and Frank, Ber., 1905, 38, p. 812). The atomic weight was determined by Marignac to be 90·03; Bailey (Proc. Roy. Soc., 1890, 46, p. 74) deduced the value 89·95. 