ZIRCON JOSEPH W. HOWARD Montana State University, Missoula, Montana
Zircon is chiefly zirconium silicate (ZrSi04)although most s$ecimens contain traces of many other elements. I t has a wide geogra$hical distribution. Its chief use i s as a gem stone, its clearness, fire and adamantine luster makin8 it a close riaal to the diamond.
INTRODUCTION
I
T IS difficult to understand why a mineral having all the excellent gem stone properties of zircon should have been so slow in winning its present popular favor. Perhaps it is because there has been some question as to the eenuineness of certain colored specim'ens, or i t may b; because there have been so many unanswered questions about the composition and properties of this "gem of mystery." The name zircon is generally said to come from the Arabic words zirk (a jewel) or zarqun (vermillion). Others trace it to the Persian word zargun (gold colored) (1). I n Sanskrit i t is known as Rahuratna, signifying the stone of the nodes of the moon. The red varieties are called Hyacinth for the youth Hyakinthos, beloved of Apollo. A yellow species is known as Jacinth while "jargoon" includes many colors. The Ceylonese call the colorless variety "Matura Diamond." Recently the name "Starlite" has become vety popular for the blue and green shades (2). Early Sanskrit books mention a ring called the Nava-ratna, set with the nine most precious stokes of the time, one of which was zircon. ff This mineral was one of the twelve stones in the breast plate of the Hebrew High Priest as well as forming one of the twelve foundations of the New Jerusalem. It was placed with other jewels in the Daras-Salem, the Arabian garden of peace. Zircon has also played its part in the history of chemistry. I n 1789 Klaproth analyzed a specimen of zircon from Ceylon and reported that it contained seventy per cent. of a new earth which he called zirconderde. From this "earth" Berzelius later obtained the element zirconium. SOURCES
The reported deposits of zircon are of such a wide geographical distribution that it is very likely that new finds may be reported a t any time. Known to the miners as "Dutch Boort" i t is found in the Kimberly diamond mines of South Africa (3). The grease used in the "jiggers" to separate the diamonds from other minerals also collects the zircons,
and the two must later be separated by hand-picking methods (4). 'Other occurrences of note are Expially, France (5), the Ilmen and Ural Mountains in Russia (6),Mugee, New South Wales, Australia (7), Canada (a), India ( 9 ) , Upper Burma (lo), Madagascar (11), Ceylon (12), (13), (14), and Siam (15). Colorado, New York, New Jersey, Maine, North Carolina, South Carolina and Idaho have also reported deposits (2). Zircon is said to be the only gem stone of natural occurrence in France (16). For many years Ceylon was always listed with the chief producers but a t present, "There is no mining
for zircons as such in Ceylon. They are found but only incidentally in mining for other stones" (17). Venable (7) says that Mugee, New South Wales, is the "source of the finest zircon gems." The Canadian reports state that many crystals of enormous size and remarkable beauty have been found there. Some of the individual crysials weigh about fifteen pounds and are more than four inches in diameter. I n India and Burma the excavated mud containing the zircons is dumped on iron grates to remove the larger rocks present. The material going through the grates is dumped into water-fed trammels. The finest material from here goes into washing pans and a series of jiggs. The final product representing only about one-fourth of one per cent. of the original mud, goes
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on sorting tables. The sorters are required to wear large gauze-fronted boxes over their heads to prevent them from secreting any stones in their mouths. In addition they arecarefully watched by a foreman. Siam is now one of the chief countries supplying the gem stone trade. "The zircons are mined in Chantaboon and along the western border of Siam. The mining is done by individuals rather than companies, and principally by Chinese. The rough stones are delivered in Bangkok for cutting or shipment to foreign
high accuracy. The zirconium and silicon atoms are fixed in position by the symmetry requirements of the tetragonal lattice. The oxygen atoms build a regular tetrahedron around the silicon atoms with an oxygensilicon distance of 1.64 A. and an oxygen-oxygen distance of 2.64 A. (21). When fused, zircon was found to dissociate into Zr02 and a high SiOoglass (22). Various samples of zircon have shown such differences in their physical properties that i t has been proposed that they should be divided into three classes (4). The f ~ s class t favors green, sky-blue, and golden-yellow colors. It never shows any trace of crystal faces, has nearly single refraction and the refractive index varies from 1.790 to 1.840, the specific gravity from 4.00 to 4.14 and the hardness about 7.5. A second type is characterized by honey-yellow, light green, blue, and red shades. It shows crystalline faces with tetragonal symmetry. Its double refraction runs from 1.923 to 1.932 while the extraordinary is from 1.967 to 1.993. The specific gravity varies from 4.67 to 4.71. It is slightly harder than 7.5. Neither of these types is changed, except in color, by heating.
I Courterr af Anaricon Museum of Nalural History. Ncvl Fork Cily
HYACINTH ZIRMN
FROM
CEYLON
countries, and there are quite a number of small shops, generally under Chinese proprietorships, engaged in cutting the gems. While ,Bangkok is an important market for rough zircons becaus~thecutters are available here, the greater portion of the rough zircons delivered here are said to come from Indo-China; some stones mined in Burma are also delivered here for cutting or marketing. The greater portion of the zircons shipped from Siam go to the United States, with smaller quantities being sent to Europe and India. Shipments to the invoice value of about one hundred forty thousand dollars a year are being sent to the United States (IS)." COMPOSITION AND PROPERTIES
Zircon is chiefly the silicate of zirconium (ZrSiOJ although it is seldom found in the pure state. Small amounts of many metals have been reported in various analyses (19). A specimen from Monte Somma Vesuvius, for example, was found to contain Hf, Ti, Ce, La, Nd, Pr, Sm, Yt, Dy, Ho, and Er (20). Due to the simplicity of the structure of zirconium silicate it has been possible to determine the situation of the oxygen atoms around the silicon atoms with
Between these two classes is a third which is mostly yellowish green, cloudy blue or green. Heat has a striking effect on it. I t contracts in size so that the specific gravity increases as much as three units in the &st decimal place and corresponding increases are tound in the refractive index and double refraction. I t is usually characterized by a banded or zonal structure. Color is also a basis for classification and valuation. The hyacinth variety is generally designated as having an aurora red, but Whitlock (16) believes the color of the juice of a ripe plum is a closer approximation. The blue and colorless types are the most popular in the gem trade. There has been much difference of
opinion as to whether these varieties occur as such in nature or are the result of some sort of treatment. The fact that there was such questioning is probably one cause of the delayed popularity of zircon. Any treated stone, even if i t is essentially genuine, is considered by some people to be a deception. Whatever may be the case as to natural color the fact remains that colors can be and are changed by proper treatment. And many of those who contend they have seen the natural blue stones feel that the artificially blued ones surpass them in color and fire. The simplest method of decolorizing is by firing. The usual practice is to heat the stone between 700 and 1500°C. very slowly and cool at the same rate (2). Kunz (23) states the brown variety can be changed to blue by heating in a crucible out of contact with fire but exposed to the fumes from a mixture of potassium femcyanide and cobalt nitrate for six to eight hours. After cutting, an additional five- to twenty-minute treatment is used. Other methods than these may be in use as many of those engaged in this work will not reveal the processes used. SUBSTITUTES
Synthetic spinel is the gem stone most commonly employed as a substitute for zircon. An easy means of distinguishing these two minerals is given by Wade (24). "Hold the synthetic stone in bright sunlight and allow the colored reflections to fall on an opaque white card. The card should, of course, be held in.the direction of the sun but so as not to shade the stone. The true zircon gives double images, pairs of colored spots that move as pairs when the stone is slowly moved. The synthetic substitute gives single colored spots that move about independently of each other as the stone is moved." Due to its clearness, fire, and adamantine luster a
properly cut and polished zircon is a very close rival to the diamond, and occasionally they are palmed off as diamonds. However determinations of specific gravity and hardness give easy means of distinguishing between the two. USES
Chemically purified zircon and electrically fused zircon oxide have been used for sem-permanent foundry molds, refractory bricks, cements, ladle nozzles, crucibles, and porous refractory diaphragms with good results (25). Zircon crystals have found considerable use as watch jewels and supporting edges for analytical balances. This mineral is becoming increasingly popular as a gem stone which is its chief use. The colorless stones are generally cut as brilliants and the colored ones with brilliant cut fronts and step-cut backs. Rose and table forms are also used. Wade (24) has made a vety careful study of the angles which the lapidary should use to give a zircon brilliant its greatest effectiveness. Of the one hundred sixty-nine rings of the British Museum set with gems cut from a single stone fortyfive are zircons. For ages this gem stone has been worn as a talisman. Cardanus, writing in the sixteenty century said he was accustomed to carry a zircon with him in order to induce sleep'which "it did seem somewhat to confer, but not much." It was also believed to render the wearer pleasant and agreeable, be conducive to obtaining money, cheer the heart, strengthen the mind, preserve the traveler from all perils, bring bountiful harvests to the farmer, procure honor and,wisdom, act as a charm against plague, 'thunderbolts, and evil spirits, sharpen the appetite, help digestion, and take away sin. What more could one ask?
LITERATURE CITED
(1) BUY AND BAILY, "Etymological dictioilary of chemistry and mineralogy," Edward Arnold and Co., London, 1929, p. 307. "Zircon (the gem)," Dept. of Commerce, (2) YOUNGMAN, 11. . . S. -~ Bureau of Mines. Information Circular 6465, July, 1931, 20 PP. HARGAR, "Diamond pipes and fissures of South Africa," Trans. Geol. Soc., 8, 1 1 W 4 (1905). SMITH,"Gem stones and their distinctive chsracters," Methuen & Co., London, 1931, pp. 149,228-32. DAUS AND HOLDEN, "Gems and gem materials." McGrawHill Book Co., Inc., New York City. 1931, pp. 141. Punwcro~,"Precious stones in the Urals," Mining Mag., 15, 24-5 (July. 1916). VBNABLE, "Zirconinm and its compounds," Reinhold publish in^ Corn.. New York City. 1922, pp. 99-102. PARSONS. A: L.. ''Mode of occurr&ce of the giant zircons from ilrud&ll Township, Ontario." UnivGsity of Toronto Geological Studies No. 30, 2 1 4 (1931). "Zircon from Cardiff Township, Ontario," Jid., Nu. 39. 49-56 (1931). BROWN, "India's mineral wealth." India of Today, 4, 104-5 (1923). ADAMS, "A visit to the gem districts of Ceylon and Burma," Ann. Report Smithsonian Institution, 1926, pp. 297-318. C-m, "The African and Madagascar deposits of zircon," Rev. melkriaur construction trau. publics. 1932, pp. 61-2. ~~
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(12) SPENCER,"Regularly developed crystals of zircon from . 14,43 (1907). Ceylon," Min. M G ~J., (13) Cnuncx, "Zircons from Ceylon, Chem. Nms, 85, 270 (June 6, 1902). (14) . . AD AM^, "Geology of Ceylon," Can. J. Research, 1, 461-3 (1929). (15) ALBRECT. "Zircons," Consular Report, July 17. 1926. Bureau of Mines Foreign file'No. 9203. "The story of the gems." Lee Fumam, New (16) WHITLOCK, . York City, 1936. pp. 116-17. American Consul at Colombo, Ceylon; private (17) BUELL, communication to the author. (18) American Consul at Bangkok, Siam; private . . CHAPMAN. communication to the author. (19) MELLOR,"A comprehensive treatise on inorganic and theoretical chernistrv." Lonarnans. Green and Co.. London. 1925, VOI.vii, pp. 98% nmlysis of zircon from Vesu(20) C ~ y y '.Spccrrugraphic . v u s . Bol. Sur. .Vd .Vupolt.41. 1W (1931). ASD HESDRICKS. Z.Krirt.. 66. 73 ,1927). 121) WYCKOPF (22) BARTLETT,"X-Ray and m~noscapicstudies of silicate melts containing zirconia," J. Am. Ceram. Soc., 14, 83743 (1931). (23) KUNZ,"Zircon," Mineral I d , 35, 581 (1927). (24) WADE. "Some interesting notes on mcon. The Jewelers ~ i r & l e r .97, 5 3 4 (Sepi. 13, 1928). "Same , experiments with zircon and zirconia (25) C o ~ s r o c ~ refractories."J. Am. Ceram. Soc., 16, 1%35 (1933).
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