Zircon

Zircon is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is ZrSiO4. An empirical formula showing some of the range of substitution in zircon is (Zr1–y, REEy)(SiO4)1–x(OH)4x–y. Zircon precipitates from silicate melts and has relatively high concentrations of high field strength incompatible elements. For example, hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, red, brown, blue, and green.

The name derives from the Persian zargun, meaning "gold-hued". This word is changed into "jargoon", a term applied to light-colored zircons. The English word "zircon" is derived from Zirkon, which is the German adaptation of this word. Yellow, orange, and red zircon is also known as "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.

Properties

Zircon is common in the crust of Earth. It occurs as a common accessory mineral in igneous rocks (as primary crystallization products), in metamorphic rocks and as detrital grains in sedimentary rocks. Large zircon crystals are rare. Their average size in granite rocks is about 0.1–0.3 mm (0.0039–0.0118 in), but they can also grow to sizes of several cm, especially in mafic pegmatites and carbonatites. Zircon is fairly hard (with a Mohs hardness of 7.5) and chemically stable, and so is highly resistant to weathering. It also is resistant to heat, so that detrital zircon grains are sometimes preserved in igneous rocks formed from melted sediments. Its resistance to weathering, together with its relatively high specific gravity (4.68), make it an important component of the heavy mineral fraction of sandstones.

Because of their uranium and thorium content, some zircons undergo metamictization. Connected to internal radiation damage, these processes partially disrupt the crystal structure and partly explain the highly variable properties of zircon. As zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, and the color changes.

Zircon occurs in many colors, including reddish brown, yellow, green, blue, gray, and colorless. The color of zircons can sometimes be changed by heat treatment. Common brown zircons can be transformed into colorless and blue zircons by heating to 800 to 1,000 °C (1,470 to 1,830 °F). In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers. Color in this red or pink series is annealed in geological conditions above temperatures of around 400 °C (752 °F).

Structurally, zircon consists of parallel chains of alternating silica tetrahedra (silicon ions in fourfold coordination with oxygen ions) and zirconium ions, with the large zirconium ions in eightfold coordination with oxygen ions.

Classification

Already in the outdated, but still in use 8th edition of the mineral systematics according to Strunz, zircon belonged to the mineral class of “silicates and germanates” and there to the division of “ island silicates (nesosilicates)”, where it formed the name giver of the “zircon group” with the system number VIII/A.09 and the other members coffinite, hafnon, reidite, thorite and thorogummite.

The 9th edition of Strunz's mineral classification, valid since 2001 and used by the IMA, also classifies zircon in the division of "island silicates (nesosilicates)". However, this division is further subdivided according to the possible presence of other anions and the coordination of the cations, so that the mineral, according to its composition, can be found in the subdivision of "island silicates without other anions with cations in octahedral and usually larger coordination", where it forms the "zircon group" with the system number 9.AD.30 together with coffinite, hafnone, stetindite, thorite and thorogummite.

Dana's classification of minerals, which is predominantly used in English-speaking countries, also classifies zircon in the class of "silicates and germanates" and within the division of "island silicate minerals." Here, it is found as the namesake of the " zircon group " with the system number 51.05.02, along with the other members hafnone, thorite, coffinite, thorogummite, and stetindite, within the subdivision of "island silicates: SiO 4 groups only with cations in > coordination."

Chemistry

The formula of pure zircon with end-member composition Zr requires contents of 67.1 wt.% (weight percent) ZrO 2 and 32.9 wt.% SiO 2. Natural zircons often contain a wide spectrum of foreign elements and inclusions of various other minerals, including segregations, intercalations and zoned intergrowths.  The most important accompanying elements are hafnium, thorium, uranium, yttrium, cerium and other rare earth metals as well as phosphorus, niobium, tantalum, aluminium, iron and calcium. The isotypy of zircon (Zr) and xenotime-(Y) (Y) is the cause of the coupled (heterovalent) substitution of Zr 4+ and Si 4+ by Y 3+ and P 5+. However, the majority of the sometimes very high Y contents are not due to a diadochic incorporation of yttrium for zircon, but to zoned, sometimes even epitaxial intergrowths with the discrete foreign mineral xenotime (compare the adjacent SEM image and under varieties). 

Hafnium was first detected in zircons from Norway by X-ray spectroscopy by the physicists Dirk Coster and George de Hevesy in Copenhagen in 1923. It also quickly became clear that hafnium is always present in zirconium-containing minerals – and thus in all zircons – because Hf 4+ ions have an ionic radius comparable to that of their lighter homologue Zr 4+ due to lanthanide contraction and thus fit perfectly into the crystal structures of zirconium compounds. With its hafnium-dominant analogue hafnone (Hf), zirconium thus forms a continuous solid solution series. Contents of 45.30 wt.% hafnium dioxide (HfO 2) and 27.69 wt.% zirconium dioxide (ZrO 2) characterize the midpoint of the solid solution series with the formula (Zr 0.50 Hf 0.50) Σ=1.00 SiO 4. Crystals with hafnium dioxide contents > 45.30 wt.% are classified as hafnones; if the value is less than 45.30 wt.%, they are zircons. The HfO 2 content of zircons is normally about 1 to 1.5 wt.%, and the Hf/Zr ratio is 0.02–0.04. 

In extreme cases, zircon can also contain up to 12 wt% thorium dioxide (ThO 2) or 1.5 wt% uranium(V,VI) oxide (U 3 O 8). An yttrium-bearing zircon variety has been called ribeirite and contains 7.45 wt% Y 2 O 3 (“yttrian earths”). A gray-green to gray-brown zircon from Hayamadake, Fukushima Prefecture, Japan, was found to contain 10.14 wt% Y 2 O 3. 

The sometimes considerable contents of uranium and thorium make zircon the main carrier of radioactivity in the rocks. However, even formula-pure zircon is weakly radioactive, as it consists of 2.8% of the isotope 96 Zr, which decays to 96 Mo with an extremely long half-life of 24 10 18 years by double beta decay. 

Crystal structure

Zircon crystallizes tetragonally in space group I 4 1 / amd (space group no. 141) with lattice parameters a = 6.61 Å and c = 5.98 Å and four formula units per unit cell. 

The structure of zircon contains island-like 4− tetrahedra in a body - centered unit cell of Zr 4+ ions, with each Zr 4+ ion surrounded by eight O 2− ions. The 4− tetrahedra are mirror-symmetric and arranged along fourfold screw axes. The latter have opposite directions of rotation parallel through the center of the four quarter cells.  The principal structural element in zircon are zigzag chains of alternating, edge-sharing ZrO 8 dodecahedra parallel, which are connected to the 4− tetrahedra by common corners and edges to form a three-dimensional framework. Zircon is isotypic to xenotime-(Y), béhierite (Ta), chernovite-(Y), hafnone, thorite and wakefieldite-(Y) as well as a number of artificial compounds, i.e. it crystallizes with the same structure as these minerals and phases. 

In some zircons, the lattice structure is partially destroyed (metamictized) by the action of high-energy radiogenic particles (from the radioactive decay of the elements uranium and thorium contained in the zircon) – such crystals usually have darker, brown colors. Metamictization can cause water to be incorporated into the crystal lattice. The result is a noticeable reduction in refractive index, density, and hardness. Birefringence is no longer present at all. In this respect, zircons are differentiated according to their stages into

High zircons (normal, crystalline zircons),

Deep zircons (metamict zircons),

intermediate zircons,

whose properties lie between the first two groups. By heating to over 1000 °C, the low-grade zircons can recrystallize back into high-grade zircons.

Characteristics

Morphology

Zircon almost always forms circumferentially developed, but only rarely intergrown, crystals, often square in cross-section, whose average size, e.g., in granitoid rocks, is between 100 and 300 µm. Occasionally, however, they also reach sizes of several centimeters, especially in pegmatites or heavy mineral placers. The largest known zircon in the world to date measured 10 cm × 10 cm × 30 cm, weighed over 7 kg, and was found near Brudenell in the Canadian province of Ontario. 

Zircon crystals are in most cases terminated at both ends. Their length/width ratios, which reflect the rate of crystallization, vary between 1 and 5. Indeed, acicular crystals are frequently found in rapidly crystallized, porphyritic, subvolcanic intrusions, as well as near-surface intruded granites and gabbros. 

Zircons essentially occur in three different basic types with the main surface shapes {100}, {110}, {101}, and {301} . These three basic morphological types of zircon include a pyramidal habit with {101} and/or {211}, a prismatic habit with {100} and/or {110}, and an elongated habit with prismatic and pyramidal surface shapes. The pyramidal habit includes the typical dipyramidal crystals, which display the pyramid {101} alone or with narrow faces of the prism II position {100}. Crystals with a prismatic habit are considerably more common. Here, the pattern-determining prisms of position II {100} and/or position I {110} are joined by the tetragonal pyramids of position II {101} and {301}, the tetragonal pyramid of position I {112}, and the tetragonal dipyramid {211}. Very characteristic are short-prismatic crystals with {110} and {101}, which exhibit a pseudorhombic dodecahedral habit (so-called hyacinth habit) and are reminiscent of corresponding garnet crystals ("garnetohedrons") .

When comparing with historical crystal drawings, it should be noted that the arrangement of the crystals in modern drawings is rotated by 45° compared to the earlier morphological orientation. Thus, the pyramid formerly indexed as {111} is now indexed as {101}. 

In contrast to the similarly shaped minerals cassiterite and rutile, zircon only rarely forms knee-shaped twins with (112) as the twin plane. Such twins have been described from the Meredeth Freeman Zircon Mine in Henderson County, North Carolina, as well as cruciform twins according to (101) and visor-pearl-like twins according to (111). However, Georges Friedel had already cast doubt on the regularity of cruciform twins in 1904. Large twins according to (112), but not cruciform, but as knee-shaped twins, are known primarily from Brudenell Township, Renfrew County, Ontario, Canada. 

Zircon also occurs in clustered, kidney-shaped, and radially radiating aggregates, as well as irregular grains. Due to its resistance to weathering, zircon is found in unconsolidated sediments and placers in the form of loose, uncoiled crystals, in scoria and xenoliths associated with basaltic rocks, and in skeletal and tree-shaped aggregates.

Intergrowths with other minerals such as xenotime-(Y) are characteristic, including perfectly oriented (epitaxial) intergrowths (see the adjacent crystal drawing). Intergrowths with baddeleyite are called "zircon favas" or "caldasite". Since thorite and zircon have completely analogous structures, epitaxial intergrowths of zircon with thorite are also possible. Such intergrowths are known, among others, from Bassano Romano, Province of Viterbo, Lazio, from the San Vito quarry near San Vito not far from Ercolano, Monte Somma, Somma- Vesuvius complex, Metropolitan City of Naples, Campania, both in Italy, as well as from the ejecta of the Laacher See volcano in the Vulkaneifel.

Physical and chemical properties

In its pure form, zircon is colorless and crystal-clear. Due to multiple refractions of light caused by lattice defects or polycrystalline formation, it can also be translucent white, and due to impurities, it can take on a brown and brownish-red, and more rarely yellow, green, or blue, color. The streak color of zircon, however, is always white. The surfaces of the transparent to opaque crystals exhibit a strong glass- to diamond-like luster on all surfaces, while fracture surfaces and the metamict state exhibit a greasy luster. Some zircons also exhibit chatoyancy (cat's eye effect). 

Zircon has a very imperfect cleavage according to {100}, but, due to its brittleness, breaks similarly to quartz, with conchoidal fracture surfaces. With a Mohs hardness of 7.5, zircon is one of the hard minerals, placing it between the reference minerals quartz (hardness 7) and topaz (hardness 8). The measured density for zircon is, depending on the author, 4.6 to 4.7 g/cm³; the calculated density is 4.714 g/cm³. During metamictization (isotropy), the density of the mineral decreases to values of 3.9 to 4.2 g/cm³ (“low density zircons”). 

At normal pressure, zirconium is stable up to a temperature of 1676 °C. Above this temperature, it decomposes into tetragonal zirconium dioxide (ZrO 2) and silicon dioxide (SiO 2) in the modification β- cristobalite (high cristobalite), thus having no congruent melting point.

From 1689 °C onwards, a SiO 2 - rich melt (~95 mol-% SiO 2) forms, which becomes increasingly richer in ZrO 2 with further increasing temperatures. 

In thin sections, zircon is colorless to pale brown and exhibits pronounced pleochroism in strongly colored grains. For example, brownish-pearl-gray grains exhibit a pleochroism ranging from ω = clove brown to ε = asparagus green, pale clove-brown grains exhibit a pleochroism ranging from ω = gray-violet blue to ε = gray-olive green, and yellowish-white grains exhibit a pleochroism ranging from ω = pale blue to ε = pale yellow. The mineral is characterized by high light refraction (strong relief with a dark border) and high birefringence (δ = 0.044 to 0.055) with vivid red, blue, and green interference colors of the second and third orders. Metamictic zircons can be anomalously biaxial, exhibiting axial angles of 2V = 10°, while their birefringence decreases to values of δ = 0.000. Other characteristics are the often present zonal structure and the pleochroic halos, which are best seen when the zircon occurs as an inclusion in colored minerals such as biotite and tourmaline. Inclusions of apatite, monazite, xenotime-(Y), rutile, hematite, ilmenite, magnetite, biotite, cassiterite, quartz, tourmaline, and glass have been observed in the zircon itself, which always cause a certain amount of turbidity (gray coloration). 

Zirconium is infusible in front of a blowpipe, even in a warm air stream. With oxygen it turns white without melting. Only with heated oxygen does a white enamel form on the surface; the latter also occurs when the zirconium begins to melt when heated in a stream of oxyhydrogen gas. Zirconium is not perceptibly attacked by phosphorus salt. If the powder is melted together with caustic potash - or with soda on a platinum wire - and then boiled with hydrochloric acid, the dilute acidic liquid turns turmeric paper orange (reaction to zirconium). If the hydrochloric acid solution is concentrated until crystallization and then boiled with saturated potassium sulfate solution, a white precipitate of zirconium(IV) oxide forms. It is insoluble in acids. Concentrated sulfuric acid (H2SO4) attacks zirconium only in the finest powder, and it is slightly soluble in hot, concentrated hydrofluoric acid (HF). Zircon can be extracted by melting with alkali carbonates and potassium disulfate and other bisulfates, but especially with potassium fluoride and hydrogen fluoride-potassium fluoride. 

By annealing – depending on the treatment in the oxidation or reduction flame – a darker color is sometimes created, and in some cases the crystals are decolorized. Some zircon crystals exhibit thermoluminescence when annealed; particularly in the case of lighter transparent crystals, even “very gentle heating” produces a bright to intense green light, with the phosphorescence lasting two to three minutes.  Zircon can also exhibit cathodoluminescence  as well as yellow, orange-yellow to green-orange fluorescence in short-wave UV light (254 nm). This is caused by radiation-induced crystal defects and the incorporation of (UO 2) 2+ (uranyl ion) as an impurity, or Dy 3+, Er 3+, Nd 3+, Yb 3+. Lattice defects induced by irradiation can heal upon heating, sometimes sunlight is sufficient, which is accompanied by a loss of the coloration caused by this defect. As a result, the color changes—leaving only the coloration due to stable defects such as foreign ions—or disappears completely.

Modifications and varieties

In the past, various zircons rich in rare earth elements (REE) were described under their own names. These include alvite, hagatalite, naegite, nogizawalite, oyamalite and yamaguchilite. These mostly strongly metamict minerals originate mainly from granites and granite pegmatites in Japan. Their REE 2 O 3 and P 2 O 5 contents can reach 26 wt.% and 9.8 wt.%, for example in nogizawalite. Decades ago it was shown that these zircon "varieties" are actually (zoned) intergrowths of zircon and xenotime (Y), occasionally even in perfect epitaxial orientation. They most likely formed through the action of hydrothermal solutions enriched in yttrium, phosphorus and rare earth metals on metamict zircons. In alvite, the intergrowths with xenotime (Y) crystals up to 0.1 mm in size are relatively coarse. In hagatalite and yamaguchilite, the xenotime domains are smaller and rarer, whereas in oyamalite and naegite, no discrete phase boundaries are discernible at all. 

Alvite was the name given to a zircon from Kragerø in Norway containing up to 16% HfO 2 as well as Th and REE. Later, this name was used for metamictic, Hf-rich zircons from granite pegmatites. 

Anderbergite is a pseudododecahedral crystal-forming, altered zircon from Ytterby, Sweden, named by Christian Wilhelm Blomstrand after the pharmacist and outstanding mineral expert CW Anderberg. This zircon variety was described by Adolf Erik Nordenskiöld. Anderbergite was found growing with fergusonite and xenotime on black mica plates and proved to be a cyrtolite-like hydrous zirconium silicate with calcium and REE.  

Auerbachite was named after the Russian scientist Dr. Auerbach in Moscow. Hans Rudolph Hermann described the crystals embedded in chert from the area around the village of Anatolia near "Hutor Masurenki" near Mariupol in Ukraine. 

A zircon with an extremely dipyramidal habit, found in the sanidinite of São Miguel in the Azores, was called azorite. 

Caldasite is the name given to a mixture of baddeleyite and zircon, originally known as zircon favas ("zircon beans"). These originate from the Poços de Caldas massif in southern Brazil, which is considered a uranium-bearing zirconium ore due to its average contents of > 60% ZrO 2 and 0.3% U 3 O 8. 

Calyptolite (also caliptolith or calyptolith) is the name chosen by Charles Upham Shepard for a tiny crystal-forming zircon from the chrysoberyl locality of Haddam in Connecticut, USA. 

Cyrtolite (also Kyrtolite) from the Greek κυρτός for "crooked" because of the curved pyramid surfaces is the name given by William J. Knowlton to a zircon from the granite of Rockport in Massachusetts, USA. 

Engelhardite is a colorless to yellowish-white, transparent, diamond-like crystal up to 12 mm in size from the gold fields near Tomsk, which exhibits the characteristic form {101}. 

Hyacinth (also Jacinth (us)) is still the name given to yellow and yellow-red to reddish-brown zircon varieties. 

Jargon is a straw-yellow to almost colorless variety of zircon. 

Malakon, from Greek μαλακός [ malakos ] for "soft", is the name given by Theodor Scheerer to an opaque and isotropized zircon first described from the island of Hidra (formerly Hitterø) in Norway. 

Naegite is a fully metamictized, Y-Th-U-rich zircon variety from the pegmatite district of Naegiti, Japan.  Similar is the Nb, Ta, Th, and REE-containing variety hagatalite, which, unlike naegite, is richer in REE and poorer in zirconium. 

Teikichi Kawai named a mixture of xenotime and zircon Nogizawalith. 

Oerstedtite is a metamictic zircon from Arendal, Aust-Agder, Norway, usually found on augite crystals. Johann Georg Forchhammer named the variety after Hans Christian Ørsted. 

Ostranite was named by August Breithaupt after the Germanic spring goddess Ostra and is an altered zircon probably from Arendal, Aust-Agder, Norway. 

Polycrasilite, derived from the Greek πολύς for "many" and κρᾶσις for "mixture", is the name chosen by Eduard Linnemann for zircons from North Carolina, USA, due to the large number of elements spectroscopically detected in them (Sn, Pb, Cu, Bi, Zr, Al, Fe, Co, Mn, Zn, Mg, Ur, Er, Ca, Ka, Na and Li). 

Ribeirite is an extremely yttrium-rich zircon from Macarani, Bahia, Brazil, named after the professor of mineralogy Joaquim Costa Ribeiro. 

Nils Johan Berlin named dark reddish-brown crystals in "granitic precipitations in the gneiss near Kragerö" tachyaphaltite. The name was chosen from the Greek words ταχύ for "quick" and ἄφαλτος for "jumping down", because the crystals easily jump out when the rock is broken. 

Yamaguchilite (also Yamazuchilite or Yamagulite) is a REE-bearing or REE- and P-rich zircon with 4–5 wt.% P 2 O 5 from Yamaguchi near Kiso, Japan. 

Applications

Zircon is mainly consumed as an opacifier, and has been known to be used in the decorative ceramics industry. It is also the principal precursor not only to metallic zirconium, although this application is small, but also to all compounds of zirconium including zirconium dioxide (ZrO2), an important refractory oxide with a melting point of 2,717 °C (4,923 °F).

Other applications include use in refractories and foundry casting and a growing array of specialty applications as zirconia and zirconium chemicals, including in nuclear fuel rods, catalytic fuel converters and in water and air purification systems. Zircon is one of the key minerals used by geologists for geochronology. Zircon is a part of the ZTR index to classify highly-weathered sediments.

Age determination in geology

Since the development of radiometric dating, zircons have become particularly important in geochronology because they contain traces of the radioactive nuclides 235 U, 238 U, and 232 Th (from 10 ppm to 5 wt%). All of these isotopes decay via decay series to various lead isotopes. By measuring the corresponding uranium-to-lead or thorium-to-lead ratios, the crystallization age of a zircon can be determined. Stable isotope ratios provide information about the environment in which the crystals formed. Zircons retain this information because they are extremely resistant to geological influences such as weathering and even high-grade rock metamorphism. 

Zircon has played an important role during the evolution of radiometric dating. Zircons contain trace amounts of uranium and thorium (from 10 ppm up to 1 wt%) and can be dated using several modern analytical techniques. Because zircons can survive geologic processes like erosion, transport, even high-grade metamorphism, they contain a rich and varied record of geological processes. Currently, zircons are typically dated by uranium-lead (U-Pb), fission-track, and U+Th/He techniques. Imaging the cathodoluminescence emission from fast electrons can be used as a prescreening tool for high-resolution secondary-ion-mass spectrometry (SIMS) to image the zonation pattern and identify regions of interest for isotope analysis. This is done using an integrated cathodoluminescence and scanning electron microscope. Zircons in sedimentary rock can identify the sediment source.

Zircons from Jack Hills in the Narryer Gneiss terrane, Yilgarn craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth. In addition, the oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.3 billion years ago there was already liquid water on the surface of the Earth. This interpretation is supported by additional trace element data, but is also the subject of debate. In 2015, "remains of biotic life" were found in 4.1-billion-year-old rocks in the Jack Hills of Western Australia. According to one of the researchers, "If life arose relatively quickly on Earth... then it could be common in the universe."

The Jack Hills lie south of the Murchison River on the border between the Shire of Murchison and the Shire of Meekatharra, about 800 km north of Perth. The oldest dated mineral in Europe is considered to be a 3.69 billion year old zircon crystal from gneisses that outcrop in the Øvre-Pasvik National Park in northern Norway, not far from the town of Kirkenes in the Pasvik Valley in the municipality of Sør-Varanger. Zircons in a lunar rock sample (Breccia 72215) were dated to 4.417 billion years ago, indicating a prolonged solidification process of the lunar crust after the formation of the Moon. 

Provenance analysis in sedimentary petrology

Zircon plays an important role in the analysis of the heavy mineral spectrum of sedimentary rocks. By determining the crystal pattern and crystal habit (including the length/width ratio and the degree of unwinding) of the zircons, as well as by determining their trace element content, the source areas of the sediments with their discrete rock types can be delimited or even assigned. Ideally, the transport processes, including reprocessing, mechanical abrasion, and sorting effects, leading to the sediment deposition area can also be quantified. 

Gemstone

Transparent zircon is a well-known form of semi-precious gemstone, favored for its high specific gravity (between 4.2 and 4.86) and adamantine luster. Because of its high refractive index (1.92) it has sometimes been used as a substitute for diamond, though it does not display quite the same play of color as a diamond. Zircon is one of the heaviest types of gemstone. Its Mohs hardness is between that of quartz and topaz, at 7.5 on the 10 point scale, though below that of the similar manmade stone cubic zirconia (8-8.5). Zircons may sometimes lose their inherent color after long exposure to bright sunlight, which is unusual in a gemstone. It is immune to acid attack except by sulfuric acid and then only when ground into a fine powder.

Most gem-grade zircons show a high degree of birefringence which, on stones cut with a table and pavilion cuts (i.e., nearly all cut stones), can be seen as the apparent doubling-up of the latter when viewed through the former, and this characteristic can be used to distinguish them from diamonds and cubic zirconias (CZ) as well as soda-lime glass, none of which show this characteristic. However, some zircons from Sri Lanka display only weak or no birefringence at all, and some other Sri Lanka stones may show clear birefringence in one place and little or none in another part of the same cut stone. Other gemstones also display birefringence, so while the presence of this characteristic may help distinguish a given zircon from a diamond or a CZ, it will not help distinguish it from, for example, a topaz gemstone. The high specific gravity of zircon, however, can usually separate it from any other gem and is simple to test.

Also, birefringence depends on the cut of the stone in relation to its optical axis. If a zircon is cut with this axis perpendicular to its table, birefringence may be reduced to undetectable levels unless viewed with a jeweler's loupe or other magnifying optics. The highest grade zircons are cut to minimize birefringence.

The value of a zircon gem depends largely on its color, clarity, and size. Prior to World War II, blue zircons (the most valuable color) were available from many gemstone suppliers in sizes between 15 and 25 carats; since then, stones even as large as 10 carats have become very scarce, especially in the most desirable color varieties.

Synthetic zircons have been created in laboratories. They are occasionally used in jewellery such as earrings. Zircons are sometimes imitated by spinel and synthetic sapphire, but are not difficult to distinguish from them with simple tools.

Zircon from Ratanakiri province in Cambodia is heat treated to produce blue zircon gemstones, sometimes referred to by the trade name cambolite.

Occurrence

Zircon is a common accessory to trace mineral constituent of all kinds of igneous rocks, but particularly granite and felsic igneous rocks. Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands. Zircon can occasionally be found as a trace mineral in ultrapotassic igneous rocks such as kimberlites, carbonatites, and lamprophyre, owing to the unusual magma genesis of these rocks.

Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, and within some rare alkaline volcanic rocks, for example the Toongi Trachyte, Dubbo, New South Wales Australia in association with the zirconium-hafnium minerals eudialyte and armstrongite.

Australia leads the world in zircon mining, producing 37% of the world total and accounting for 40% of world EDR (economic demonstrated resources) for the mineral. South Africa is Africa's main producer, with 30% of world production, second after Australia.

Applied Materials

Zircon is the most important ore for both zirconium and hafnium. Zirconium is used as an alloying metal (ferrocircon) and – in the form of the corrosion-resistant alloy Zirkaloy (with small amounts of iron, chromium and tin) – as a reactor material. Here it is used as a cladding material for fuel rods because of its low neutron capture cross section. Zirconium-niobium alloys exhibit superconducting properties, and most nickel- and cobalt- based superalloys contain between 0.03 and 2.2% zirconium. Glasses made from zirconium fluorides have extremely high infrared transmittance and are therefore used in fiber optic technology. Zirconium glass is used to encase radioactive waste (e.g. plutonium) for final storage; according to current research, the containers can withstand radiation for around 2000 years. However, scientists led by Ian Farnan from the British Cambridge Nuclear Energy Centre at the University of Cambridge have found in experiments that the expected durability of zirconium glass against the plutonium isotope 239 Pu is only about 210 years. 

Zirconia made from zirconium consists of artificially produced single crystals of zirconium(IV) oxide which have been stabilized in the cubic high-temperature phase and are often used as an inexpensive imitation diamond in jewelry. Zirconia is difficult to distinguish visually from diamonds - the different thermal conductivity of the two substances is used for this purpose. While diamonds are particularly good thermal conductors, zirconia conducts heat particularly poorly. Other relatively simple differences to diamonds, which can be determined using non-destructive measuring methods, are the different light refraction (refractive index of zirconia 2.18, diamond 2.42), dispersion (zirconia 0.066, diamond 0.044) and density (zirconia 5.8 g/cm³, diamond 3.5 g/cm³). Stabilized zirconium oxide is produced in various shapes and dimensions. Because the compound ZrO2 has an extremely high melting point, slip-cast bricks made of polycrystalline zirconium or crucible material made of zirconia are used to produce mechanically strong, acid-resistant, and highly refractory materials. Such highly refractory oxide ceramics exhibit low thermal conductivity and thermal expansion.

In the chemical industry, zirconium is used in the manufacture of spinnerets, pipes, stirrers, valves, and heat exchangers. Together with aluminum oxide or corundum, zirconium is used as molding sand in foundries, in the glass industry, and as an abrasives. Porous, ZrO2 - based ceramics are excellent thermal insulators—for example, high-temperature glasses and metals with high melting points can be melted in zirconia containers. Zirconia is also used to manufacture crucibles and abrasion-resistant materials such as dental implant superstructures and dental crown/bridge frameworks.

Zirconia is also used in the form of polycrystalline fibers for reinforcement in composite materials and generally for high-temperature insulation materials. The main applications of ZrO2 fibers are high -temperature furnaces and heat barriers in rockets, space shuttles, and launch pads. High-temperature laboratory furnaces insulated with such fibers can be heated up very quickly and then cooled down again very quickly. "Cemfil" glass fibers developed for the production of glass-fiber-reinforced cement contain a high proportion of zirconium and are therefore particularly alkali-resistant. Although these fibers do not achieve the same reinforcing effects as asbestos, they are good substitutes for asbestos fibers because of their harmlessness. 

Other zirconium compounds are used for glazes in the ceramic and glass industries. Specialty ceramic products made with zircon include zirconium porcelain, zirconium steatite, zirconium glazes, and zirconium enamels. The flame produced by the combustion of zirconium has a temperature of 4660 °C and emits a pure white, sun-like light. Therefore, zirconium is used in flashbulbs, fireworks, and tracer ammunition. Airbag gas generators and pyrotechnic seatbelt pretensioners also contain zirconium.  

Similar minerals

Hafnon (HfSiO4), xenotime (YPO4), béhierite, schiavinatoite ((Ta,Nb)BO4), thorite (ThSiO4), and coffinite (USiO4) all share the same crystal structure (IVX IVY O4, IIIX VY O4 in the case of xenotime) as zircon.

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