Zircon

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Zircon
Zircon-dtn1a.jpg
A lustrous crystal of zircon perched on a tan matrix of calcite from the Gilgit District of Pakistan
General
Category Nesosilicates
Formula
(repeating unit)		 zirconium silicate  (ZrSiO4)
IMA symbol Zrn [1]
Strunz classification 9.AD.30
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space group I41/amd (No. 141)
Unit cell a = 6.607(1), c = 5.982(1) [Å]; Z = 4
Identification
ColorReddish brown, yellow, green, blue, gray, colorless; in thin section, colorless to pale brown
Crystal habit tabular to prismatic crystals, irregular grains, massive
Twinning On {101}. Crystals shocked by meteorite impact show polysynthetic twins on {112}
Cleavage {110} and {111}
Fracture Conchoidal to uneven
Tenacity Brittle
Mohs scale hardness7.5
Luster Vitreous to adamantine; greasy when metamict.
Streak White
Diaphaneity Transparent to opaque
Specific gravity 4.6–4.7
Optical propertiesUniaxial (+)
Refractive index nω = 1.925–1.961
nε = 1.980–2.015, 1.75 when metamict
Birefringence δ = 0.047–0.055
Pleochroism Weak
Fusibility close to 2,550 °C depend on Hf,Th,U,H,etc... concentrations.
Solubility Insoluble
Other characteristics Fluorescent and Radioactive.svg Radioactive,
May form pleochroic halos,
Relief: high
References [2] [3] [4] [5] [6]

Zircon ( /ˈzɜːrkɒn,-kən/ ) [7] [8] [9] 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 Zr SiO4. An empirical formula showing some of the range of substitution in zircon is (Zr1–y, REE y)(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.

Contents

The name derives from the Persian zargun, meaning "gold-hued". [10] 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. [11] Yellow, orange, and red zircon is also known as "hyacinth", [12] from the flower hyacinthus , whose name is of Ancient Greek origin.

Properties

Optical microscope photograph; the length of the crystal is about 250 um Zircon microscope.jpg
Optical microscope photograph; the length of the crystal is about 250 µm

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. [2] 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. [2] 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. [13] 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. [5]

Because of their uranium [14] 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. [15]

Zircon occurs in many colors, including reddish brown, yellow, green, blue, gray, and colorless. [2] 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). [16] 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). [17]

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. [18]

Applications

Sand-sized grains of zircon ZirconUSGOV.jpg
Sand-sized grains of zircon

Zircon is mainly consumed as an opacifier, and has been known to be used in the decorative ceramics industry. [19] 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). [20]

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. [21]

Zircon is one of the key minerals used by geologists for geochronology. [22]

Zircon is a part of the ZTR index to classify highly-weathered sediments. [23]

As gemstone

A pale blue zircon gemstone weighing 3.36 carats ZirkonBlau.jpg
A pale blue zircon gemstone weighing 3.36 carats

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 the heaviest of any gem, readily sinking in even highly viscous liquids. 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 (9). 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. [24]

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. [25] 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. [26]

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. [26]

Synthetic zircons have been created in laboratories [27] but they are only of scientific interest and are never encountered in the jewellery trade. Zircons are sometimes imitated by spinel and synthetic sapphire, but are not difficult to distinguish from them with simple tools.

Occurrence

World production trend of zirconium mineral concentrates Zirconium mineral concentrates - world production trend.svg
World production trend of zirconium mineral concentrates

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. [28] [29] 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.[ citation needed ]

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 [30] 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. [31] South Africa is Africa's main producer, with 30% of world production, second after Australia. [32]

Radiometric dating

SEM-CL image of Zircon grain showing zonations and poly-cycles (core-rim structure) Zircon grain (CL-SEM imaging).tiff
SEM-CL image of Zircon grain showing zonations and poly-cycles (core-rim structure)

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%) [14] 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, cathodoluminescence, and U+Th/He techniques. For instance, 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. [33] Zircons in sedimentary rock can identify the sediment source. [34]

Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, [35] 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 4.4 billion years ago there was already water on the surface of the Earth. [35] [36] This interpretation is supported by additional trace element data, [37] [38] but is also the subject of debate. [39] [40] In 2015, "remains of biotic life" were found in 4.1-billion-year-old rocks in the Jack Hills of Western Australia. [41] [42] According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe." [41]

Similar minerals

Hafnon (HfSiO4), xenotime (YPO4), béhierite, schiavinatoite ((Ta,Nb)BO4), thorite (ThSiO4), and coffinite (USiO4) [14] all share the same crystal structure (IVX IVY O4, IIIX VY O4 in the case of xenotime) as zircon.

See also

Related Research Articles

<span class="mw-page-title-main">Kyanite</span> Aluminosilicate mineral

Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.

<span class="mw-page-title-main">Mafic</span> Silicate mineral or igneous rock that is rich in magnesium and iron

A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian.

<span class="mw-page-title-main">Spinel</span> Mineral or gemstone

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl
2O
4 in the cubic crystal system. Its name comes from the Latin word spinella, which means spine in reference to its pointed crystals.

<span class="mw-page-title-main">Titanite</span> Nesosilicate mineral

Titanite, or sphene (from the Greek sphenos (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals including cerium and yttrium; calcium may be partly replaced by thorium.

<span class="mw-page-title-main">Zirconium</span> Chemical element, symbol Zr and atomic number 40

Zirconium is a chemical element with the symbol Zr and atomic number 40. The name zirconium is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian zargun. It is a lustrous, grey-white, strong transition metal that closely resembles hafnium and, to a lesser extent, titanium. Zirconium is mainly used as a refractory and opacifier, although small amounts are used as an alloying agent for its strong resistance to corrosion. Zirconium forms a variety of inorganic and organometallic compounds such as zirconium dioxide and zirconocene dichloride, respectively. Five isotopes occur naturally, four of which are stable. Zirconium compounds have no known biological role.

<span class="mw-page-title-main">Garnet</span> Mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

<span class="mw-page-title-main">Rutile</span> Oxide mineral composed of titanium dioxide

Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.

<span class="mw-page-title-main">Plagioclase</span> Type of feldspar

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

<span class="mw-page-title-main">Baddeleyite</span>

Baddeleyite is a rare zirconium oxide mineral (ZrO2 or zirconia), occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high indices of refraction, and ranges from colorless to yellow, green, and dark brown. See etymology below.

<span class="mw-page-title-main">Zirconium dioxide</span> Chemical compound

Zirconium dioxide is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.

<span class="mw-page-title-main">Peridotite</span> Coarse-grained ultramafic igneous rock type

Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

<span class="mw-page-title-main">Thorite</span>

Thorite, (Th,U)SiO4, is a rare nesosilicate of thorium that crystallizes in the tetragonal system and is isomorphous with zircon and hafnon. It is the most common mineral of thorium and is nearly always strongly radioactive. It was named in 1829 to reflect its thorium content. Thorite was discovered in 1828 on the island of Løvøya, Norway, by the vicar and mineralogist, Hans Morten Thrane Esmark, who sent the first specimens of this black mineral to his father, Jens Esmark, who was a professor of mineralogy and geology.

<span class="mw-page-title-main">Xenotime</span> Phosphate mineral

Xenotime is a rare-earth phosphate mineral, the major component of which is yttrium orthophosphate (YPO4). It forms a solid solution series with chernovite-(Y) (YAsO4) and therefore may contain trace impurities of arsenic, as well as silicon dioxide and calcium. The rare-earth elements dysprosium, erbium, terbium and ytterbium, as well as metal elements such as thorium and uranium (all replacing yttrium) are the expressive secondary components of xenotime. Due to uranium and thorium impurities, some xenotime specimens may be weakly to strongly radioactive. Lithiophyllite, monazite and purpurite are sometimes grouped with xenotime in the informal "anhydrous phosphates" group. Xenotime is used chiefly as a source of yttrium and heavy lanthanide metals (dysprosium, ytterbium, erbium and gadolinium). Occasionally, gemstones are also cut from the finest xenotime crystals.

<span class="mw-page-title-main">Eudialyte</span> Cyclosilicate mineral

Eudialyte, whose name derives from the Greek phrase Εὖ διάλυτος, eu dialytos, meaning "well decomposable", is a somewhat rare, nine member ring cyclosilicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites. Its name alludes to its ready solubility in acid.

<span class="mw-page-title-main">Painite</span> Borate mineral

Painite is a very rare borate mineral. It was first found in Myanmar by British mineralogist and gem dealer Arthur C.D. Pain who misidentified it as ruby, until it was discovered as a new gemstone in the 1950s. When it was confirmed as a new mineral species, the mineral was named after him. Due to its rarity, painite can cost in the range of between US$50,000 to $60,000 per carat.

Provenance in geology, is the reconstruction of the origin of sediments. The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks. Rocks exposed to the surface are sooner or later broken down into sediments. Sediments are expected to be able to provide evidence of the erosional history of their parent source rocks. The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.

<span class="mw-page-title-main">Titanium in zircon geothermometry</span>

Titanium in zircon geothermometry is a form of a geothermometry technique by which the crystallization temperature of a zircon crystal can be estimated by the amount of titanium atoms which can only be found in the crystal lattice. In zircon crystals, titanium is commonly incorporated, replacing similarly charged zirconium and silicon atoms. This process is relatively unaffected by pressure and highly temperature dependent, with the amount of titanium incorporated rising exponentially with temperature, making this an accurate geothermometry method. This measurement of titanium in zircons can be used to estimate the cooling temperatures of the crystal and infer conditions during which it crystallized. Compositional changes in the crystals growth rings can be used to estimate the thermodynamic history of the entire crystal. This method is useful as it can be combined with radiometric dating techniques that are commonly used with zircon crystals, to correlate quantitative temperature measurements with specific absolute ages. This technique can be used to estimate early Earth conditions, determine metamorphic facies, or to determine the source of detrital zircons, among other uses.

<span class="mw-page-title-main">Detrital zircon geochronology</span> Science of analyzing the age of zircons

Detrital zircon geochronology is the science of analyzing the age of zircons deposited within a specific sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead ratio. Zircon is a common accessory or trace mineral constituent of most 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. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques.

<span class="mw-page-title-main">Hadean zircon</span> Oldest-surviving crustal material from the Earths earliest geological time period

Hadean zircon is the oldest-surviving crustal material from the Earth's earliest geological time period, the Hadean eon, about 4 billion years ago. Zircon is a mineral that is commonly used for radiometric dating because it is highly resistant to chemical changes and appears in the form of small crystals or grains in most igneous and metamorphic host rocks.

Zigrasite is a phosphate mineral with the chemical formula of MgZr(PO4)2(H2O)4. Zigrasite was discovered and is only known to occur in the Dunton Quarry at Oxford County, Maine. Zigrasite was specifically found in the giant 1972 gem tourmaline-bearing pocket at the Dunton Quarry. Zigrasite is named after James Zigras who originally discovered and brought the mineral to attention.

References

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  42. ↑ Bell, Elizabeth A.; Boehnke, Patrick; Harrison, T. Mark; Mao, Wendy L. (2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". Proceedings of the National Academy of Sciences. 112 (47): 14518–14521. Bibcode:2015PNAS..11214518B. doi: 10.1073/pnas.1517557112 . PMC   4664351 . PMID   26483481.

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