zircon

Zircon
Zircon crystal from Tocantins, Brazil (2cm x 2cm)
General
Category	Mineral
Chemical formula	zirconium silicate ZrSiO4
Crystal symmetry	I41/amd
Z	4
Identification
Color	Reddish brown, yellow, green, blue, gray, colorless; in thin section, colorless to pale brown
Crystal habit	dipyramidal prismatic
Crystal system	Tetragonal 4/m 2/m 2/m
Twinning	On {101}
Cleavage	indistinct, on {110} and {111}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	7.5
Luster	Vitreous to adamantine; greasy when metamict.
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	4.6–4.7
Optical properties	Uniaxial (+)
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	Infusible
Solubility	Insoluble
Other characteristics	Fluorescent and radioactive, may form pleochroic halos
References	[1][2][3][4]

Zircon (including hyacinth or yellow zircon) is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO₄. A common empirical formula showing some of the range of substitution in zircon is (Zr_1-y,REE_y)(SiO₄)_1-x(OH)_4x-y. Zircon forms in silicate melts with concentrated incompatible elements and accepts high field strength elements into its structure. 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. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond". It is not to be confused with cubic zirconia, a synthetic substance with a completely different chemical composition.

The name either derives from the Arabic word zarqun, meaning vermilion, or from the Persian zargun, meaning golden-colored. These words are corrupted into "jargoon", a term applied to light-colored zircons. Yellow zircon is called "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin; in the Middle Ages all yellow stones of East Indian origin were called hyacinth, but today this term is restricted to the yellow zircons.

Zircon is regarded as the traditional birthstone for December.

Properties

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

Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). Large zircon crystals are seldom abundant. Their average size, e.g. in granite rocks, is about 100–300 µm, but they can also grow to sizes of several centimeters (a few inches), especially in pegmatites.

Owing to their uranium and thorium content, some zircons may undergo metamictization. The processes, related to internal radiation damage, 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 is a common accessory mineral that occurs worldwide. Noted occurrences include: Australia; Russia (Ural Mountains); Trentino, Monte Somma, and Vesuvius, Italy; Arendal, Norway; Sri Lanka; India; Indonesia , Java, Kalimantan, Sulawesi; Thailand; Ratanakiri, Cambodia; the Kimberley mines, Republic of South Africa; Madagascar; Renfrew County, Ontario, and Grenville, Quebec, Canada; and Litchfield, Maine; Chesterfield, Massachusetts; Essex, Orange, and St. Lawrence counties, New York; Henderson County, North Carolina; the Pikes Peak district of Colorado; and Llano County, Texas in the United States. 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. Thorite (ThSiO₄) is an isostructural related mineral.

Zircon occurs in many different colors, including red, pink, brown, yellow, hazel, black, or colorless. The color of zircons sometimes can be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, and golden-yellow zircons can be made. In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years provided the crystal has sufficient trace elements to produce color centers. Color in this red or pink series is annealed in geological conditions above about 350°C.

Uses

Sand-sized grains of zircon

• Zircons are commercially mined for the metal zirconium, and are used for abrasive and insulating purposes.
• It is the source of zirconium oxide(ZrO₂), one of the most refractory materials known.
• Crucibles of ZrO₂ are used to fuse platinum at temperatures in excess of 1755 ^oC.
• Zirconium metal is used in nuclear reactors due to its neutron absorption properties.
• Large specimens are appreciated as gemstones, owing to their high refractive index. (Zircon has a refractive index of approximately 1.95; diamond's is approximately 2.4.)
• 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.

Occurrence

World production trend of zirconium mineral concentrates

Zircon is a common accessory to 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. Zircon is rare within mafic rocks and very rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites, carbonatites, and lamprophyre, where zircon can occasionally be found as a trace mineral 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^[5] in association with the zirconium-hafnium minerals eudialyte and armstrongite.

Zircons and radiometric dating

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.

Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years,^[6] 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.^[6][7] This interpretation is supported by additional trace element data^[8][9], but is also the subject of debate^[10][11].

Gallery

Crystal structure of zircon	A unit cell	Zircon collection at the National Museum of Natural History	Polished surface of the oldest zircon so far found on Earth[6]
SEM image of zircon

Similar Minerals

Hafnon (HfSiO₄), Xenotime (YPO₄), Béhierite, Schiavinatoite ( (Ta,Nb)BO₄ ), Thorite, (ThSiO₄), and Coffinite (USiO₄) all share the same crystal structure (^VIIIX ^IVY O₄) as Zircon.

See also

• Baddeleyite, ZrO₂
• Cool Early Earth
• Heavy mineral sands ore deposits
• History of Earth
• Ilmenite
• List of minerals
• List of English words of Persian origin
• Oldest rock
• Radiometric dating

References

Wikimedia Commons has media related to: Zircon

• Geochemistry of old zircons
• Mineral galleries

Further reading

• John M. Hanchar & Paul W. O. Hoskin (eds.) (2003). "Zircon". Reviews in Mineralogy and Geochemistry, 53. ISBN 093995065-0 (Mineralogical Society of America monograph).

• D. J. Cherniak and E. B. Watson (2000). "Pb diffusion in zircon". Chemical Geology 172: 5–24. doi:10.1016/S0009-2541(00)00233-3. 
• A. N. Halliday (2001). "In the beginning…". Nature 409: 144–145. doi:10.1038/35051685. 
• Hermann Köhler (1970). "Die Änderung der Zirkonmorphologie mit dem Differentiationsgrad eines Granits". Neues Jahrbuch Mineralogische Monatshefte 9: 405–420. 
• K. Mezger and E. J. Krogstad (1997). "Interpretation of discordant U-Pb zircon ages: An evaluation". Journal of Metamorphic Geology 15: 127–140. doi:10.1111/j.1525-1314.1997.00008.x. 
• J. P. Pupin (1980). "Zircon and Granite petrology". Contributions to Mineralogy and Petrology 73: 207–220. doi:10.1007/BF00381441. 
• Gunnar Ries (2001). "Zirkon als akzessorisches Mineral". Aufschluss 52: 381–383. doi:10.1007/BF00381441. 
• P. Tondar (1991). Zirkonmorphologie als Charakteristikum eines Gesteins. Dissertation an der Ludwig-Maximilians-Universität München, 87 pp.
• G. Vavra (1990). "On the kinematics of zircon growth and its petrogenetic significance: a cathodoluminescence study". Contributions to Mineralogy and Petrology 106: 90–99. doi:10.1007/BF00306410. 
• John W. Valley, William H. Peck, Elizabeth M. King, Simon A. Wilde (2002). "A Cool Early Earth". Geology 30: 351–354. doi:10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2. http://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html. Retrieved 2005-04-11. 
• G. Vavra (1994). "Systematics of internal zircon morphology in major Variscan granitoid types". Contributions to Mineralogy and Petrology 117: 331–344. doi:10.1007/BF00307269.