Zircon

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For the spy satellite of this codename see Zircon (satellite).

Zircon
Zircon crystal from Tocantins, Brazil
General
Category	Mineral
Chemical formula	zirconium silicate ZrSiO₄
Identification
Color	brown, red, yellow, green, black, and colorless
Crystal habit	dipyramidal prismatic
Crystal system	Tetragonal; 4/m 2/m 2/m
Cleavage	indistinct, two directions
Fracture	Subconchoidal to uneven - brittle
Mohs Scale hardness	7.5
Luster	Adamantine
Refractive index	nω=1.967 - 2.015 nε=1.920 - 1.960
Birefringence	δ=0.047 - 0.055
Streak	White
Specific gravity	4.6–4.7
Fusibility	Infusible
Solubility	Insoluble
Other Characteristics	Fluorescent and radioactive

Zircon is a mineral belonging to the group of nesosilicates. Its chemical formula is Zr Si O₄. Hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal class. The natural color of zircon varies between colorless, yellow-golden, red, brown, and green. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond" (but note that cubic zirconia is a completely different synthetic mineral with a different chemical composition).

The name derives from the Arabic word zarqun, meaning vermilion, or perhaps 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 a word of East Indian 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.

1 Properties
2 Uses
3 Zircons and the age of Earth
4 See also
5 Further reading
6 References
7 External links

[edit] 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 is found in igneous rocks (as primary crystallization products), in metamorphic rocks (as recrystallized grains) 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, especially in pegmatites.

Owing to their uranium and thorium content, some zircons may undergo metamictization. This partially disrupts the crystal structure and explains the highly variable properties of zircon.

Zircon is a common accessory mineral and found worldwide. Noted occurrences include: in the Ural Mountains; Trentino, Monte Somma; and Vesuvius, Italy; Arendal, Norway; Sri Lanka, India; Thailand; Ratanakiri, Cambodia; at the Kimberley mines, Republic of South Africa; Madagascar; and in Canada in Renfrew County, Ontario, and Grenville, Quebec. In the United States: Litchfield, Maine; Chesterfield, Massachusetts; in Essex, Orange, and St. Lawrence Counties, New York; Henderson County, North Carolina; the Pikes Peak district of Colorado; and Llano County, Texas.

Thorite (ThSiO₄) is an isostructural related mineral.

[edit] Uses

Zircon dust

Commercially, zircons are mined for the metal zirconium which is used for abrasive and insulating purposes. It is the source of zirconium oxide , one of the most refractory materials known. Crucibles of ZrO 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 around 1.95, diamond around 2.4). The color of zircons that do not have gem quality can be changed by heat treatment. Depending on the amount of heat applied, colorless, blue and golden-yellow zircons can be made.

[edit] Zircons and the age of Earth

A piece of zircon estimated to be the oldest object on Earth

The pervasive occurrence of zircon has become more important since the discovery of radiometric dating. Zircons contain amounts of uranium and thorium (from 10 ppm up to 1 wt%) and can be dated using modern analytical techniques. Since zircons have the capability to survive geologic processes like erosion, transport, even high-grade metamorphism, they are used as protolith indicators. The oldest minerals found so far are zircons from the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, with an age of 4.404 billion years. This age is interpreted to be the age of crystallization. These zircons might not only be the oldest minerals on earth, they also show another interesting feature. Their oxygen isotopic composition has been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. This is a spectacular interpretation that has been published in top scientific journals but is currently the subject of debate. It may be that the oxygen isotopes, and other compositional features (the rare earth elements), record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization.

[edit] See also

list of minerals

[edit] Further reading

The structure of zircon has a space group I41/amd

The most comprehensive and up-to-date work on zircon and its related disciplines is the Mineralogical Society of America monograph published in late 2003: Hanchar & Hoskin (2003). Zircon. Reviews in Mineralogy and Geochemistry, volume 53, 500 pages. http://www.minsocam.org/MSA/RIM/Rim53.html
D. J. Cherniak and E. B. Watson (2000). "Pb diffusion in zircon". Chemical Geology 172: pp. 5-24.
A. N. Halliday (1999). "In the beginning…". Nature 409: pp. 144-145.
Hermann Köhler (1970). "Die Änderung der Zirkonmorphologie mit dem Differentiationsgrad eines Granits". Neues Jahrbuch Mineralogische Monatshefte 9: pp. 405 - 420.
K. Mezger and E. J. Krogstad (1997). "Interpretation of discordant U-Pb zircon ages: An evaluation". Journal of Metamorphic Geology 15: pp. 127-140.
J. P. Pupin (1980). "Zircon and Granite petrology". Contributions to Mineralogy and Petrology 73: pp. 207-220.
Gunnar Ries (2001). "Zirkon als akzessorisches Mineral". Aufschluss 52: pp. 381-383.
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: pp. 90-99.
G. Vavra (1994). "Systematics of internal zircon morphology in major Variscan granitoid types". Contributions to Mineralogy and Petrology 117: pp. 331-344.
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 10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2. A Cool Early Earth. Zircons Are Forever. Retrieved on April 11, 2005.