Zircon

From New World Encyclopedia
Zircon
Zircão.jpeg
Zircon crystal from Tocantins, Brazil
General
CategoryMineral
Chemical formulazirconium silicate ZrSiO4
Identification
Colorbrown, red, yellow, green, black, and colorless
Crystal habitdipyramidal prismatic
Crystal systemTetragonal; 4/m 2/m 2/m
Cleavageindistinct, two directions
FractureSubconchoidal to uneven - brittle
Mohs Scale hardness7.5
LusterAdamantine
Refractive indexnω=1.967 - 2.015 nε=1.920 - 1.960
Birefringenceδ=0.047 - 0.055
StreakWhite
Specific gravity4.6–4.7
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FusibilityInfusible
SolubilityInsoluble
Other CharacteristicsFluorescent and radioactive

Zircon is a mineral belonging to a subgroup of silicate minerals called nesosilicates. Chemically, it is known as zirconium silicate, with the formula ZrSiO4. Although its crystals are seldom large, zircon is remarkable in that it is nearly ubiquitous in the Earth's crust. Its natural color varies from colorless to golden yellow, red, brown, green, and black. Colorless specimens of gem quality are a popular substitute for diamond and are known as "Matura diamond." Zircon should not be confused with cubic zirconia, which is zirconium oxide (ZrO2), a different chemical substance that is rare in nature but produced synthetically and used as a diamond simulant.

Etymology

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.

Occurrence

Zircon is found worldwide as a common accessory mineral. Notable occurrences include the Ural Mountains; Trentino, Monte Somma, and Vesuvius, Italy; Arendal, Norway; Sri Lanka; India; Thailand; Ratanakiri, Cambodia; the Kimberley mines, Republic of South Africa; and Madagascar. In Canada, it is found in Renfrew County, Ontario, and Grenville, Quebec. In the United States, it occurs in 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.

Properties

Optical microscope photograph; the length of the crystal is about 250 micrometers.
A model of the structure of zircon.

Zircon is found in igneous rocks (as primary crystallization products), metamorphic rocks, and sedimentary rocks (as detrital grains). The average size of zircon crystals, as in granite rocks, is about 100–300 micrometers (µm), but they can also grow to sizes of several centimeters, especially in pegmatites.

In terms of crystal structure, zircon belongs to the tetragonal crystal class. Hafnium is almost always present, in quantities ranging from 1 to 4 percent. Also, based on their uranium and thorium content, some zircons undergo metamictization. This partially disrupts the crystal structure and explains the highly variable properties of zircon.

Thorite, or thorium silicate (ThSiO4), is a related mineral with a similar structure.

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.

Zircon is regarded as the traditional birthstone for December.

Zircons and the age of Earth

File:Zircon crystal.jpg
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 parts per million (ppm) up to 1 percent by weight) 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 Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, with an age of 4.404 billion years [1]. 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 spectacular interpretation 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.

See also

Further reading

  • D. J. Cherniak and E. B. Watson (2000). Pb diffusion in zircon. Chemical Geology 172: pp. 5-24.
  • K. Mezger and E. J. Krogstad (1997). Interpretation of discordant U-Pb zircon ages: An evaluation. Journal of Metamorphic Geology 15: pp. 127-140.

References
ISBN links support NWE through referral fees

  • Hanchar & Hoskin. 2003. Zircon. Reviews in Mineralogy and Geochemistry. vol. 53. Mineralogical Society of America. [2] Retrieved April 4, 2007.
  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412 and ISBN 978-0754815419.
  • Valley, John W., William H. Peck, Elizabeth M. King, and Simon A. Wilde. 2002. A Cool Early Earth. Geology 30:251-254. Retrieved April 5, 2007.

External links

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