Definitions

zircon light

Zircon

[zur-kon]

Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. 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, 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 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.

Properties

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 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: Australia; the Ural Mountains; Trentino, Monte Somma, and Vesuvius, Italy; Arendal, Norway; Sri Lanka; India; 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 can come in red, brown, yellow, hazel, black, or colorless. The color of zircons below gem quality can be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, and golden-yellow zircons can be made.

Uses

  • Zircons are commercially mined for the metal zirconium, and are 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 approximately 1.95; diamond's is approximately 2.4.)

Occurrence

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 in association with the zirconium-hafnium minerals eudialyte and armstrongite.

Zircons and radiometric dating

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 can 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, 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 interpretation has been published in respected scientific journals but is the subject of debate.

See also

References

Further reading

  • 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: John M. Hanchar & Paul W. O. Hoskin (eds.) (2003). "Zircon" Reviews in Mineralogy and Geochemistry, 53. ISBN 093995065-0.
  • D. J. Cherniak and E. B. Watson (2000). "Pb diffusion in zircon". Chemical Geology 172 5–24.
  • A. N. Halliday (2001). "In the beginning…". Nature 409 144–145.
  • 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.
  • J. P. Pupin (1980). "Zircon and Granite petrology". Contributions to Mineralogy and Petrology 73 207–220.
  • Gunnar Ries (2001). "Zirkon als akzessorisches Mineral". Aufschluss 52 381–383.
  • 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 90–99.
  • John W. Valley, William H. Peck, Elizabeth M. King, Simon A. Wilde (2002). "A Cool Early Earth". Geology 30 351–354. Retrieved on 2005-04-11.
  • G. Vavra (1994). "Systematics of internal zircon morphology in major Variscan granitoid types". Contributions to Mineralogy and Petrology 117 331–344.

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