Zircon

see also: Etymology of Zircon

The word zircon comes is Arab zarqun , “Cinabre”, or Perse zargun , gilded. One finds this etymology in English jargoon , who indicates zircons of clear color. Yellow zircons with garnet are called hyacinthes (of the Greek meaning “hyacinth”). The transparent specimens are used in jewelry for uses similar to those of the Diamant.

Zircon is a mineral of the group of the Silicate S, more precisely of the Nésosilicate S. Its chemical formula is Zr If O ₄, it acts of Silicate of natural Zirconium.

The term of zircon is sometimes used wrongly to indicate the Zircone, another oxide of zirconium, formula (ZrO₂). This last is in the Baddeleyite and is produced industrially to be used as not very expensive substitute with the Diamant.

Zircon crystallizes in crystalline Système tétragonal (crystalline Classe: 4/m2/m2/m) and presents a Dureté relative from 6,5 to 7,5 on the scale of hardness Mohs. Sometimes colorless, zircons have a natural color which varies gilded, with red and brown, but they can also be green, blue or black. The zircon dust is white. Zircon can be chatoyant, i.e. to show an effect “cats eye” on the stones cut out of cabochons. The specimens, which by their size and their purity are regarded as invaluable stones, are appreciated like substitute of the Diamant S, with which one often confuses them.

Zircon is the oldest known terrestrial rock (cf will infra) and one of most common of the Earth's crust. It seems one of the early products of the primary crystallization of the magmatic rocks like the Granite and the rocks Alcalin be the such Pegmatite or the Syénite. In the metamorphic rocks zircon is presented in recristallized or epitactic form. One finds in the Sédiments detrital zircons, i.e. grains transported and carted by the erosion. Zircons have an intermediate size ranging between 100 and 300 µm, e.g. in the granitic rocks . However they occasionally reach the size of several centimetres, especially in the Pegmatite S or the Alluvions.

The analysis of the form and the crystalline building of zircons informs about their conditions of formation and their future growth.

Chemistry

Zircon frequently contains impurities and various bodies or minerals in the form of inclusions. The form oxidizes theoretical zircon is made up of 67,1 % ZrO₂ and of 32,9 % of SiO₂. According to Rösler (1991) it can, in certain extreme cases, to contain until 30 % of oxide of Hafnium (HfO₂), 12 % of oxide of Thorium (ThO₂) or 1,5 % of oxide of Uranium (U₃O₈). The density falls correlatively with 4,3-4,8 g/cm ³. Zircons of the granites almost always contain U and Th in Zr replacement isomorphic and the determination of the reports/ratios Th/U or Pb/U is used to determine the age of the granites, this group is thus very important in geology. Thorite and uranothorite are easily hydrated without that destroying the structure, one a:

thorogummite: (Th, U) (If, H₄) O₄;
the coffinite: U (If, H₄) O₄.

Structure

Of general formula XSiO4 or X 4+ can be Zr 4+ of diameter 0,79 Angstroem or Th 4+ of diameter 1,02 or U 4+ of diameter 0,97. Quadratic, a mesh contains 4 molecules. The crystalline building of many zircons is locally destroyed by the action of rays of high energy (state known as “métamicte”): these crystals generally exhibent a dark color brown. With the state métamicte, water can be absorptive by the matrix, with for consequence a collapse characteristic of the density and hardness of the rock (see the table).

Dating using its isotopes

With the development of the Radiochronologie, zircons play a particularly important part in the Géochronologie.

The silicated shape of zircon is of major importance for the absolute Datation. The method of absolute dating requires that radioactive Isotope S in the course of disintegration be contained inside the rocks. Zircon, when it is subjected to extreme temperatures and pressures, does not change a Phase. It is practically inalterable except by the radioactivity which causes a state métamicte (disordered state in a crystal with possible change of color and deterioration until appearance of an amorphous state). That with the advantage of making so that zircon has the same structure as with its creation. Moreover, the substitution of its zirconium atoms by atoms of Uranium (U) is frequent, which is not the case with the majority of the other elements. It is a rare in the lava and the tuffs but relatively frequent mineral in the granites, the gneisses, pegmatites often in inclusions in the biotite contained in these rocks. One can find sometimes it very abundant, in syenites.

Zircons contain with the state of trace of the radioactive Isotope S of ²³⁵U, of ²³⁸U which have the advantage of having a very long half-life, about 4,5 billion years; they also contain ²³²Th. These isotopes represent 10 ppm with 5% in weight. They disintegrate according to precise periods in various isotopes of the Plomb. The proportion uranium-lead or thorium-lead makes it possible to estimate the age of a zircon crystal and consequently often the age of the rock which contains it.

Zircons are refractory with the most severe Altération S and with the geological aggressions such attrition, and this characteristic leaves its print on the rock matrix support.

The oldest terrestrial rocks found on the sphere are zircons of Narryer Gneiss Terrane ( Yilgarn Craton ), in Western Australia, with an age estimated at 4,404 billion years. This age is interpreted like that of the formation of zircon in question.

Use

Zircon is the main thing Minerai of Zirconium and Hafnium. The zirconium oxide (ZrO₂) has a melting point of approximately 1852 °C (2125 K) and is employed for the manufacture of materials at high point melting and abrasion resistant, e.g. the Plombage S and the dental bridge S. The Zirconium finds to him also its utility, inter alia in the nuclear reactors. One generally finds zircons in the metalliferous alluvia, where the gems free of any gangue are occasionally found. The alluvia richest in zircons are in India, with the the United States, in Australia, with Ceylon or in South Africa.

By their optical Index high (optical Index of 1,95, compared with diamond: 2,4, with the Zirconia: 2,2 and with the quartz: the 1,5) largest specimens are cut in Cabochon. By heat treatment, the brown or turbid zircon color can be modified, and pass according to the degree of heating to translucent, blue or gilded.

Zircon glass is used like sarcophagus of radioactive waste (e.g. of the Plutonium) for the storage of waste, sarcophagus which, according to current research, contains the radioactivity at least 2000 years.

References

Hanchar & Hoskin (2003): Zircon. Reviews in Mineralogy and Geochemistry , 53 , 500 p., http://www.minsocam.org/MSA/RIM/Rim53.html. - exhaustive and recent article on the zircon, published by Mineralogical Society off America.
D.J. Cherniak und E.B. Watson (2000): Pb diffusion in zircon. Chemical Geology 172 , p. 5-24.
A.N. Halliday (1999): In the beginning… . Natural 409 , pp. 144-145.
K. Mezger und E.J. Krogstad (1997): Interpretation off unmatched old U-Pb zircon: Year evaluation. Newspaper off metamorphic Geology 15 , pp. 127-140.
J.P. Pupin (1980): Zircon and Granite petrology. Contributions to Mineralogy and Petrology 73 , pp. 207-220.
H.J. Rösler (1991): Lehrbuch DER Mineralogy , Deutscher Verlag für Grundstoffindustrie, Leipzig, 5th ED., ISBN 3-342-00288-3
G. Vavra (1990): One the kinematics off zircon growth and its petrogenetic significance: cathodoluminescence study has. Contrib. Mineral. Petrol. 106 , pp. 90-99.
G. Vavra (1994): Systematics off internal zircon morphology in major Variscan granitoid standard. Contrib. Mineral. Petrol. 117 , pp. 331-344.

See too

External bonds

Mineralienatlas - Information and images (see also page “UV”)
http://www.geocities.com/CapeCanaveral/Launchpad/1008/Zirkon.html - the article of Gunnar Ries, seek on Internet for other references
http://www.a-m.de/deutsch/lexikon/mineral/inselsilicate/zirkon-bild1.htm - Photograph of a zircon

Sources

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