Zircão
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Para o satélite espião deste codename veja Zircon (satelite).
Zircão ou Zircon (do Persa: sarkun, dourado) é um mineral pertencente ao grupo dos silicatos. Sua formula química é ZrSiO4. A estrutura cristalina do zircon é tetragonal (cristal classe: 4/m 2/m 2/m). A coloração natural do zircão varia entre incolor, amarelo dourado, vermelho, marrom ou verde . Espécies que exibem gemas de qualidade são um substituto popular do diamante ( porém observe que a zirconia cúbica é um mineral sintético completamente diferente, com uma composição química diferente ).
O Zircão é um mineral notável, apenas por sua quase ubiqüitousa presença na crosta terrestre. Está presente em muitas rochas ígneas ou magmáticas(como produto primário da cristalização), nas rochas metamórficas(como grãos recristalizados) e nas rochas sedimentares (como grãos detríticos). É raro encontrar cristais de zircão de grandes dimensões. O seu tamanho médio no granito por exemplo, é de cerca de 100-300 µm, contundo é possível encontrar em rochas minerais com vários centímetros, especialmente na pegmatite.
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 5 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 are not only the oldest minerals on earth, they also show another interesting feature. Their oxygen isotopic composition is interpreted by some 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 widely disputed. It is most likely that the oxygen isotopes, and other compositional features (the rare-earth elements), simply record hydrothermal alteration. The timing of the alteration is uncertain, but this negates the necessity for ancient liquid-water oceans.
Owing to their uranium and thorium content, some zircons may undergo metamictization. This explains the highly variable properties of zircon.
Commercially, zircons are mined for the metal zirconium which is used for abrasive and isolating purposes. Bigger specimen are appreciated gemstones, owing to their high refraction (zicon has a refraction of around 1.95, diamond of around 2.4). The color of zircons that don't have gem quality can be changed by treating with heat. Depending on the amount of heat applied, colorless, blue and golden-yellow zircons can be made.
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.
See also: list of minerals
Physical properties:
- Hardness (Mohs): 7.5
- Density (g/cm3): 3.9-4.7
- Color: mostly brown, colorless, yellow, red
- Refraction: 1.93-1.98
[editar] 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:
- 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 Mineralogie and Petrololgy 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. Contrib. Mineral. Petrol. 106, pp. 90-99.
- G. Vavra (1994): Systematics of internal zircon morphology in major Variscan granitoid types. Contrib. Mineral. Petrol. 117, pp. 331-344.
Also available about geochemistry of old zircons http://www.geology.wisc.edu/zircon/zircon_home.html
