Zircon

Optical microscope photography. The length of the crystal is about 250µm.

Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It is found in magmatic 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.

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 crystallisation. These zircons are not only the oldest minerals on earth, they also show another interesting feature. Their oxygen isotopic composition suggests that more than 4.4 billion years ago there was already water on the surface of the earth.

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 vermillion, 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

Further Reading

• 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.
• S. A. Wilde et al. (2001): ????. Nature 409, pp. 175-178.

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