Zircon

Zircon (from Persian: sarkun, golden) is a mineral belonging to the group of silicates. Its chemical formula is ZrSi[[Oxygen|O] 4 ]. The crystal structure of zircon is tetragonal (crystal class: 4/m 2/m 2/m). The natural color of zircon varies between colorless, yellow-golden, red, brown or green. Specimen that show gem quality are a popular substitute for diamond . But zircons are not to be confused with cubic zirconia which is a synthetic substitute for diamond.

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 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 (zircon 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.

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