Zircon

Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It occurs 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 0.1 - 0.3 mm, but they can also grow to sizes of several centimeters (a few inches), especially in pegmatites.

Owing to their uranium and thorium content, some zircons may undergo metamictization. The processes, related to internal radiation damage, partially disrupt the crystal structure and partly explain the highly-variable properties of zircon. As zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, and the color changes.

Zircon occurs in many different colors, including red, pink, brown, yellow, hazel, black, or colorless. The color of zircons sometimes can be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, and golden-yellow zircons can be made. In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years provided the crystal has sufficient trace elements to produce color centers. Color in this red or pink series is annealed in geological conditions above the temperature about 350 C.

 

Knowledge center

Zircon (including hyacinth or yellow zircon) is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is (Zr1-y,REEy)(SiO4)1-x(OH)4x-y. Zircon forms in silicate melts with concentrated incompatible elements and accepts high field strength elements into its structure. For example, 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, blue, and green. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond".

Zircon has played an important role during the evolution of radiometric dating. Zircons contain trace amounts of uranium and thorium (from 10 ppm up to 1 wt%) and can be dated using several modern analytical techniques. Because zircons can survive geologic processes like erosion, transport, even high-grade metamorphism, they contain a rich and varied record of geological processes. Currently, zircons are typically dated by uranium-lead (U-Pb), fission-track, and U+Th/He techniques.

(Source : Wikipedia.org)