common in the crust of Earth
Zircon is common in the crust of Earth. It occurs as a common accessory mineral in igneous rocks(as primary crystallization products), in metamorphic rocks and as detrital grains in sedimentary rocks. Large zircon crystals are rare. Their average size in granite rocks about 0.1–0.3 mm (0.0039–0.0118 in), but they can also grow to sizes of several cm, especially in mafic pegmatities and carbonatities. Zircon is fairly hard (with a Mohs hardness of 7.5) and chemically stable, and so is highly resistant to weathering. It also is resistant to heat, so that detrital zircon grains are sometimes preserved in igneous rocks formed from melted sediments. ts resistance to weathering, together with its relatively high specific gravity (4.68), make it an important component of the heavy mineral fraction of sandstones. Connected to internal radiation damage, these processes 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. Structurally, zircon consists of parallel chains of alternating silica tetrahedra (silicon ions in fourfold coordination with oxygen ions) and zirconium ions, with the large zirconium ions in eightfold coordination with oxygen ions.