Zircon

Zircon
Zircon crystal from Tocantins, Brazil (2×2 cm)
General
Category	Nesosilicates
Formula (repeating unit)	zirconium silicate ZrSiO4
Strunz classification	09.AD.30
Crystal symmetry	Tetragonal ditetragonal dipyramidal H-M symbol: (4/m 2/m 2/m) Space group: I 41/amd
Unit cell	a = 6.607(1) Å, c = 5.982(1) Å; Z=4
Identification
Color	Reddish brown, yellow, green, blue, gray, colorless; in thin section, colorless to pale brown
Crystal habit	tabular to prismatic crystals, irregular grains, massive
Crystal system	Tetragonal
Twinning	On {101}
Cleavage	indistinct, on {110} and {111}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	7.5
Luster	Vitreous to adamantine; greasy when metamict.
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	4.6–4.7
Optical properties	Uniaxial (+)
Refractive index	nω = 1.925–1.961 nε = 1.980–2.015, 1.75 when metamict
Birefringence	δ = 0.047–0.055
Pleochroism	Weak
Fusibility	close to 2,550 °C depend on Hf,Th,U,H,etc... concentrations.
Solubility	Insoluble
Other characteristics	Fluorescent and radioactive, may form pleochroic halos
References	[1][2][3][4]

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 (Zr_1–y, REE_y)(SiO₄)_1–x(OH)_4x–y. Zircon forms in silicate melts with large proportions of high field strength incompatible elements. 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 and are also known as "Matura diamond".

The name derives from the Persian zargun (زرگون), meaning golden-colored.^[5] This word is corrupted into "jargoon", a term applied to light-colored zircons. The English word "zircon" is derived from "Zirkon," which is the German adaptation of this word.^[6] Red zircon is called "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.

Properties

Optical microscope photograph; the length of the crystal is about 250 µm.

Zircon is ubiquitous 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 rare. Their average size in granite rocks is about 0.1–0.3 mm, but they can also grow to sizes of several centimeters, especially in pegmatites.

Because of their uranium and thorium content, some zircons undergo metamictization. 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.

Zircon occurs in many colors, including red, pink, brown, yellow, hazel, or black. It can also be colorless. The color of zircons can sometimes be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, or golden-yellow zircons can be made. In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years, if 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.

Applications

Sand-sized grains of zircon

Zircon is mainly consumed as an opacifier, and has been known to be used in the decorative ceramics industry.^[7] It is also the principal precursor not only to metallic zirconium, although this application is small, but also to all compounds of zirconium including zirconium dioxide (ZrO₂), one of the most refractory materials known.

Occurrence

World production trend of zirconium mineral concentrates

Zircon is a common accessory to trace mineral constituent of most granite and felsic igneous rocks. Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands. Zircon is rare within mafic rocks and very rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites, carbonatites, and lamprophyre, where zircon can occasionally be found as a trace mineral owing to the unusual magma genesis of these rocks.

Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, and within some rare alkaline volcanic rocks, for example the Toongi Trachyte, Dubbo, New South Wales Australia^[8] in association with the zirconium-hafnium minerals eudialyte and armstrongite.

Australia leads the world in zircon mining, producing 37% of the world total and accounting for 40% of world EDR (economic demonstrated resources) for the mineral.

Radiometric dating

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.

Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years,^[9] interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth. In addition, the oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth.^[9][10] This interpretation is supported by additional trace element data,^[11][12] but is also the subject of debate.^[13][14]

Gallery

• Crystal structure of zircon

• A unit cell

• Zircon collection at the National Museum of Natural History

• SEM image of zircon

• A small sample of zircon.

Similar minerals

Hafnon (HfSiO₄), xenotime (YPO₄), béhierite, schiavinatoite ((Ta,Nb)BO₄), thorite (ThSiO₄), and coffinite (USiO₄) all share the same crystal structure (^VIIIX ^IVY O₄) as zircon.

See also

References