Zircon

Zircon

This article is about the mineral. For other uses, see Zircon (disambiguation).

Zircon
Zircon crystal from Tocantins, Brazil (unknown scale)
General
Category	Mineral
Chemical formula	zirconium silicate ZrSiO4
Identification
Color	brown, red, yellow, green, black, and colorless
Crystal habit	dipyramidal prismatic
Crystal system	Tetragonal; 4/m 2/m 2/m
Cleavage	indistinct, two directions
Fracture	Subconchoidal to uneven—brittle
Mohs Scalehardness	7.5
Luster	Adamantine
Streak	White
Specific gravity	4.6–4.7
Refractive index	nω=1.967–2.015 nε=1.920–1.960
Birefringence	δ=0..047–0.055
Fusibility	Infusible
Solubility	Insoluble
Other characteristics	Fluorescent and radioactive

Crystal structure of zircon

Zircon (including hyacinth or yellow zircon) is a mineral belonging to the group ofnesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO₄. 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, and green. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond". It is not to be confused with cubic zirconia, a synthetic substance with a completely different chemical composition.

The name either derives from the Arabic word zarqun, meaning vermilion, or from the Persianzargun, meaning golden-colored. These words are corrupted into "jargoon", a term applied to light-colored zircons. Yellow zircon is called "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek 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.

Zircon is regarded as the traditional birthstone for December.

Properties

Optical microscope photograph; 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 occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and insedimentary 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 (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 is a common accessory mineral that occurs worldwide. Noted occurrences include:Australia; Russia (Ural Mountains); Trentino, Monte Somma, and Vesuvius, Italy; Arendal,Norway; Sri Lanka; India; Indonesia , Java, Kalimantan, Sulawesi; Thailand; Ratanakiri,Cambodia; the Kimberley mines, Republic of South Africa; Madagascar; Renfrew County,Ontario, and Grenville, Quebec, Canada; and Litchfield, Maine; Chesterfield, Massachusetts; Essex, Orange, and St. Lawrence counties, New York; Henderson County, North Carolina; thePikes Peak district of Colorado; and Llano County, Texas in the United States. 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. Thorite (ThSiO₄) is an isostructural related mineral.

Zircon occurs in many different colors, including red, pink, brown, yellow, hazel, black, or colorless. The color of zircons 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 100's 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 about 350°C.

Uses

Sand-sized grains of zircon

• Zircons are commercially mined for the metal zirconium, and are used for abrasive and insulating purposes.
• It is the source of zirconium oxide(ZrO₂), one of the most refractory materials known.
• Crucibles of ZrO₂ are used to fuse platinum at temperatures in excess of 1755 ^oC.
• Zirconium metal is used in nuclear reactors due to its neutron absorption properties.
• Large specimens are appreciated as gemstones, owing to their high refractive index. (Zircon has a refractive index of approximately 1.95; diamond's is approximately 2.4.)
• Zircon is one of the key minerals used by geologists for geochronology .

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 askimberlites, 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 certainpegmatites, and within some rare alkaline volcanic rocks, for example the Toongi Trachyte, Dubbo, New South Wales Australia^[1] in association with the zirconium-hafnium mineralseudialyte and armstrongite.

Zircons and 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 likeerosion, transport, even high-grade metamorphism, they contain a rich and varied record of geological processes. Currently, zircons are typically dated by U/Pb, fission-track dating, and U+Th/He dating techniques.

The oldest minerals dated so far by the U/Pb technique are zircons from Jack Hills in theNarryer Gneiss Terrane, Yilgarn Craton, Western Australia, with an age of 4.404 billion years,^[2] interpreted to be the age of crystallization. These zircons might be the oldest minerals on earth. In addition, the oxygen isotopic composition has been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. This interpretation has been published in scientific journals^[3][4] but is the subject of debate.^{[citation needed]}

Zircons and Oxygen Isotope Dating

Oxygen isotope dating is also used to help to identify the age of zircons in many metamorphic, igneous, and clastic sedimentary rocks. Unlike some minerals, zircons are able to preserve oxygen isotope ratios through many environmental processes, even through metamorphism and hydrothermal alteration.. The oxygen ratio of oxygen-18 to oxygen-16 is also a way of helping to better understand the type of atmospheric conditions when the minerals were being formed. Zircons that date back to the Archean are found in the Canadian Shield, and an area where zircons are extracted heavily for dating.^[5]