Zircon

Zircon
Zircão.jpeg
Zircon crystal from Tocantins, Brazil(unknown scale)
General
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 Scale hardness 7.5
Luster Adamantine
Refractive index nω=1.967–2.015
nε=1.920–1.960
Birefringence δ=0.047–0.055
Streak White
Specific gravity 4.6–4.7
Fusibility Infusible
Solubility Insoluble
Other Characteristics Fluorescent and radioactive

Zircon is a mineral belonging to the group of nesosilicates. 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 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.

Zircon is regarded as the traditional birthstone for December.

Contents

Properties

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

Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It is found 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 100–300 µm, but they can also grow to sizes of several centimeters, especially in pegmatites.

Owing to their uranium and thorium content, some zircons may undergo metamictization. This partially disrupts the crystal structure and explains the highly variable properties of zircon.

Zircon is a common accessory mineral and found worldwide. Noted occurrences include: Australia; the Ural Mountains; Trentino, Monte Somma, and Vesuvius, Italy; Arendal, Norway; Sri Lanka; India; 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; the Pikes 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 can come in red, brown, yellow, hazel, black, or colorless. The color of zircons below gem quality can be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, and golden-yellow zircons can be made.

Uses

Zircon dust

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[1] in association with the zirconium-hafnium minerals eudialyte and armstrongite.

Zircons and radiometric dating

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 1 wt%) and can be dated using modern analytical techniques. Since zircons can 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 Jack Hills in the Narryer 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 not only be the oldest minerals on earth, they also show another interesting feature. Their 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 respected scientific journals[3][4] but is the subject of debate.

See also

References

  1. ↑ "Dubbo Zirconia Project Fact Sheet June 2007" (06/2007). Retrieved on 2007-09-10.
  2. ↑ http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf
  3. ↑ Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust
  4. ↑ Ancient mineral shows early Earth climate tough on continents Physorg.com, June 13, 2008

Further reading

The structure of zircon has a space group I41/amd.