Zircon (pronunciation: /ˈzɜːrkɒn/[5][6] or /ˈzɜːrkən/;[7]) 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 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.
Detailed description
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 meaning gold-hued.[8] 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.[9] Yellow, orange and
red zircon is also known as "hyacinth",[10] from the flower hyacinthus, whose name is of
Ancient Greek origin.
Properties
Zircon is ubiquitous 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.[1]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 mafic pegmatites and carbonatites.[1] Zircon is also very
resistant to heat and corrosion.
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 reddish
brown, yellow, green, blue, gray and colorless.[1] The color of zircons
can sometimes be changed by heat treatment. Common brown zircons can be
transformed into colorless and blue zircons by heating to 800 to 1000 °C.[11] 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
Zircon is mainly consumed as an opacifier, and has been known
to be used in the decorative ceramics industry.[12] 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.
Other applications include use in refractories and
foundry casting and a growing array of specialty applications as zirconia and
zirconium chemicals, including in nuclear fuel rods, catalytic fuel converters
and in water and air purification systems.[13]
Occurrence
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[14] 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. South
Africa is Africa’s main producer, with 30% of world production, second after
Australia.
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, cathodoluminescence, and U+Th/He techniques. For instance, imaging the
cathodoluminescence emission from fast electrons can be used as a prescreening
tool for high-resolution secondary-ion-mass spectrometry (SIMS) to image the
zonation pattern and identify regions of interest for isotope analysis. This is
done using an integrated cathodoluminescence and scanning electron microscope.[17]Zircons in
sedimentary rock can identify the
sediment source.
Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404
billion years,[18] 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. This interpretation is supported by
additional trace element data, but
is also the subject of debate. In
2015, "remains of biotic life" were found in
4.1 billion-year-old rocks in the Jack Hills of Western Australia. According to one of the researchers,
"If life arose relatively
quickly on Earth ... then it could be
common in the universe."
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