Template:Distinguish Template:Infobox mineral Corundum is a crystalline form of aluminium oxide (Template:Chem2) typically containing traces of iron, titanium, vanadium and chromium.[1][2] It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.[3] Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present.[3] A rare type of sapphire, padparadscha sapphire, is pink-orange.
The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).[4][5]
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.[6]
In addition to its hardness, corundum has a density of Template:Cvt, which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.[7]
Geology and occurrence
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low-silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.[6] It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.[6] The largest documented single crystal of corundum measured about Template:Convert, and weighed Template:Convert.[8] The record has since been surpassed by certain synthetic boules.[9]
Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario.[6] Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.[6]
Four corundum axes dating to 2500 BC from the Liangzhou culture have been discovered in China.[10]
Synthetic corundum
- In 1837, Marc Antoine Gaudin made the first synthetic rubies by reacting alumina at a high temperature with a small amount of chromium as a colourant.[11]
- In 1847, J. J. Ebelmen made white synthetic sapphires by reacting alumina in boric acid.
- In 1877, Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing Template:Chem2 and Template:Chem2 with a little chromium at temperatures above Template:Convert.
- In 1903, Verneuil announced that he could produce synthetic rubies on a commercial scale using this flame fusion process.[12]
The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market at a fraction of the cost of natural stones.[13]
Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components. For example, the KAGRA gravitational wave detector's main mirrors are Template:Cvt sapphires,[14] and Advanced LIGO considered Template:Cvt sapphire mirrors.[15] Corundum has also found use in the development of ceramic armour thanks to its high hardiness.[16]
Structure and physical properties
Corundum crystallizes with trigonal symmetry in the space group [[hexagonal crystal family#Crystal classes|Template:MvarTemplate:OverlineTemplate:Mvar]] and has the lattice parameters Template:Mvar = 4.75 Å and Template:Mvar = 12.982 Å at standard conditions. The unit cell contains six formula units.[17][2]
The toughness of corundum is sensitive to surface roughness[18][19] and crystallographic orientation.[20] It may be 6–7 MPa·mTemplate:Sup for synthetic crystals,[20] and around 4 MPa·mTemplate:Sup for natural.[21]
In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions.[22] The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry to Template:MvarTemplate:OverlineTemplate:Mvar and the crystal class to trigonal.[23] The structure of corundum is sometimes described as a pseudohexagonal structure.[24]
Generalization
Template:Main Because of its prevalence, corundum has also become the name of a major structure type (corundum type) found in various binary and ternary compounds.[25]
See also
- Aluminium oxynitride
- Gemstone
- Spinel – natural and synthetic mineral often mistaken for corundum
References
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedHandbook - ↑ 2.0 2.1 Cite error: Invalid
<ref>tag; no text was provided for refs namedMindat - ↑ 3.0 3.1 Gem Corundum{{#if:|, {{{last}}}}. Mineralogical Association of Canada(2014). ISBN 978-0-921294-54-2
- ↑ Template:OEtymD
- ↑ Intriguing minerals: corundum in the world of rubies and sapphires with special attention to Macedonian rubies. ChemTexts({Template:Date).
- ↑ 6.0 6.1 6.2 6.3 6.4 Cite error: Invalid
<ref>tag; no text was provided for refs namedHurlbut - ↑ The Mineral Corundum. galleries.com.
- ↑ The largest crystals. American Mineralogist({Template:Year).
- ↑ Rubicon Technology grows 200 kg "super boule". LED Inside(April 21, 2009).
- ↑ Chinese made first use of diamond. BBC(May 2005).
- ↑ Untreated yellowish orange sapphire exhibiting its natural colour. Journal of Gemmology({Template:Year).
- ↑ A Handbook of Precious Stones. Retrieved 2007-08-19.
- ↑ The commodification of fetishes: Telling the difference between natural and synthetic sapphires. American Ethnologist({Template:Date).
- ↑ Sapphire mirror for the KAGRA gravitational wave detector. Physical Review D({Template:Year).
- ↑ Advanced Ligo Core Optics Components – Downselect, Billingsley, GariLynn. LIGO Laboratory. Retrieved 2020-02-06.
- ↑ Defense World.Net, Russia’s Armored Steel-Comparable Ceramic Plate Clears Tests, 5th September 2020, Retrieved 29th December 2020
- ↑ Refinement of the α Al2O3, Ti2O3, V2O3 and Cr2O3 structures*. Zeitschrift für Kristallographie({Template:Date).
- ↑ Effect of machining on fracture toughness of corundum. Journal of Materials Science({Template:Year).
- ↑ Fracture-Strength Anisotropy of Sapphire. Journal of the American Ceramic Society({Template:Year).
- ↑ 20.0 20.1 Fracture of Sapphire. Journal of the American Ceramic Society({Template:Year).
- ↑ Corundum, Aluminum Oxide, Alumina, 99.9%, Al2O3. www.matweb.com.
- ↑ Introduction to mineralogy{{#if:|, {{{last}}}}. Oxford University Press(2000). ISBN 9780195106916
- ↑ Crystallography{{#if:|, {{{last}}}}. Springer(1995). ISBN 3540594787
- ↑ The Formation Al 2 O 3 /V 2 O 3 Multilayer Structures by High-Dose Ion Implantation. MRS Proceedings({Template:Date).
- ↑ The major ternary structural families{{#if:|, {{{last}}}}. Springer-Verlag(1974). ISBN 0-387-06430-3