Corundum#Synthetic corundum

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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. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about {{convert|65|×|40|×|40|cm|in|abbr=on}}, and weighed {{convert|152|kg|lb|abbr=on}}.{{cite journal |author=Rickwood, P. C. |year=1981 |title=The largest crystals |journal=American Mineralogist |volume=66 |pages=885–907 |url=http://www.minsocam.org/ammin/AM66/AM66_885.pdf |archive-url=https://web.archive.org/web/20090620081033/http://www.minsocam.org/ammin/AM66/AM66_885.pdf |archive-date=2009-06-20 |url-status=live}} The record has since been surpassed by certain synthetic boules.{{cite web |url=http://www.ledinside.com/news/2009/4/Rubicon_Technology_Grows_Super_boule_of_200kg_Weight_20090421 |title=Rubicon Technology grows 200 kg "super boule" |website=LED Inside |date=April 21, 2009 |df=dmy-all}}

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. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.

Four corundum axes dating to 2500 BC from the Liangzhu culture and Sanxingcun culture (the latter of which is located in Jintan District) have been discovered in China.{{cite web |url=http://news.bbc.co.uk/2/hi/science/nature/4555235.stm |title=Chinese made first use of diamond |publisher=BBC |date=May 2005 |work=BBC News}}{{cite web |last1=Alexandra |first1=Goho |title=In the Buff: Stone Age tools may have derived luster from diamond |url=https://www.sciencenews.org/article/buff-stone-age-tools-may-have-derived-luster-diamond#:~:text=He%20found%20that%20all%20four,Earth%3B%20only%20diamond%20is%20harder |website=Science News |date=16 February 2005 }}

• 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.{{cite journal |url=http://www.gem-a.com/media/94808/duroc%20danner%20website.pdf |journal=Journal of Gemmology |volume=32 |pages=175–178 |year=2011 |title=Untreated yellowish orange sapphire exhibiting its natural colour |author=Duroc-Danner, J. M. |issue=5 |doi=10.15506/jog.2011.32.5.174 |url-status=dead |archive-url=https://web.archive.org/web/20130516231326/http://www.gem-a.com/media/94808/duroc%20danner%20website.pdf |archive-date=2013-05-16 |df=dmy-all}}
• 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 {{chem2|link=barium fluoride|BaF2}} and {{chem2|link=aluminium oxide|Al2O3}} with a little chromium at temperatures above {{convert|2000|C|abbr=on}}.
• In 1903, Verneuil announced that he could produce synthetic rubies on a commercial scale using this flame fusion process.{{cite web |url=http://farlang.com/books/bahadur-a-handbook-of-precious-stones |author=Bahadur |title=A Handbook of Precious Stones |year=1943 |access-date=2007-08-19 |df=dmy-all}}

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.{{cite journal |last1=Walsh |first1=Andrew |title=The commodification of fetishes: Telling the difference between natural and synthetic sapphires |journal=American Ethnologist |date=February 2010 |volume=37 |issue=1 |pages=98–114 |doi=10.1111/j.1548-1425.2010.01244.x}}

Synthetic corundum has a lower environmental impact than natural corundum by avoiding destructive mining and conserving resources.{{cite web |url=https://www.preciseceramic.com/blog/comparing-alumina-and-corundum.html |title=Comparing Alumina and Corundum: From Raw Form to Crystal Clarity |date=March 1, 2024 |website=Advanced Ceramic Materials |access-date=Oct 7, 2024}}{{cite journal |last=Walsh |first=Andrew |year=2010 |title=The commodification of fetishes: Telling the difference between natural and synthetic sapphires |journal=American Ethnologist |volume=37 |issue=1 |pages=98–114 |doi=10.1111/j.1548-1425.2010.01244.x}} However, its production is energy-intensive, contributing to carbon emissions if fossil fuels are used, and involves chemicals that can pose risks.{{cite journal |last1=Sudiro |first1=Maria |last2=Bertucco |first2=Alberto |year=2007 |title=Synthetic Fuels by a Limited CO2 Emission Process Which Uses Both Fossil and Solar Energy |journal=Energy Fuels |volume=21 |issue=6 |pages=3668–3675 |doi=10.1021/ef7003255}}

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 {{cvt|50|lb|adj=on|order=flip}} sapphires,{{cite journal |first1=Eiichi |last1=Hirose |display-authors=etal |year=2014 |title=Sapphire mirror for the KAGRA gravitational wave detector |journal=Physical Review D |volume=89 |issue=6 |page=062003 |doi=10.1103/PhysRevD.89.062003 |bibcode=2014PhRvD..89f2003H |url=https://authors.library.caltech.edu/45938/1/PhysRevD.89.062003.pdf |archive-url=https://web.archive.org/web/20180724161640/https://authors.library.caltech.edu/45938/1/PhysRevD.89.062003.pdf |archive-date=2018-07-24 |url-status=live}} and Advanced LIGO considered {{cvt|40|kg}} sapphire mirrors.{{cite web |first=GariLynn |last=Billingsley |title=Advanced Ligo Core Optics Components – Downselect |url=https://labcit.ligo.caltech.edu/~gari/LIGOII/Downselect/ |publisher=LIGO Laboratory |year=2004 |access-date=2020-02-06 |df=dmy-all}} Corundum has also found use in the development of ceramic armour thanks to its high hardiness.Defense World.Net, [https://www.defenseworld.net/news/27805/Russia___s_Armored_Steel_Comparable_Ceramic_Plate_Clears_Tests Russia’s Armored Steel-Comparable Ceramic Plate Clears Tests], 5 September 2020, Retrieved 29 December 2020

File:Corundum.png

File:Corundum-pV.svg

Corundum crystallizes with trigonal symmetry in the space group hexagonal crystal family#Crystal classes and has the lattice parameters {{nowrap|{{mvar|a}} {{=}} 4.75 Å}} and {{nowrap|{{mvar|c}} {{=}} 12.982 Å}} at standard conditions. The unit cell contains six formula units.{{cite journal |last1=Newnham |first1=R. E. |last2=de Haan |first2=Y. M. |title=Refinement of the α Al₂O₃, Ti₂O₃, V₂O₃ and Cr₂O₃ structures* |journal=Zeitschrift für Kristallographie |date=August 1962 |volume=117 |issue=2–3 |pages=235–237 |doi=10.1524/zkri.1962.117.2-3.235|bibcode=1962ZK....117..235N }}

The toughness of corundum is sensitive to surface roughness{{cite journal |url=https://rd.springer.com/article/10.1007/BF00638054?no-access=true |title=Effect of machining on fracture toughness of corundum |first1=Farrokh |last1=Farzin-Nia |first2=Terry |last2=Sterrett |first3=Ron |last3=Sirney |journal=Journal of Materials Science |year=1990 |volume=25 |issue=5 |pages=2527–2531 |doi=10.1007/bf00638054|bibcode=1990JMatS..25.2527F |s2cid=137548763 }}{{cite journal |doi=10.1111/j.1151-2916.1976.tb09390.x |volume=59 |title=Fracture-Strength Anisotropy of Sapphire |journal=Journal of the American Ceramic Society |year=1976 |pages=59–61|last1=Becker |first1=Paul F. |issue=1–2 }} and crystallographic orientation.{{cite journal |doi=10.1111/j.1151-2916.1969.tb09199.x |volume=52 |title=Fracture of Sapphire |journal=Journal of the American Ceramic Society |year=1969 |pages=485–491|last1=Wiederhorn |first1=S. M. |issue=9 }} It may be 6–7 MPa·m{{sup|1/2}} for synthetic crystals, and around 4 MPa·m{{sup|1/2}} for natural.{{cite web |title=Corundum, Aluminum Oxide, Alumina, 99.9%, Al2O3 |website=www.matweb.com |url=http://www.matweb.com/search/datasheet_print.aspx?matguid=c8c56ad547ae4cfabad15977bfb537f1}}

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.{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |pages=363–364}} 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 {{mvar|R}}{{overline|3}}{{mvar|c}} and the crystal class to trigonal.{{cite book |last1=Borchardt-Ott |first1=Walter |last2=Kaiser |first2=E. T. |title=Crystallography |date=1995 |publisher=Springer |location=Berlin |isbn=3540594787 |page=230 |edition=2nd}} The structure of corundum is sometimes described as a pseudohexagonal structure.{{cite journal |last1=Gea |first1=Laurence A. |last2=Boatner |first2=L. A. |last3=Rankin |first3=Janet |last4=Budai |first4=J. D. |title=The Formation Al 2 O 3 /V 2 O 3 Multilayer Structures by High-Dose Ion Implantation |journal=MRS Proceedings |date=1995 |volume=382 |pages=107 |doi=10.1557/PROC-382-107|url=https://digital.library.unt.edu/ark:/67531/metadc618580/ }}

The Young's modulus of corundum (sapphire) has been reported by many different sources with values varying between 300 and 500 GPa, but a commonly cited value used for calculations is 345 GPa.{{Citation |last1=Dobrovinskaya |first1=Elena R. |title=Properties of Sapphire |date=2009 |work=Sapphire: Material, Manufacturing, Applications |pages=55–176 |editor-last=Pishchik |editor-first=Valerian |url=https://doi.org/10.1007/978-0-387-85695-7_2 |access-date=2024-05-12 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-0-387-85695-7_2 |isbn=978-0-387-85695-7 |last2=Lytvynov |first2=Leonid A. |last3=Pishchik |first3=Valerian |editor2-last=Lytvynov |editor2-first=Leonid A. |editor3-last=Dobrovinskaya |editor3-first=Elena R.}} The Young's modulus is temperature dependent, and has been reported in the [0001] direction as 435 GPa at 323 K and 386 GPa at 1,273 K. The shear modulus of corundum is 145 GPa,{{Cite book |last=Ramdas |first=Roshan L. Aggarwal, Anant K. |url=https://www.taylorfrancis.com/books/mono/10.1201/9780429283260/physical-properties-diamond-sapphire-roshan-aggarwal-anant-ramdas |title=Physical Properties of Diamond and Sapphire |date=2019-05-03 |publisher=CRC Press |isbn=978-0-429-28326-0 |location=Boca Raton |doi=10.1201/9780429283260}} and the bulk modulus is 240 GPa.

Single crystal corundum fibers have potential applications in high temperature composites, and the Young's modulus is highly dependent on the crystallographic orientation along the fiber axis. The fiber exhibits a max modulus of 461 GPa when the crystallographic c-axis [0001] is aligned with the fiber axis, and minimum moduli ~373 GPa when a direction 45° away from the c-axis is aligned with the fiber axis.{{Cite journal |last1=Wadley |first1=Haydn N. G. |last2=Lu |first2=Yichi |last3=Goldman |first3=Jeffrey A. |date=1995-03-01 |title=Ultrasonic determination of single crystal sapphire fiber modulus |url=https://doi.org/10.1007/BF00735669 |journal=Journal of Nondestructive Evaluation |language=en |volume=14 |issue=1 |pages=31–38 |doi=10.1007/BF00735669 |issn=1573-4862}}

The hardness of corundum measured by indentation at low loads of 1-2 N has been reported as 22-23 GPa{{Cite journal |last1=Sinani |first1=A. B. |last2=Dynkin |first2=N. K. |last3=Lytvinov |first3=L. A. |last4=Konevsky |first4=P. V. |last5=Andreev |first5=E. P. |date=2009-10-01 |title=Sapphire hardness in different crystallographic directions |url=https://doi.org/10.3103/S1062873809100177 |journal=Bulletin of the Russian Academy of Sciences: Physics |language=en |volume=73 |issue=10 |pages=1380–1382 |doi=10.3103/S1062873809100177 |issn=1934-9432}} in major crystallographic planes: (0001) (basal plane), (10{{overline|1}}0) (rhombohedral plane), (11{{overline|2}}0) (prismatic plane), and (10{{overline|1}}2). The hardness can drop significantly under high indentation loads. The drop with respect to load varies with the crystallographic plane due to the difference in crack resistance and propagation between directions. One extreme case is seen in the (0001) plane, where the hardness under high load (~1 kN) is nearly half the value under low load (1-2 N).

Polycrystalline corundum formed through sintering and treated with a hot isostatic press process can achieve grain sizes in the range of 0.55-0.7 μm, and has been measured to have four-point bending strength between 600 and 700 MPa and three-point bending strength between 750 and 900 MPa.{{Cite journal |last1=Krell |first1=Andreas |last2=Blank |first2=Paul |last3=Ma |first3=Hongwei |last4=Hutzler |first4=Thomas |last5=van Bruggen |first5=Michel P. B. |last6=Apetz |first6=Rolf |date=2003 |title=Transparent Sintered Corundum with High Hardness and Strength |url=https://ceramics.onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.2003.tb03270.x |journal=Journal of the American Ceramic Society |language=en |volume=86 |issue=1 |pages=12–18 |doi=10.1111/j.1151-2916.2003.tb03270.x |issn=0002-7820}}

{{Main|Corundum (structure)}}

Because of its prevalence, corundum has also become the name of a major structure type (corundum type) found in various binary and ternary compounds.{{Cite book |last1=Muller |first1=Olaf |last2=Roy |first2=Rustum |url=https://www.worldcat.org/oclc/1056558 |title=The major ternary structural families |date=1974 |publisher=Springer-Verlag |isbn=0-387-06430-3 |location=New York |oclc=1056558}}

{{Commons category|Corundum}}

• Aluminium oxynitride
• Gemstone
• Spinel – natural and synthetic mineral often mistaken for corundum

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Category:Abrasives

Category:Aluminium minerals

Category:Corundum varieties

Category:Hematite group

Category:Industrial minerals

Category:Luminescent minerals

Category:Oxide minerals

Category:Polymorphism (materials science)

Category:Superhard materials

Category:Trigonal minerals

Category:Minerals in space group 167