Corundum is acrystalline form ofaluminium oxide (Al2O3) typically containing traces ofiron,titanium,vanadium, andchromium.[3][4] It is arock-formingmineral. It is a naturallytransparent material, but can have different colors depending on the presence oftransition metal impurities in its crystalline structure.[7] Corundum has two primary gem varieties:ruby andsapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present.[7] A rare type of sapphire,padparadscha sapphire, is pink-orange.
The name "corundum" is derived from theTamil-Dravidian wordkurundam (ruby-sapphire) (appearing inSanskrit askuruvinda).[8][9]
Because of corundum's hardness (pure corundum is defined to have 9.0 on theMohs scale), it can scratch almost all other minerals. It is commonly used as anabrasive onsandpaper 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 withmagnetite,hematite, orhercynite.[6]
In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elementsaluminium andoxygen.[10]
Corundum fromBrazil, size about 2 cm × 3 cm (0.8 in × 1 in)
Corundum occurs as a mineral in micaschist,gneiss, and somemarbles inmetamorphicterranes. It also occurs in low-silicaigneoussyenite andnepheline syeniteintrusives. Other occurrences are as masses adjacent toultramafic intrusives, associated withlamprophyredikes and as large crystals inpegmatites.[6] It commonly occurs as adetrital mineral in stream and beach sands because of its hardness and resistance to weathering.[6] The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb).[11] The record has since been surpassed by certain syntheticboules.[12]
Four corundum axes dating to 2500 BC from theLiangzhu culture and Sanxingcun culture (the latter of which is located inJintan District) have been discovered in China.[13][14]
In 1877, Frenic and Freil made crystal corundum from which small stones could be cut. Frimy andAuguste Verneuil manufactured artificial ruby by fusingBaF2 andAl2O3 with a little chromium at temperatures above 2,000 °C (3,630 °F).
In 1903,Verneuil announced that he could produce synthetic rubies on a commercial scale using thisflame fusion process.[16]
TheVerneuil process allows the production of flawless single-crystalsapphire andruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth andhydrothermal 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.[17]
Synthetic corundum has a lower environmental impact than natural corundum by avoiding destructive mining and conserving resources.[18][19] However, its production is energy-intensive, contributing tocarbon emissions if fossil fuels are used, and involves chemicals that can pose risks.[20]
Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistantwatch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), andlaser components. For example, theKAGRA gravitational wave detector's main mirrors are 23 kg (50 lb) sapphires,[21] andAdvanced LIGO considered 40 kg (88 lb) sapphire mirrors.[22] Corundum has also found use in the development of ceramic armour thanks to its high hardiness.[23]
Crystal structure of corundumMolar volume vs. pressure at room temperature
Corundum crystallizes with trigonal symmetry in the space groupR3c and has the lattice parametersa = 4.75 Å andc = 12.982 Å at standard conditions. The unit cell contains six formula units.[4][24]
The toughness of corundum is sensitive to surface roughness[25][26] and crystallographic orientation.[27] It may be 6–7 MPa·m1/2 for synthetic crystals,[27] and around 4 MPa·m1/2 for natural.[28]
In the lattice of corundum, the oxygen atoms form a slightly distortedhexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions.[29] The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry toR3c and the crystal class to trigonal.[30] The structure of corundum is sometimes described as a pseudohexagonal structure.[31]
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.[32] 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.[32] The shear modulus of corundum is 145 GPa,[33] and the bulk modulus is 240 GPa.[33]
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.[34]
The hardness of corundum measured by indentation at low loads of 1-2 N has been reported as 22-23 GPa[35] in major crystallographic planes: (0001) (basal plane), (1010) (rhombohedral plane), (1120) (prismatic plane), and (1012). 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).[35]
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.[36]
Because of its prevalence, corundum has also become the name of a major structure type (corundum type) found in variousbinary andternary compounds.[37]
^"Mohs' scale of hardness".Collector's corner. Mineralogical Society of America. Retrieved10 January 2014.
^abAnthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1997). "Corundum".Handbook of Mineralogy(PDF). Vol. III Halides, Hydroxides, Oxides. Chantilly, VA, US: Mineralogical Society of America.ISBN0962209724.Archived(PDF) from the original on 5 September 2006.
^abGiuliani, Gaston; Ohnenstetter, Daniel; Fallick, Anthony E.; Groat, Lee; Fagan; Andrew J. (2014). "The Geology and Genesis of Gem Corundum Deposits".Gem Corundum. Research Gate: Mineralogical Association of Canada. pp. 37–38.ISBN978-0-921294-54-2.
^Walsh, Andrew (February 2010). "The commodification of fetishes: Telling the difference between natural and synthetic sapphires".American Ethnologist.37 (1):98–114.doi:10.1111/j.1548-1425.2010.01244.x.
^Walsh, Andrew (2010). "The commodification of fetishes: Telling the difference between natural and synthetic sapphires".American Ethnologist.37 (1):98–114.doi:10.1111/j.1548-1425.2010.01244.x.
^Sudiro, Maria; Bertucco, Alberto (2007). "Synthetic Fuels by a Limited CO2 Emission Process Which Uses Both Fossil and Solar Energy".Energy Fuels.21 (6):3668–3675.doi:10.1021/ef7003255.
^Becker, Paul F. (1976). "Fracture-Strength Anisotropy of Sapphire".Journal of the American Ceramic Society.59 (1–2):59–61.doi:10.1111/j.1151-2916.1976.tb09390.x.
^abDobrovinskaya, Elena R.; Lytvynov, Leonid A.; Pishchik, Valerian (2009), Pishchik, Valerian; Lytvynov, Leonid A.; Dobrovinskaya, Elena R. (eds.),"Properties of Sapphire",Sapphire: Material, Manufacturing, Applications, Boston, MA: Springer US, pp. 55–176,doi:10.1007/978-0-387-85695-7_2,ISBN978-0-387-85695-7, retrieved12 May 2024
