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Moissanite engagement ring

Moissanite is the rare mineral form of silicon carbide (SiC) which has been found in meteorites and in mantle derived igneous rocks.[1] It is classed in the element group in both the Dana and mineral class. It crystallizes in the hexagonal system.[2] Synthetic moissanite is used by lapidaries.


[edit] Background

Moissanite was discovered by Henri Moissan while examining rock samples from a meteor crater located in Canyon Diablo, Arizona in 1893. At first, he mistakenly identified the crystals as diamonds, but in 1904 he identified the crystals as a new mineral and called it silicon carbide (SiC).[3][4] The mineral form of silicon carbide was named moissanite in honor of Moissan later on in his life.[5]

Until the 1950s no other source, apart from meteorites, had been found. Later it was found as inclusion in kimberlite from a diamond mine in Yakutia in 1959, and in the Green River Formation in Wyoming in 1958.[6] The discovery in the Canyon Diablo meteorite and other places was challenged for a long time to be carborundum contamination from human abrasive tools. The existence of moissanite in nature was questioned even in 1986 by Charles Milton, an American geologist.[7]

[edit] Geological occurrence

Moissanite, in its natural form, is very rare. It has only been discovered in a small variety of places from upper mantle rock to meteorites. Discoveries have shown that moissanite occurs naturally as inclusions in diamonds, xenoliths, and ultramafic rocks such as kimberlite and lamproite.[5] They have also been identified in carbonaceous chondrite meteorites as presolar grains.[8]

[edit] Composition

All applications of silicon carbide today use synthetic material, as the natural material is incredibly scarce. Silicon carbide was first synthesized by Jons Jacob Berzelius, who is best known for his discovery of silicon.[9] Years later, Acheson produced viable minerals that could substitute diamond as an abrasive and cutting material. This was possible as moissanite is one of the hardest substances known to man, falling only behind rhenium diboride and diamond in hardness. Acheson mixed coke and silica in a furnace and found a crystalline product characterized by a great hardness, refractability, and infusibility, which was shown to be a compound of carbon and silicon.[9] Since naturally occurring moissanite is so rare, lab-grown moissanite is the only commercially viable version of the mineral.

[edit] Structure

The structure of moissanite is one of its greatest properties. Similar to the diamond structure, moissanite’s structure gives it great strength, making it useful for testing applications and microelectronics. The crystalline structure is held together with strong covalent bonding that gives moissanite its strength along with other properties that rival diamond.[3] Moissanite has little to no anisotropies occurring with in the crystal structure, thus giving it the ability to withstand high pressures and temperatures.[10] On the Mohs scale of hardness, moissanite is graded at 9.25, second in strength only to diamond. Moissanite is harder than rubies and sapphires which come in at a hardness of 9, and significantly harder than cubic zirconia, which is a brittle material and takes damage relatively easily.

[edit] Physical properties

Physical properties for moissanite include a hexagonal crystal system, the H-M symbol is 6 mm; space group is P 63mc; cleavage is indistinct; refractive index of 2.65–2.69 (Higher than Diamond 2.4175); density of 3.22 g/cm³; hardness of 9.25 and varies in colours usually being graded in the I-J-K range on the Diamond Color Grading Scale.[11] Moissanite can withstand pressures up to 52.1 gigapascals.[3] Silicon carbide has a wide, adjustable bandgap, or a space where electrons can or cannot jump giving the mineral variable conductance abilities which is useful in nanotechnology.[12]

[edit] Modern uses

Moissanite has many common uses today, aside from jewelry, and experiments in the nanoscale are being performed to help use silicon carbide for the future. Because moissanite has strong covalent bonds, it is useful for high-pressure experiments.[3] Also the cost of natural diamonds increases with size, therefore if one wants to test the hardness on a large scale, much of the funding would go to purchase the natural diamond.[3] In contrast, synthetic moissanite minerals like synthetic diamonds are less expensive and more readily available in many different sizes, making them ideal for a scientific testing instrument along with many other synthetic materials. This allows scientists to test the hardness of other minerals without using natural diamonds. Silicon carbide is interesting for electronic purposes, because it is a semiconductor of heat and electricity just like diamonds.[10] High power SiC devices are expected to play an enabling and vital role in the design of protection circuits used for motors, actuators, and energy storage or pulse power systems.

[edit] Jewelry use

Having a higher refractive index than diamond, moissanite is the most brilliant of colorless jewels, due to its double refraction. Moissanite also has a lower specific gravity than diamonds; it is 13% lighter than diamonds by volume (moissanite has a specific gravity of 3.20, and diamond has a specific gravity of 3.52). These optical and physical properties translate to a physically larger gem of equivalent weight and clarity. While a 1 carat (200 mg) round brilliant cut diamond is typically 6.5 mm wide when cut to ideal proportions, a one carat moissanite should come out to be about 6.7 mm in diameter. Moissanite clarity is comparable to diamonds depending on the make-up of the material. As most synthetics have low amounts of foreign material present inside them synthetic diamonds, synthetic cubic zirconium and moissanite all have similar clarity scales. But just like synthetic diamonds, synthetic moissanite, the most common type of moissanite, can have a greenish or yellow tint visible to the naked eye.

Because moissanite and diamonds look similar and have a few similar properties, jewelry stores today market moissanite as a diamond alternative. SiC used in jewelry today is manufactured by multiple companies across the world.

[edit] References

  1. ^ Handbook of Mineral
  2. ^ Webmineral
  3. ^ a b c d e Xu J. and Mao H. (2000). "Moissanite: A window for high-pressure experiments". Science 290: 783–787. doi:10.1126/science.290.5492.783. 
  4. ^ Henri Moissan (1904). "Nouvelles recherches sur la météorité de Cañon Diablo". Comptes rendus 139: 773–786. 
  5. ^ a b Di Pierro S., Gnos E., Grobety B.H., Armbruster T., Bernasconi S.M., and Ulmer P. (2003). "Rock-forming moissanite (natural α-silicon carbide)". American Mineralogist 88: 1817–1821. 
  6. ^ J. Bauer J. Fiala, R. Hřichová (1963). "Natural α–Silicone Carbide". American Mineralogist 48: 620–634. 
  7. ^ H. E. Belkin, E. J. Dwornik (1994). "Memorial of Charles Milton April 25 1896 – October 1990". American Mineralogist 79: 190–192. 
  8. ^ Schönbächler et al. (March 2007). "Nucleosynthetic Os Isotropic Anomalies in Carbonaceous Chondrites.". 38th Lunar and Planetary Science Conference. 
  9. ^ a b Saddow S.E and Agarwal A. (2004). Advances in Silicon Carbide Processing an Applications. Boston. Artech House Inc.. pp. –. ISBN 1580537405. 
  10. ^ a b Zhang J., Wang L., Weidner D.J., Uchida T. and Xu J. (2002). "The strength of moissanite" (PDF). American Mineralogist 87: 1005–1008. 
  11. ^ Read P. (2005). Gemmology. Massachusetts: Elsevier Butterworth-Heinemann. pp. –. ISBN 0750664495. 
  12. ^ Melinon P., Masenelli B., Tournus F. and Perez P. (2007). "Playing with carbon and silicon at the nanoscale". Nature Materials 6: 479–490. doi:10.1038/nmat1914. 

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