Moissanite

Moissanite
General
Category	Mineral species
Chemical formula	SiC
Strunz classification	01.DA.05
Identification
Color	transparent, green, yellow
Crystal habit	Generally found as inclusions in other minerals
Crystal system	Most common: 6H hexagonal (6mm), space group: P63mc
Cleavage	(0001) Indistinct
Fracture	Conchoidal – fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz)
Mohs scale hardness	9.5
Luster	Adamantine to metallic
Streak	greenish gray
Diaphaneity	transparent
Specific gravity	3.218–3.22
Refractive index	nω=2.654 nε=2.967, Birefringence 0.313 (6H form)
Ultraviolet fluorescence	green or yellow
Melting point	2730 °C (decomposes)
Solubility	none
Other characteristics	Not radioactive, non-magnetic
References	[1][2][3]

Moissanite /ˈmɔɪsənaɪt/^[4] originally referred to a rare mineral discovered by Henri Moissan having a chemical formula SiC and various crystalline polymorphs. Earlier, this material had been synthesized in the laboratory and named silicon carbide (SiC).

Background

Mineral 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 silicon carbide.^[5][6] The mineral form of silicon carbide was named moissanite in honor of Moissan later on in his life. The discovery in the Canyon Diablo meteorite and other places was challenged for a long time as carborundum contamination from human abrasive tools.^[7]

Geological occurrence

Until the 1950s no other source, apart from meteorites, had been encountered. Later moissanite was found as inclusion in kimberlite from a diamond mine in Yakutia in 1959, and in the Green River Formation in Wyoming in 1958.^[8] The existence of moissanite in nature was questioned even in 1986 by Charles Milton, an American geologist.^[9]

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.^[7] They have also been identified in carbonaceous chondrite meteorites as presolar grains.^[10]

Meteorites

Analysis of SiC grains found in the Murchison carbonaceous chondrite meteorite has revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system.^[11] 99% of these SiC grains originate around carbon-rich Asymptotic giant branch stars. SiC is commonly found around these stars as deduced from their infrared spectra.

Composition

All applications of silicon carbide today use synthetic material, as the natural material is very scarce. Silicon carbide was first synthesized by Jöns Jacob Berzelius, who is best known for his discovery of silicon.^[12] 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, with a hardness below that of diamond and comparable with those of cubic boron nitride and boron. Since naturally occurring moissanite is so rare, lab-grown moissanite is the only commercially viable version of the mineral. More recently, pure synthetic Moissanite has been made from thermal decomposition of the preceramic polymer poly(methylsilyne), requiring no binding matrix (e.g. cobalt metal powder).

Physical properties

Main article: Silicon carbide

The crystalline structure is held together with strong covalent bonding similar to diamonds,^[5] that allows moissanite to withstand high pressures up to 52.1 gigapascals.^[5][13] Colours vary widely and are graded in the I-J-K range on the diamond color grading scale.^[14]

Applications

Main article: Silicon carbide

Gem-cut moissanite set in a ring

Moissanite has several applications. Since 1998, when the first moissanite reached the jewelry market, it has been regarded as an excellent fine jewel, with optical properties exceeding those of diamond. Because it has its own unique appearance, it can not be truly called a diamond simulant. Its ethical production, however, does make it a popular alternative to diamonds. In many developed countries, the use of moissanite in jewelry has been patented; these patents expire in 2015.

Because of its hardness, it is useful for high-pressure experiments (e.g., using diamond anvil cell) competing there with diamond.^[5] Large diamonds, used for anvils, are prohibitively expensive. Therefore for large-volume experiments, synthetic moissanite is a more realistic choice. Synthetic moissanite is also interesting for electronic and thermal applications because its thermal conductivity is similar to that of diamonds.^[13] High power SiC electronic 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.^[15]

See also

• Blood diamond
• Engagement ring
• Fair Trade
• Silicon carbide

References

1. ^ Moissanite at Webmineral
2. ^ Moissanite at Mindat
3. ^ Moissanite in Handbook of Mineralogy
4. ^ "Moissanite". Oxford English Dictionary. Oxford University Press. 2nd ed. 1989.
5. ^ ^{a b c d} Xu J. and Mao H. (2000). "Moissanite: A window for high-pressure experiments". Science 290 (5492): 783–787. Bibcode 2000Sci...290..783X. doi:10.1126/science.290.5492.783. PMID 11052937. 
6. ^ Henri Moissan (1904). "Nouvelles recherches sur la météorité de Cañon Diablo". Comptes rendus 139: 773–786. http://gallica.bnf.fr/ark:/12148/bpt6k30930/f773.table. 
7. ^ ^{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. http://www.geoscienceworld.org/cgi/georef/2004018181. 
8. ^ J. Bauer J. Fiala, R. Hřichová (1963). "Natural α–Silicon Carbide". American Mineralogist 48: 620–634. 
9. ^ H. E. Belkin, E. J. Dwornik (1994). "Memorial of Charles Milton April 25 1896 – October 1990". American Mineralogist 79: 190–192. 
10. ^ Schönbächler et al. (March 2007). "Nucleosynthetic Os Isotropic Anomalies in Carbonaceous Chondrites.". 38th Lunar and Planetary Science Conference. 
11. ^ The Astrophysical Nature of Silicon Carbide
12. ^ Saddow S.E and Agarwal A. (2004). Advances in Silicon Carbide Processing an Applications. Artech House Inc.. ISBN 1580537405. http://books.google.com/?id=2jSPO_JtQwEC. 
13. ^ ^{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. http://www.minsocam.org/msa/AmMin/toc/Abstracts/2002_Abstracts/July02_Abstracts/Zhang_p1005_02.pdf. 
14. ^ Read P. (2005). Gemmology. Massachusetts: Elsevier Butterworth-Heinemann. ISBN 0750664495. http://books.google.com/?id=t-OQO3Wk-JsC. 
15. ^ Bhatnagar, M.; Baliga, B.J. (1993). "Comparison of 6H-SiC, 3C-SiC, and Si for power devices". IEEE Transactions on Electron Devices 40 (3): 645–655. Bibcode 1993ITED...40..645B. doi:10.1109/16.199372. http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel1/16/5182/00199372.pdf?tp=&isnumber=5182&arnumber=199372. 

External links

• Whatismoissanite.com Journal of Gemmology, 1999
• U.S. Patent 5,762,896, June 9, 1998 : Silicon Carbide Gemstones by Charles Eric Hunter et. al