Moissanite

Moissanite or silicon carbide (SiC) is a rare mineral that can be found in meteorites and in terrestrial samples. It belongs to the carbon group and because of its similarity to diamonds it is used as a replacement for diamonds in fashion and in science applications.

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Background

Moissanite was discovered by Dr. Ferdinand Frederick Henri Moissan, a French chemist best known for his Nobel Prize winning procedure on the isolation of fluorine. He discovered moissanite while examining rock samples from a meteor crater located in Canyon Diablo, Arizona. He later identified this new mineral and called it silicon carbide (Xu J. and Mao H., 2000). Silicon carbide was named moissanite in honor of Dr. Moissan later on in his life (Di Pierro et al., 2003).

Geological occurrence

Moissanite has been discovered in a 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 (Di Pierro et al., 2003). They have also been identified in presolar meteorites formed with grains from supernovas or red giants, called carbonaceous chondrites (Schonbachler et al, 2007).

Composition

All applications of silicon carbide used today are man-made in laboratories. Silicon carbide was first synthesized by Jons Jacob Berzelius, who is best known for his discovery of silicon (Saddow and Agarwal, 2004). Years later, Acheson produced suitable minerals that could substitute diamond as an abrasive and cutting material. He 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 (Saddow and Agarwal, 2004). Since naturally existing moissanite is so rare, synthetic silicon carbide is used for scientific applications and in jewelry sales as well.

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 ^[1]. The structure of elements is held together with strong covalent bonding that gives moissanite its distinctive strength along with other properties that rival diamond (Xu J. and Mao H., 2000). All SiC minerals for testing purposes are synthetically made, due to the rarity of the natural existence of the mineral. Moissanite has little to no anisotropies occurring with in the crystal structure, thus giving it the ability to withstand high pressures and temperatures (Zhang J et al., 2002).

Physical properties

Moissanite belongs to the carbon group with a chemical formula of SiC. 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; density of 3.22 g/cm³; hardness of 9.25 and varies in colors including clear, greyish green, greyish yellow, grey, greenish yellow, brownish yellow and yellow green (Read P., 2005). Moissanite is stable to temperatures up to 1127 °C (1400 K) and can withstand pressures up to 52.1 gigapascals (Xu J. and Mao H., 2000). A few properties that make moissanite unique from its rival the diamond include, a hardness of 0.75 less than the diamond; SiC floats in diiodomethane while the diamond does not; diamonds burn at 847 °C (1120 K) (Zhang J et al., 2002), much lower than moissanite. 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 (Melinon P. et al., 2007).

Modern uses

Moissanite has many common uses today and experiments in the nanoscale are being preformed to help use silicon carbide for the future. Because moissanite has strong covalent bonds, it is useful for high-pressure experiments (Xu J. and Mao H., 2000). Also the cost of 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 diamond itself (Xu J. and Mao H., 2000). In contrast, moissanite minerals are less expensive and more readily available in many different sizes, making them ideal for a scientific testing instrument. This allows scientist to test the hardness of other minerals without using diamonds. Silicon carbide is better suited than diamond for electronic purposes, because it is a semiconductor of temperature and electricity (Zhang J et al., 2002). 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. Because moissanite and diamonds look similar and have a few similar properties, jewelry stores today market moissanite as the “other diamond”.

References

• Di Pierro S., Gnos E., Grobety B.H., Armbruster T., Bernasconi S.M., and Ulmer P. (2003) Rock-forming moissanite (natural α-silicon carbide), Am. Mineralogist 88, 1817-1821.
• Melinon P., Masenelli B., Tournus F. and Perez P. (2007) Playing with carbon and silicon

at the nanoscale, Nature 6, 479-490.

• Read P. (2005) Gemmology, Third Edition. Elsevier Butterworth-Heinemann, Massachusetts.
• Saddow S.E and Agarwal A. (2004) Advances in Silicon Carbide Processing an Applications. Artech House Inc., Boston.
• Schonbachler et al. (2007) Nucleosynthetic Os Isotropic Anomalies in Carbonaceous Chondrites. 38th Lunar and Planetary Science Conference, March 2007.
• Xu J. and Mao H. (2000) Moissanite: A window for high-pressure experiments, Science 290, 783-787.
• Zhang J., Wang L., Weidner D.J., Uchida T. and Xu J. (2002) The strength of moissanite. Am. Mineralogist 87, 1005-1008.