August, 2016 Page 1 from 6 B"H Natural Moissanite Properties and Terrestrial Occurrence Griffin, W. L. (1), Toledo, V. (2), Gain, S.E.M. (1), Huang, J-X. (1) 1. ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Macquarie University, NSW 2109, Australia 2. Shefa Yamim (A.T.M.) Ltd., Akko, Israel Composition: Silicon Carbide (SiC). Silicon 70%, Carbon 30% by weight. Crystal Structure Hexagonal - Dihexagonal Pyramidal H-M Symbol (6mm) Space Group: P 63mc Properties Cleavage: {0001} Indistinct Color: Blue, Colorless, Green, Green yellow, Yellow. Density: 3.218 - 3.22, Average = 3.21 Diaphaneity: Transparent to Opaque Fracture: Conchoidal - Fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz). Habit: Usually microscopic crystals, hexagonal plates Hardness: 9.5 - Silicon Carbide Luster: Metallic Refractive index: 2.691-2.648; slightly higher than diamond (2.42) Streak: greenish gray Dichroism (e): weak blue. Dichroism (w): weaker blue. Optical Data: Uniaxial (+), w=2.654, e=2.967, bire=0.3130. The historical background of Moissanite and its use as a gemstone In 1893, the Nobel Prize-winning French scientist Dr. Henri Moissan discovered minute quantities of a new mineral, natural silicon carbide. The mineral was discovered in an iron meteorite (Canyon Diablo) in Arizona. It was later named "moissanite" in honor of Dr. Moissan. More than a century later, Cree (a North Carolina-based R&D lab) developed a process for producing large, single, crystals of synthectic moissanite. In 1995, a master diamond cutter saw samples of the silicon carbide crystals and suggested to the founders of Charles & Colvard that, if properly cut, the crystals could make a beautiful jewel. Charles & Colvard recognized the mineral's potential. They also realized that in order for the moissanite gems to be used, they would have to be manufactured, since there is essentially no natural supply for this stone. In 1995, Charles & Colvard collaborated with Cree to develop larger gemstones for Charles & Colvard to use in the Cree colorless development program. In conjunction with Cree, Charles & Colvard is the exclusive worldwide manufacturer and marketer of lab- created moissanite gemstones. Charles & Covald homepage link: http://www.charlesandcolvard.com/
Transcript
August, 2016 Page 1 from 6
B"H
Natural Moissanite Properties and Terrestrial Occurrence
Griffin, W. L. (1), Toledo, V. (2), Gain, S.E.M. (1), Huang, J-X. (1)
1. ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Macquarie University, NSW
The historical background of Moissanite and its use as a gemstone In 1893, the Nobel Prize-winning French scientist Dr. Henri Moissan discovered minute quantities of a new mineral, natural silicon carbide. The mineral was discovered in an iron meteorite (Canyon Diablo) in Arizona. It was later named "moissanite" in honor of Dr. Moissan. More than a century later, Cree (a North Carolina-based R&D lab) developed a process for producing large, single, crystals of synthectic moissanite. In 1995, a master diamond cutter saw samples of the silicon carbide crystals and suggested to the founders
of Charles & Colvard that, if properly cut, the crystals could make a beautiful jewel. Charles & Colvard recognized the mineral's potential. They also realized that in order for the moissanite gems to be used, they would have to be manufactured, since there is essentially no natural supply for this stone. In 1995, Charles & Colvard collaborated with Cree to develop larger gemstones for Charles & Colvard to use in the Cree colorless development program. In conjunction with Cree, Charles & Colvard is the exclusive worldwide manufacturer and marketer of lab-created moissanite gemstones. Charles & Covald homepage link: http://www.charlesandcolvard.com/
On the basis of this background story, it seems that special attention should be given to the unique natural moissanite found by Shefa Yamim, a pioneering gem and industrial mineral exploration company in Northern Israel. Read more in the following paragraphs. Type Locality Canyon Diablo iron meteorite, Arizona. Moissanite also occurs in a range of other meteorite types, and their unusual Si- and C- isotope compositions indicate that some grains are pre-solar. Terrestrial occurrence Natural occurrences are manifold (Trumbull et al. 2009) but many fall into three categories: kimberlites (Leung et al., 1990; Shiryaev et al., 2011); metasomatic rocks (Lyakhovich, 1980; Di Pierro et al., 2003); peridotites, serpentinites (Xu et al., 2008) and podiform chromitites (Bai et al., 2000). 1. Moissanite is a relatively common trace
mineral in kimberlites; it has been extensively studied in samples of Siberian kimberlites, but was long overlooked in South African ones because of different sample-processing practices used to extract diamonds. Crystals of moissanite found in kimberlite-related samples are typically <1mm in size. They contain metallic inclusions that consist mainly of native Si and an iron silicide, usually FeSi2. (Shiryaev et al., 2011). We have separated moissanite from eclogite xenoliths in South African kimberlites (Greau et al., unpublished work). Moissanite is also a rare inclusion in kimberlitic diamonds. It has been recovered from most of the cratons (China, Australia, North America, Brasil, South Africa; Trumbull et al., 2009).
2. A single beach cobble from the Turkish
coast consists mainly of moissanite crystals (up to 1 mm) in a matrix of magnesite, calcite and low-T silicates (Di Pierro et al., 2003). The cobble is believed to be derived from Tertiary volcanic rocks that crop out in
the area. Before the Shefa Yamim discoveries, this was the occurrence with the largest natural moissanite crystals.
3. Moissanite has been reported in limestones
in Russia, as crystals up to 50 µm across. The source is not clear; the grains might be related to distant volcanic eruptions.
4. Moissanite occurs as small grains in the
vesicles of mafic pumice erupted from the Tolbachick volcano (Kamchatka Peninsula) in 2011-2012. The same rocks contain abundant microdiamonds (Karpov et al., 2014).
5. Moissanite has been found in mineral
separates from chromitites, peridotites and pyroxenites of the Luobusa and Zedang ophiolites in southeastern Tibet, and from similar bodies in the Polar Urals (e.g. Ray-Iz). These grains are typically ≤100 µm across. (Yang et al. 2014; our unpublished data). Other ophiolitic occurrences include the Semail ophiolite in the United Arab Emirates (Oman).
6. The largest known moissanite crystals have
been recovered from Cretaceous volcanic rocks and associated alluvial deposits in the Mt Carmel area, Israel (Shefa Yamim project). These are euhedral crystals up to 4.14 mm long. They contain inclusions of Fe2Si3, FeSi2, a Cu-Zn alloy and native Si (our unpublished work). Moissanite is found intergrown with the Carmel Sapphire (yellow-brown corundum with many melt inclusions) from this locality, and possibly with some grains of ruby.
7. As above-mentioned, the two largest
Moissanite crystals found to date worldwide have been recovered from alluvial samples (see red star and highlighted rectangle in the maps for location). A 4.1mm crystal was recovered from a bulk sample of 400 tons, sample SY-982, reported during August 2012. Sample SY-982 yielded a total of 6 carats of moissanite.
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The World Record crystal (4.14mm) was recovered from a bulk sample of 400 tons, sample SY-1124, reported on December 14th, 2015.
8. Up until now Shefa Yamim recovered more
than 3,000 moissanite crystals (0.1 to 4.14mm) from alluvial and rock samples. This is by far the largest occurrence known to date worldwide in terms of both size and quantity.
Conditions of Formation: Oxygen fugacity (fO2): Moissanite is only stable under extremely reducing conditions (fO2 at least 6-8 log units below the Iron-Wustite buffer (the reaction FeO = Fe +O). These conditions were common in the early stages of the Solar System’s evolution (hence the occurrence of moissanite in meteorites), but are not normally expected in the Earth’s crust or mantle. However, the occurrence of moissanite in a variety of rock types and settings (see above) indicates that such conditions must be generated at least locally in Earth’s mantle. The Shefa Yamim material is an essential contribution to understanding these processes, which may indicate the action of highly reduced (methane-bearing) volatiles from the deep interior. Some authors (Schmidt et al. 2014) have suggested that moissanite is formed at very low temperatures during the alteration of ultramafic rocks, where hydrogen liberated by reactions involving serpentine would induce low fO2. This is clearly inconsistent with the evidence
from the common metallic inclusions in moissanite, which appear to have been trapped as melts (see below). Temperature: SiC is stable over a range of temperatures; synthetic SiC (carborundum) is produced at temperatures of 1500-2500 °C. The melting point of native silicon, common as inclusions in moissanite, is 1414 °C, while the iron silicide inclusions in the native silicon are molten down to ca 1200 °C. These inclusions therefore indicate high (magmatic) temperatures for the formation of the moissanite found in kimberlites, ophiolites and the Shefa Yamim material. It should be noted that Shefa Yamim recovers Moissanite both from rock samples of the primary sources for gem and heavy minerals (volcanic bodies) and from alluvial samples from throughout the Kishon River catchment (secondary sources for gem and heavy minerals). Pressure: There are no independent constraints on the pressure of formation for any of the moissanite occurrences, because synthetic SiC can be produced at atmospheric pressure, and is stable to high pressures. The eclogites from which we have separated moissanite crystallized at depths of 180-200 km and temperatures of 1000-1250 °C (Greau et al., 2011); the moissanite inclusions in diamonds probably represent similar pressures. The Mt Carmel samples probably crystallized at depths between 30 and 100 km (Griffin et al., 2016).
Record-Size Natural Moissanite Crystals Discovered in Israel GEMS & GEMOLOGY, SUMMER 2014, VOL. 50, NO.
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Shefa Yamim's moissanite findings distribution according to number of crystals found per sample:
SY Moissanite, examples of samples from different localities: Sample 059, Backfall breccia from Eocene rocks northern part of the Kishon catchment.
Sample 901, alluvial mini-bulk sample from the Mid Reach of the Kishon River.
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References:
Bai W, Robinson PT, Fang Q, Yang J, Yan B, Zhang Z, Hu XF, Zhou MF, Malpas J., (2000) The PGE and base-metal alloys in the podiform chromitites of the Luobasa ophiolite, southern Tibet. Can
Mineral 38:585–598. Di Pierro, S., Gnos, E., Grobety, B.H.,
Fritsch, E., Toledo, V., Antoinette, M., Record-Size Natural Moissanite Crystals Discovered in Israel GIA - GEMS & GEMOLOGY, SUMMER 2014, VOL. 50, NO. 2
Gnos, E., 2012. Material attached to SiC mineral grains (Shefa Yamim project). Internal Report by Edwin Gnos, October April 23rd, 2012.
Gnos, E., 2010. Modal composition of SiC-Bearing sample SY 059 (Shefa Yamim project). Internal Report by Edwin Gnos, March 29th, 2010.
Griffin, W.L., Gain, S.E.M., Adams, D.T., Huang, J-X., Saunders, M., Toledo, V., Pearson, N.J. and O’Reilly, S.Y., 2016. First terrestrial occurrence of tistarite (Ti2O3): Ultra-low oxygen fugacity in the upper mantle beneath Mt Carmel, Israel. Geology (in press, July 2016).
Karpov, G.A., Silaev, V.I., Anikin, L.P., Rakin, V.I., Vasil’ev, E.A., Filatov, S.K., Petrovskii, V.A. and Flerov, G.B., 2014, Diamonds and accessory minerals in products of the 2012-2013 Tolbachik fissure eruption. Journal of Volcanology and Seismology 8, 323-339.
Roup, A., Kalmanovich, E., Baykov, Y., Toledo V., 2009. Moissanite Discovery. Israel Geological Society, Annual Meeting, March 2009.
Leung, I.S., Guo, W., Freidman, I., and Gleanson, J. (1990) Natural occurrence of silicon carbide in a diamondiferous kimberlite from Fuxian. Nature, 346, 352–354.
Lyakhovich, V.V.,1980. Origin of accessory moissanite. International Geology Review 22, 961–970.
Schmidt, M.W., Gao, C., Golubkova, A., Rohrbach, A. and Connolly, J.A.D., 2014, Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH fluids. Progress Earth and Planetary Science 2014, 1:27.
Shiryaev, A.A., Griffin,W.L. and Stoyanov, E., 2011, Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations on origin. Lithos 122,152-164.
Yang, J.S., Meng,. F., Xu, S., Robinson, P.T., Dilek, Y., Makeyev, A.B., Wirth, R., Wiedenbeck, M. and Cliff, J., 2015, Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Research 27, 459-485.
Xu, S., Wu, W., Xiao, W., Yang, J., Chen, J., Ji, S., Liu, Y., 2008. Moissanite in serpentinite from the Dabie Mountains in China. Mineralogical Magazine, 72(4), 899–908.
Huang, J-X., Griffin, W.L., Martin, L., Toledo, V., O’Reilly, S.Y., 2016. Deep carbon: SiC in mantle-and mantle-generated rocks. Goldschmidt 2016 Conference Abstract.
