Not all moissanites are created equal: New constraints on moissanite from metamorphic rocks of Bulgaria

Journal Article · Sat Sep 01 00:00:00 EDT 2018 · Earth and Planetary Science Letters

DOI:https://doi.org/10.1016/j.epsl.2018.07.009· OSTI ID:1479087

Machev, P. ^[1]; O'Bannon, E. F. ^[2]; Bozhilov, K. N. ^[3]; Wang, Q. ^[4]; Dobrzhinetskaya, L. ^[3]

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1. Sofia Univ. "St. Kliment Ohridski", Sofia (Bulgaria)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
3. Univ. of California at Riverside, Riverside, CA (United States)
4. Nanjing Univ., Nanjing (China)

Here, terrestrial moissanite (SiC) is widely reported as an ultra-high pressure mineral occurring in kimberlites, diamonds and ultramafic/mafic rocks of mantle origin. However, the conditions of crystallization remain largely unknown. Moreover, dozens of SiC occurrences have been reported from continental crust sources such as granitoids, andesite–dacite volcanic rocks and their breccia, metasomatic and metamorphic rocks, and even limestones. The validity of many of these reports is still debated primarily due to possible contaminations from the widespread use of synthetic SiC abrasives in samples preparation. Indeed, reports of well-documented in-situ occurrences of moissanite in association with co-existing minerals are still scarce. The only condition of moissanite formation that is agreed upon is that extremely reducing media are required (e.g. 4.5–6 log units below the iron-wustite buffer). Here, we report the new occurrence of moissanite that was found in-situ within the garnet–staurolite–mica schists of Topolovgrad metamorphic group of Triassic age in Southern Bulgaria. The 10–300 μm moissanite crystals are situated within 0.1–1.2 mm isolated clusters, filled with amorphous carbon and nanocrystalline graphite. Most of moissanite crystals are 15R (rhombohedral) and 6H (hexagonal) polytypes, and one prismatic crystal, found within them, exhibits unusual concentric polytypical zoning with core (15R), intermediate zone (6H) and rim (3C-cubic). Experimental data show that this type of polytypical zonation is likely due to a decrease in temperature (or/and pressure?) and changes in Si/C ratio. Indeed, amphibolite facies metamorphism (500–580°C – garnet–staurolite zone) followed by a subsequent cooling during the retrograde stage of green schist facies metamorphism (~400–500°C) could have provided a change in temperature. The SiC containing clusters exhibit evidence that they are pre-metamorphic, and we hypothesize that their protolith was a “black shale” material likely rich in carbon, hydrocarbon and terrigenous silica. The latter served as a source of isolated chemically-reduced media, which is required for SiC formation. Other concepts to explain moissanite occurrences in metasedimentary rocks are also discussed. Importantly, our findings show that the formation conditions of moissanite are likely more variable than previously recognized.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344; EAR-1620423; 41590623
• OSTI ID: 1479087
• Alternate ID(s): OSTI ID: 1603396
• Report Number(s): LLNL-JRNL-750652; 936184
• Journal Information: Earth and Planetary Science Letters, Vol. 498, Issue C; ISSN 0012-821X
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
moissanite (SiC)
metamorphic rocks
polytypical zonation of SiC
15R-SiC
6H-SiC
3C-SiC polytypes