Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations on origin

An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes of Yakutia has been characterized in terms of structural perfection, defects and the major- and trace-element chemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced by various methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (< 10%) show extremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning in impurities, C-isotope composition and cathodoluminescence brightness.Inclusions are heterogeneously distributed within the natural SiC; their size varies from a few nanometers to hundreds of microns. The most abundant inclusions in SiC are Si metal and iron silicide (FeSi2); a Si–C–O phase with stoichiometry close to Si4(C,O)7 probably is related to the silicon oxycarbides. FeSi2 commonly appears to have exsolved from Si metal; in some cases Ti metal then has exsolved from FeSi2 to form symplectites. Trace elements are strongly concentrated in the inclusions of FeSi2 and Si4(C,O)7. The trace-element patterns of these phases are generally similar in the different kimberlites, but there are some consistent minor differences between localities. The trace-element patterns of FeSi2 and Si4(C,O)7 are strongly enriched in LREE/HREE and are broadly similar to the patterns of kimberlites, carbonatites and some diamond-forming fluids. However, extreme negative anomalies in Eu (and Sm) suggest highly reducing conditions. Yb also shows strong negative anomalies in FeSi2 from all three localities, and in Si4(C,O)7 from Aikhal and Mir, but not in those from Udachnaya. Trace-element chemistry and the nature of the inclusions provide a reliable basis for distinguishing natural and synthetic SiC. Textural and chemical features and the presence of oxidation products (Si4(C,O)7 and SiO2) suggest that moissanite grew at high temperatures and elevated pressures and was subsequently partly oxidised, also at high T. Several important features of moissanite grains from kimberlites are consistent with the formation of natural SiC by electrochemical processes in carbonate-silicate melts.

Research Highlights
► Moissanite definitely differs from synthetic SiC in trace-element composition.
► Natural moissanite contains inclusions of silicides, Si metal and oxycarbides.
► Trace element patterns point to a relationship of SiC with diamond-forming fluids.
► Moissanite might have been formed by electrochemical processes.