Moissanite, α-SiC, has been reported from a variety of geological occurrences

Moissanite, α-SiC, has been reported from a variety of geological occurrences (cf. review by Lyakhovich, 1980), including in mineral separates from kimberlites and as inclusions in diamonds. The earlier reports from kimberlite were controversial because of the possibility of contamination during crushing and processing of the mineral separates. The discovery of moissanite in inclusions in diamond (Klein-BenDavid et al., 2007b; Leung, 1990; Leung et al., 1996; Moore and Gurney, 1989; Otter and Gurney, 1989) and studies that excluded any contact of bulk kimberlite samples with carbide (Leung et al., 1990; Mathez et al., 1995; Shiryaev et al., 2011) has confirmed the presence of SiC in diamond and kimberlite. Stability of SiC requires very reducing conditions, with fO2 approximately four log units below the IW buffer (Mathez et al., 1995; Ulmer et al., 1998) for plausible reactions that might control SiC stability in the upper mantle such as SiC + O2 = C + SiO2 or SiC + Mg2SiO4 + O2 = C + Mg2Si2O6 (Mathez et al., 1995; Ulmer et al., 1998; Woermann and Rosenhauer, 1985). The experimental study of Ulmer et al. (1998) confirmed the earlier calculations based on thermochemical data.

Such extremely reduced conditions contrast with the view of relatively oxidizing conditions in most regions of the upper mantle, with a progressive decrease in relative oxidation state with depth (cf. review by Frost and McCammon, 2008). Haggerty (1994) suggested SiC and its host protokimberlite magma originated at the core–mantle boundary. More recently, Trumbull et al. (2009) studied moissanite from ophiolites and suggested that it formed in the lower mantle. If the lower mantle is dominated by the mineral assemblage ferropericlase plus magnesium silicate perovskite, the reaction controlling the stability of moissanite would be SiC + MgO + O2 =MgSiO3 + C. This reaction would shift to higher fO2 in the Fe-bearing system because of the preferential partitioning of Fe into ferropericlase relative to perovskite (Fei et al., 1991; Kesson et al., 2002). Whether this effect is sufficient to stabilize SiC is not known, especially given the possible countervailing effect of decreasing the activity of carbon if the C-bearing phase is a carbide rather than diamond (e.g., Dasgupta and Hirschmann, 2010).

An alternative is that moissanite forms in select ‘microenvironments’ that do not reflect ‘normal’ mantle conditions. Mathez et al. (1995) suggested that SiC may have formed by metamorphism of reduced, carbon-bearing sediments during subduction. Ulmer et al. (1998) argued for local regions at >300 km depth that are sufficiently reduced to stabilize SiC. The observation of moissanite in cavities in diamonds that form via dissolution (Klein-BenDavid et al., 2007b) supports the idea that precipitation of moissanite is a response to a local situation. Shiryaev et al. (2011) proposed that moissanite forms by electrochemical deposition from carbonate–silicate melts in the locally reduced environments in the deep lithosphere.

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• Moissanite vs Diamond