Carbon is ubiquitous in the Universe, and on Earth it circulates between the atmosphere, land, ocean, biological organisms and plants, and it is subducted from the shallow continental crust to the very deep mantle. Recently, the geological pathway for incorporating carbon into silicates was summarized by Alexandra Navrotsky [1]; this study combined existing geologic hypotheses and observations on carbon-bearing mineralogical assemblages with experimental work on Si-O-C nanoceramics [1]. Here, we present detailed observations on moissanite (SiC) collected from a Miocene volcanic tuff (Yizre’el Valley, Israel) associated with inter-plate alkaline basalt volcanism [2,3] and from metamorphic rocks of Triassic age in Southern Bulgaria [4]. The samples were studied with optical microscopy, Raman spectroscopy, synchrotron X‐ray Laue microdiffraction, scanning electron microscopy, focused ion beam, and transmission electron microscopy.

SiC from volcanic tuff of Israel. The crystals belong to 4H and 6H polytypes of hexagonal symmetry, wurtzite structure, with 4H-SiC as the dominant phase. Larger crystals of SiC (0.2-1.8 mm) contain nanometric inclusions of Si, and metal-silicides. Several metal-silicide phases were found: Si₅₈V₂₅Ti₁₂Cr₃Fe₂, Si₄₁Fe₂₄Ti₂₀Ni₇V₅Zr₃, and Si₄₃Fe₄₀Ni₁₇. Their crystal structure parameters match Si₂TiV₅ (cubic space group Pmm), FeSi₂Ti (orthorhombic space group Pbam), and FeSi₂ (orthorhombic space group Cmca) respectively. Since the silicide inclusions exhibit spherical morphology, this provides evidence of their crystallization from a melt. Our observations suggest that SiC formed because of an interaction of a small volume of reducing fluids (H₂O–CH₄–H₂–C₂H₆) and crustal materials (SiO₂) that were possibly available from the walls of the alkaline basalt reservoir. 

SiC from metamorphic rocks (black shell) of Bulgaria. Moissanite crystals ranging from 10–300 μm in size were found within 0.1–1.2 mm isolated clusters, filled with amorphous carbon and nanocrystalline graphite. They belong to 15R (rhombohedral) and 6H (hexagonal) polytypes. One prismatic crystal was found within them which exhibits an unusual concentric polytypical zoning. The core is polytype 15R with an intermediate zone of 6H and the outer rim is 3C-cubic. SiC crystals formed during the cooling of the metamorphic reservoir from 580 °C to ∼400 °C [4]. The SiC crystals associated with amorphous carbon and nanocrystalline graphite are not in equilibrium with the surrounding mineralogical assemblage, which crystallized under a high oxygen fugacity environment. Moissanites from terrestrial rocks show that their carbon has an organic origin, e.g. δ13 C ‰ = -20 to -30 [5]. Therefore, the geologic Si-O-C pathway suggests that any existing geological or organic carbon source that forms a reducing fluid may interact with Si-bearing rocks providing local “isolated” chemical conditions where SiC, Si, metal silicides, and nanographite can form. These refractory minerals do not equilibrate with other mineralogical assemblages that normally crystallize under high oxygen fugacity. Instead, they remain as indicators of the local reducing fluid/melt environments.