Moissanite (SiC) with metal-silicide and silicon inclusions from tuff of Israel: Raman spectroscopy and electron microscope studies

Here, we present studies of natural SiC that occurs in situ in tuff related to the Miocene alkaline basalt formation deposited in northern part of Israel. Raman spectroscopy, SEM and FIB-assisted TEM studies revealed that SiC is primarily hexagonal polytypes 4H-SiC and 6H-SiC, and that the 4H-SiC polytype is the predominant phase. Both SiC polytypes contain crystalline inclusions of silicon (Si^o) and inclusions of metal-silicide with varying compositions (e.g. Si₅₈V₂₅Ti₁₂Cr₃Fe₂, Si₄₁Fe₂₄Ti₂₀Ni₇V₅Zr₃, and Si₄₃Fe₄₀Ni₁₇). The silicides crystal structure parameters match Si₂TiV₅ (Pm-3m space group, cubic), FeSi₂Ti (Pbam space group, orthorhombic), and FeSi₂ (Cmca space group, orthorhombic) respectively. We hypothesize that SiC was formed in a local ultra-reduced environment at respectively shallow depths (60–100 km), through a reaction of SiO₂ with highly reducing fluids (H₂O–CH₄–H₂–C₂H₆) arisen from the mantle “hot spot” and passing through alkaline basalt magma reservoir. SiO₂ interacting with the fluids may originate from the walls of the crustal rocks surrounding this magmatic reservoir. This process led to the formation of SiC and accompanied by the reducing of metal-oxides to native metals, alloys, and silicides. The latter were trapped by SiC during its growth. Hence, interplate “hot spot” alkali basalt volcanism can now be included as a geological environment where SiC, silicon, and silicides can be found.