Stability of SiC polytypes at mantle conditions: constraints on the origin of natural moissanite

Although silicon carbide (SiC, moissanite) is rarely found in geological samples, it may provide essential insights into the global volatile cycle and redox state of the deep Earth. The stability of SiC polytypes under high-pressure conditions has large potential for understanding the structure and dynamics of the interior of the Earth and planets. However, the stability of the polytypes remains poorly constrained, as the strong kinetic barrier hinders precise determination of their stability at high temperatures and pressures. Here, we studied the stability of 6H- and 3C-SiC under deep-Earth and planetary conditions using a Kawai-type multi-anvil apparatus and examined the kinetics of the phase transition at 5 – 20 GPa and 1500 – 2500 K over 1 – 4320 minutes. We determined the phase abundances as a function of heating duration under high-pressure conditions. Although the transformation kinetics were very sluggish, we clearly observed the phase transition from 6H- to 3C-SiC at 5 – 20 GPa and 2000 – 2500K, achieving high transformation ratios of up to 70 % after 60 minutes of heating. In contrast, the transformation ratio was nearly 0 % at 1500 K and 15 – 20 GPa, regardless of heating duration up to 4320 minutes. We re-determined the stability of SiC polytypes, reconciling all plausible previous results by taking heating duration into account. Current results show that 6H-SiC is stable over a broader range of pressure and temperature than previously reported. This is consistent with the dominant 6H polytype population in natural SiC, contrary to prior knowledge indicating a wide stability pressure-temperature range of 3C-SiC. Rarely found 3C-SiC may originate from a deep subducted slab at least above approximately 20 GPa. This imposes a fundamental constraint on the formation of natural SiC and suggests that SiC polytypes can serve as indicators not only of redox conditions but also of formation depth.