The interiors of reduced rocky exoplanets are expected to differ significantly from their oxygen-rich counterparts due to variations in mineralogy. Carbon-rich exoplanets, in particular, are hypothesized to contain phases such as SiC, CaS, MgS, and graphite. The stability of SiC under planetary interior conditions remains uncertain, as it is predicted to oxidize to quartz at fO2 exceeding ΔIW - 6. Previous studies on metal-silicate partitioning have focused on sulfur, carbon, or silicon; however, the behaviour of quaternary mixtures of these light elements with iron under core-mantle differentiation conditions is poorly understood. Additionally, immiscibility between Fe-S and Fe-Si alloys has been observed at ambient pressure. Yet, its behaviour at high pressures and temperatures, particularly in the presence of carbon, remains unresolved.

In the present study, we address the gap in thermodynamic data for reduced phases at high-PT conditions. We investigate the phase equilibria and partitioning behaviour of carbon (C), sulfur (S), and silicon (Si) in the Fe-S-C-Si-O-Mg system under reducing conditions relevant to carbon-rich planetary interiors. Nebular condensation calculations were performed using FactSage (Equilib module), with host stars having C/O > 0.8 serving as proxies for the nebular disk. A Gaussian feeding zone accretion model was used to calculate planetesimal bulk compositions, which were then subjected to high-pressure, high-temperature experiments.

Using an end-loaded piston-cylinder press, experiments conducted at 1400°C and 1 GPa in graphite capsules reveal the formation of Mg-silicates, (Fe, Mg)S, and Fe-alloys rich in carbon and silicon. By mapping phase space across varying pressures, temperatures and oxygen fugacities, this study aims to determine the metal-silicate partitioning coefficients of S and Si in the presence of carbon. These results will inform thermodynamic models of interior composition and structure, enabling the derivation of mass-radius relationships for C-rich exoplanets.