Micron-scale silicon carbide (SiC) precipitates are known to detrimentally impact silicon grown from the melt. Large precipitates reduce charge carrier lifetimes, lowering the efficiency of photovoltaic cells and can obstruct sawing wires while slicing silicon ingots into wafers, causing material losses. The size and structure of the β-SiC precipitates suggest that they form in the molten silicon phase during the solidification process, however the mechanisms and conditions underlying their formation are still not well understood. Atomistic simulations offer a unique way to study such mechanisms that might otherwise be experimentally unapproachable.A metastable, tetrahedrally coordinated low-density liquid (LDL) phase observed experimentally and in the Stillinger-Weber model of silicon was previously suggested to lower the nucleation free energy barrier of pure silicon, while it is unknown if it is involved in β-SiC nucleation at all. We investigate the formation of LDL in silicon liquids modeled by various interaction potentials including SW, Tersoff, and the Modified Embedded Atom Method. We develop a novel order parameter definition that can distinguish LDL from the high-density liquid (HDL) phase allowing us to directly measure it. Combined with a standard crystal structure order parameter in a multi-order parameter scheme, we calculate the homogeneous nucleation free energy surface (FES) for silicon using the SW and Tersoff potentials. The results reveal unambiguously how nucleation proceeds according to Ostwald’s step rule by first forming an LDL precursor that transitions to a crystallite that remains wetted by an LDL surface layer. The stability of the LDL phase is found to be the largest contributor to the free energy barrier. Next, we consider the silicon-carbon system and compute important properties relevant to solute precipitate nucleation: diffusion coefficients and solubility limits with respect to β-SiC crystallites of varying sizes. The dependence is found to follow the Ostwald-Freundlich relation with a Tolman correction on the interface free energy. Finally, we find that the silicon nucleation barrier increases in the presence of carbon impurities in the liquid. This is explained by the finding that carbons hinder the formation of LDL.