Schematic illustration of the core-segregation process. The iron core is formed from sinking iron droplets with dissolved Si, O, and S. The Si, O, and S content of the droplets is dictated by the solubility of these elements in liquid iron at the corresponding temperature and pressure conditions. As the mantle solidifies, the lowermost pressure level at which equilibration takes place moves outward. This pressure corresponds to the liquidus of the silicates. In the single-stage core-segregation model, the equilibration is assumed to have taken place at an average pressure of P_CS, which is treated as a free parameter.