Fig. 6

Structure of an Fe–S–Si core in Mercury. Cross-section through Mercury’s core, illustrating possible layered Fe–S–Si cores, depending on the temperature distribution (temperature decreases from a to c). a The immiscibility of S- and Si-rich liquids (Sanloup et al. 2000; Morard and Katsura 2010) up to 14 GPa and the lower density Fe–S-rich phase compared with the Fe–Si-rich phase (Badro et al. 2007) leads to a segregation of S-rich liquids (with almost no Si) near the top of the core, with more Si-rich liquid materials (with almost no Si) at greater depths. For pressures higher than 14 GPa, a liquid layer with Fe–Si–S can, in principle, be present, but such a configuration is unstable, since Si liquids that are located higher up are denser and sink into the core, resulting in the final structure shown. b The smaller melting point depression of Fe–Si alloys (e.g., Kuwayama and Hirose 2004) compared with Fe–S alloys (e.g., Usselman 1975; Fei et al. 1997; Li et al. 2001; Stewart et al. 2007; Chen et al. 2008) suggests that a solid Fe–Si core grows first. Alloys of Fe and Si form a solid solution with small compositional differences between the liquid and solid (Kuwayama and Hirose 2004). c A solid FeS layer forms at the CMB even when the temperature decreases below 1600–1700 K