Effect of Si on Mg Alloying in Metallic Iron at High Pressures

Abstract The incorporation of magnesium (Mg) into early planetary cores has been proposed as a source of compositional buoyancy through subsequent MgO precipitation. In the dissociation framework, Mg partitions independently of oxygen (O) into liquid Fe metal alloy, making the precipitation sensitive to the Mg budget. This study investigates the effect of silicon (Si), an important light element in the planetary cores, on the alloying behavior of Mg in iron (Fe) metal under high‐pressure and high‐temperature conditions. Using laser‐heated diamond anvil cells and synchrotron X‐ray diffraction, we observed the formation of ‐type MgFeSi at pressures below 25 GPa and 2000 K. At pressures above 50 GPa, Mg is found to be incorporated into the B2 phase of FeSi and Fe‐16Si, with a concentration of approximately 8 atom% (at.%) in FeSi and 5–10 at.% in Fe‐16Si. Compared with the nearly immiscible Fe‐Mg binary under ambient conditions and only about 4 at.% Mg in Fe metal reported around 20 GPa in previous studies, our results demonstrate that Si can enhance Mg incorporation in Fe‐rich alloys at lower pressures (therefore in smaller rocky planets), while in the stability field of bcc Mg alloyed amount appears to remain at least similar between Si‐bearing and Si‐free cases (therefore in larger rocky planets). Our result indicates that Mg can be an important light element (a few at.%) in the solid part of Fe metal‐rich cores if Si is present in the system.