The crystallization regime of Mercury’s core controlled by Silicon?

Mercure © Pixabay
Mercury being so close to the Sun, the environment of formation of the planet is considered to be depleted in oxygen, and thus very reducing, leading to the presence of significant quantities of silicon alloyed with iron in its core (Fe-Si alloy). However, the phase diagram of the Fe-FeSi system remained poorly constrained to the high pressure and high temperature conditions prevailing at the center of Mercury

A team of researchers, led mainly by members of IMPMC and ISTerre, have reconstructed the phase diagram of the Fe-Si binary system from in situ X-ray diffraction measurements (on the P02.2 light line of the German synchrotron PETRA) at the conditions expected for the Mercury core.

Moreover, the samples were recovered after the experiments and the chemical composition of the different phases studied ex situ. Under high pressure, we observe the formation of a phase with a centered cubic structure at temperatures close to fusion, and during fusion, the studied alloys evolve towards two distinct liquid compositions, one rich and the other poor in Fe. The evolution of the phase diagram with pressure and temperature thus prescribes very different possible crystallization regimes of the Mercury core, depending on the exact Si abundance in the core.
a) Fe-Si diagram reconstructed using high pressure and high temperature experiments. The arrows indicate two possible compositions for the Mercury core. B) Different crystallization scenarios of the Mercury core corresponding to different Si contents.

Référence

Edmund, E., Morard, G., Baron, M.A., A. Rivoldini, S. Yokoo, S. Boccato, K. Hirose, A. Pakhomova & D. Antonangeli. The Fe-FeSi phase diagram at Mercury’s core conditions. Nature Communications (2022). DOI

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– Guillaume Morard, ISTerre