The relatively high abundance of sulphur detected on the surface of Mercury suggests that the processes that operated during the separation of the planet's metal-rich interior into the core and the mantle and crust may be different from other terrestrial planets. Laboratory experiments suggest that the formation of sulphide minerals as Mercury's molten interior cooled may have kept sulphur and iron from being swept entirely into the planet's core.
Valérie Malavergne at the Université Paris Est Marne La Vallée, France, and colleagues melted samples of synthetic meteorites thought to be analogous to the building blocks of Mercury under pressure and temperature conditions that are believed to have occurred when the interior of the planet differentiated into the mantle and core. As the molten silicate cooled, sulphide minerals rich in magnesium and iron formed. At shallower depths, calcium-rich sulphides crystallized. The team proposes that the crystallization of magnesium- and calcium-rich sulphides, whose low density makes them unlikely to sink into the core, could result in the retention of sulphur and iron in the mantle and crust.
Compositional data from the MESSENGER orbiter lend further support to the idea that sulphur, iron and calcium are mostly stored as sulphides at Mercury's surface.
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Goldin, T. Saving sulphur. Nature Geosci 7, 328 (2014). https://doi.org/10.1038/ngeo2158