Fe5S2 identified as a host of sulfur in Earth and planetary cores

Abstract

Cosmochemical considerations suggest that sulfur is a candidate light alloying element in rocky planetary cores, such that the high pressure-temperature (P-T) Fe-S phase relations likely play a key role in planetary core crystallization thermodynamics. The iron-saturated Fe-S phase relations were investigated to 200 GPa and 3250 K using combined powder and single-crystal X-ray diffraction techniques in a laser-heated diamond anvil cell. Upon heating at 120 GPa, I-4 Fe3S is observed to break down to form iron and a novel hexagonal Fe5S2 sulfide with the Ni5As2 structure (P6cm3, Z=6). To 200 GPa, Fe5S2 and Fe are observed to coexist at high temperatures while Fe2S polymorphs are identified with Fe at lower temperatures. An updated Fe-rich Fe-S phase diagram is presented. As this hexagonal Fe5S2 expresses complex Fe-Fe coordination and atomic positional disorder, crystallization of Fe5S2 may contribute to intricate elastic and electrical properties in Earth and planetary cores as they crystallize over time. Models of a fully crystallized Fe-rich Fe-S liquid in Earth's and Venus' core establish that Fe5S2 is likely the only sulfide to crystallize and may deposit in the outer third of the planets' cores as they cool. Fe5S2 could further serve as a host for Ni and Si as has been observed in the related meteoritic phase perryite, (Fe, Ni)8(P, Si)3, adding intricacies to elemental partitioning during core crystallization. The stability of Fe5S2 presented here is key to understanding the role of sulfur in the crystallization sequences that drive the geodynamics and dictate the structures of Earth and rocky planetary cores.