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

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Claire Christine Zurkowski , Barbara Lavina, Abigail Case, Kellie Swadba, Stella Chariton, Vitali B. Prakapenka, Andrew J. Campbell


Cosmochemical considerations suggest that sulfur is a light alloying element in rocky planetary cores of varying sizes and oxidation states. High pressure-temperature (P-T) iron-sulfide phase relations therefore play a role in inner core crystallization and outer core thermochemical convection in a wide range of planetary bodies. The iron-saturated Fe-S phase relations were investigated to 200 GPa and 3250 K using single-crystal and powder X-ray diffraction techniques in a laser-heated diamond anvil cell. At high temperatures between 120 and 200 GPa, a hexagonal sulfide was synthesized with recrystallized hcp-Fe. The unit-cell parameters and diffraction intensities reveal an Fe5S2 stoichiometry adopting the Ni5As2 structure (P63cm, Z = 6). Fe5S2 is characterized by significant Fe-Fe bonding, complex coordination environments, and positional disorder and polytypism along the c-axis. Upon heating at 120 GPa, I-4 Fe3S is observed to break down into Fe5S2 + Fe, whereas with heating above 120 GPa, Fe2S + Fe reacts to form Fe5S2 + Fe at high temperatures. A C23–C37 Fe2S transition is observed above 130 GPa. These results establish that Fe5S2 is the stable Fe-rich sulfide across much of Earth’s outer core and will crystallize over an extensive depth up to the core-mantle boundary as the core cools overtime. The increased metal-metal bonding observed in Fe5S2 compared to the other high P-T iron sulfides would likely contribute to signatures of higher conductivity from regions of Fe5S2 crystallization. As Earth’s core is multicomponent, 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 inner 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.




Physical Sciences and Mathematics


Earth's core, planetary cores, iron alloys, iron sulfides, crystallography, high temperature, core processes


Published: 2021-10-22 22:18

Last Updated: 2021-11-26 09:29

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