Fe2S: The Most Fe‐Rich Iron Sulfide at the Earth's Inner Core Pressures

We examined the phase relation of Fe2S to 306 GPa and 3,000 K and found that Fe2S forms an orthorhombic structure with space group Pnma above 190 GPa. Reexamination of previous X‐ray diffraction data demonstrated that hexagonal close‐packed Fe coexists with Pnma Fe2S at >2,700 K and ~290 GPa, while it concurs with CsCl (B2)‐type stoichiometric FeS at lower temperatures. Our results indicate that Fe2S is the most Fe‐rich iron sulfide above ~250 GPa where Fe3S is not stable. It is most likely that eutectic melting occurs between Fe and Fe2S at 330 GPa corresponding to inner core boundary conditions, instead of between Fe and Fe3S below 250 GPa. The compression curve of Fe2S obtained to 294 GPa shows that it is not dense enough to account for the inner core density, suggesting that the Fe‐Fe2S eutectic liquid gives the maximum sulfur concentration in the outer core. Plain Language Summary The chemistry of the Earth's core has long remained enigmatic; the observed density and velocity profiles indicate that the core includes some light alloying elements. The present study focuses on sulfur, one of the major candidates, and experimentally investigates the phase relations in the Fe‐FeS system under high pressures relevant to the core. The main finding of this work is that 1) Fe2S adopts the orthorhombic Pnma structure above 190 GPa and 2) it is the most iron‐rich sulfide at >250 GPa where Fe3S decomposes. Our data also show that Fe2S is much lighter than the observed inner core density, suggesting that sulfur concentration in the liquid core is not high enough to crystallize Fe2S. Key Points Fe2S undergoes a phase transition to the orthorhombic Pnma phase above 190 GPa Fe and Fe2S form a eutectic system above ~250 GPa, where Fe3S is not stable Pnma Fe2S is not dense enough to account for the inner core density