Phase diagram and thermo-elastic properties of Fe-S compounds up to 15 GPa: Thermodynamic constraints on the core of medium-sized telluric planets

The Fe-FeS binary is largely seen as the archetypal system to model the properties of the core of small to medium-sized telluric planetary bodies. Noteworthy, while both at the low pressures characteristic of the Moon, and at the very high pressure pertinent to the Earth or Venus, the Fe-FeS is a simple binary eutectic, in the intermediate range relevant for planets such as such as Mercury or Mars, or satellite such as Ganymede, Io, and Europa, the Fe-S phase diagram is quite complex, with intermediate compounds of narrow stability field that incongruently melt.Properties of Fe-FeS compounds have been here studied using in situ X-ray diffraction in the pressure range of 11–15 GPa, between room temperature to solidus (around ∼1100 K). Results show that Fe + FeS mixture transforms to Fe3S2 at 850 K and 12.1 GPa, adopting an orthorhombic crystal structure. Fitting the unit-cell volumes at 940 K to 2nd order Birch-Murnaghan and Vinet equation of state yields to V0 = 372.2 ± 0.8 or 367.3 ± 0.6 Å3 and K0 = 130 ± 4 or 185 ± 4 GPa, respectively, with K′ fixed to 4. The thermal expansion around 14 GPa has been estimated to be 26.6 × 10−5 K−1 based on the temperature evolution of the unit-cell volume from 850 K to 1100 K. Moreover, the lattice parameters of FeS were obtained in the range 11–15 GPa between 470 K and 1100 K, and the compressional behavior was studied.To model the properties of the solid portion of the cores of telluric planets or exoplanets in the 12–21 GPa range, Fe3S2 should be used as the end-member together with Fe or FeS depending on whether the S content is below or above 27 wt%. While Fe3S2 is a potential crystallizing product of S-rich telluric bodies, it is not expected to form in the core of Europa, Io, or Ganymede as too small, nor in that of Mercury as most likely too poor in S. When found in a meteorite, the Fe3S2 phase could be used to infer the size and temperature of the parent body.