Phase relationships and equations of state for FeS at high pressures and temperatures and implications for the internal structure of Mars

In situ X-ray diffraction experiments on FeS up to 22 GPa and 1600 K were carried out using large volume multianvil apparatus, combined with synchrotron radiation at SPring-8. We investigated phase stability relationships of FeS and determined the straight phase boundaries between FeS III (monoclinic phase) and FeS IV (hexagonal phase) to be T (K)=20P (GPa)+170 and between FeS IV and FeS V (NiAs-type phase) to be T (K)=39.6P (GPa)+450. We also found anomalous behavior in the c/a ratio, thermal expansion, and isothermal compression of FeS V as well as FeS IV, in the pressure range 4–12 GPa. These anomalies in FeS can be attributed to the spin-pairing transition of Fe, and divides FeS IV and FeS V into the high-spin low-pressure phase (LPP) and the possibly low-spin high-pressure phase (HPP). In order to investigate the internal structure of Mars, we evaluated the equations of state for FeS IV (HPP) and FeS V (HPP). A least square fit to the experimental data yielded K0T=62.5±0.9 GPa at T=600 K and (dK0/dT)P=−0.0208±0.0028 GPa/K for FeS IV (HPP), and K0T=54.3±1.0 GPa at T=1000 K and (dK0/dT)P=−0.0117±0.0015 GPa/K for FeS V (HPP) with fixed K′=4. Thermal expansion coefficients were α=7.16×10−5+6.08×10−8T for FeS IV (HPP) and α=10.42×10−5 for FeS V (HPP), respectively. Using these equations of state, we examined the internal structure of Mars that has a model mantle composition [Meteoritics 20 (1985) 367] and Fe–FeS core. Our models show that an Mg-silicate perovskite-rich lower mantle is stable only with the Fe-rich core having less than 20 wt.% sulfur. The polar moment of inertia factor C derived from Mars Pathfinder data [Science 278 (1997) 1749] is consistent with any compositions between Fe and FeS for the Martian core, but it excludes the presence of a crust thicker than 100 km.