Abstract:
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.
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