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

Satoru Urakawa, Keiko Someya, Hidenori Terasaki, Tomoo Katsura, Syo Yokoshi, Ken ichi Funakoshi, Wataru Utsumi, Yoshinori Katayama, Yu ichiro Sueda, Tetsuo Irifune

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46 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)469-479
Number of pages11
JournalPhysics of the Earth and Planetary Interiors
Issue number1-2
Publication statusPublished - Jun 15 2004


  • Bulk modulus
  • Iron sulfide
  • Martian core
  • Spin transition
  • Thermal expansion

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science


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