EGU General Assembly 2022
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the Creative Commons Attribution 4.0 License.

Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores 

Suyu Fu1, Stella Chariton2, Vitali Prakapenka2, Andrew Chizmeshya3, and Sang-Heon Shim1
Suyu Fu et al.
  • 1Arizona State University , School of Earth and Space Exploration, United States of America (
  • 2Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA
  • 3School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA.

Light elements play a key role in the chemical and physical processes of planetary Fe-rich metallic cores [1].  H and Si are believed important candidates in planetary cores and previous estimates indicate as much as 0.6 wt% H and 13 wt% Si in the Earth’s core [2, 3]. However, existing studies are on Fe-H or Fe-Si binary systems and knowledge on Fe-Si-H ternary at high pressure and temperature is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. Fe-Si alloys with three compositions, Fe-9Si (9 wt% Si), Fe-16Si (16 wt% Si), and FeSi (33.3 wt% Si), reacted with H separately up to 125 GPa and 3700 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. Results show little H solubility in B20 and B2 phases of FeSi (0.3 wt% and <0.1 wt% H, respectively) up to 62 GPa, which is significantly smaller than H solubility in Fe metal (1.8 wt% H) [6]. The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. We found that the low-Si alloys (Fe-9Si and Fe-16Si) convert into FeHx (fcc or dhcp), FeSi (B20 or B2), and Fe-Si-H ternary phases up to 125 GPa and 3700 K. Particularly, a Fe5Si3Hx phase is stable below 43 GPa and the cubic FeH3 can appear after reactions above 100 GPa. These results indicate that H alters the behavior of the Fe-Si system severely. Considering the various sizes and masses of planets in the solar and exoplanetary systems, the planetary cores can have a wide range of Si contents. If Fe-droplets in early magma ocean contain much Si, Si could limit the amount of H incorporated in the core. On the other hand, for cores with low Si, crystallization at the solid-liquid core boundary may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially inducing heterogeneities in the region [7]. 


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How to cite: Fu, S., Chariton, S., Prakapenka, V., Chizmeshya, A., and Shim, S.-H.: Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3229,, 2022.