EQUATION OF STATE OF FE3SI AT HIGH PRESSURE AND ITS IMPLICATIONS FOR SILICON IN EARTH’S CORE

Silicon (Si) has been considered as one of the light elements in the Earth’s iron-dominated core to account for the density deficit and velocity discrepancy between pure iron and seismic value (Birch, 1952). Recent studies indicated the core could contain up to 6-8 wt.% Si (Fischer et al., 2014; Ozawa et al., 2016). To understand the effect of silicon incorporation in the Fe-rich core, knowledge on elastic property of Fe-Si alloys need to be extended to higher pressure and temperature conditions. Within the Fe-FeSi system, Fe₃Si and FeSi are the two most stable composition based on theoretical studies (e.g. Kiyokane et al., 2017). Fe₃Si is also proposed to be a candidate phase in Mercury’s core (Martin et al., 2015) and lithosphere (Strauss et al., 2016). Unlike the extensively studied FeSi with high pressure data for different structures, Fe₃Si only have two theoretical calculation results under high pressure (Fischer et al., 2014; Kiyokane et al., 2017). In this study, we conducted synchrotron X-ray diffraction experiments on Fe₃Si at high pressure in a diamond anvil cell (DAC) and room temperature condition. A third order Birch-Murnaghan equation of state fit to our XRD data yields equation of state is comparable to previous theoretical calculation. The closest well-studied member in Fe-FeSi system to Fe₃Si is Fe-16Si. Bulk modulus for Fe-16Si from literatures is smaller than our Fe₃Si result. This is consistent previous result that increase in Si content in the Fe-Si alloy will decrease the bulk modulus with similar crystal structures (Fischer et al., 2014). Further, using the Bayesian statistical methods for the equation of state fittings, the correlation between K₀ and K₀’ are determined. These results would provide new insights into the understanding of Fe-Si system in the Earth’s core and small rocky planets such as Mercury and Mars.