Inner core as a two phase mixture of the Fe-Ni-Si alloys

The Earth's core is considered to be composed of iron-nickel alloys with some light elements. Silicon is one of the most convincing candidates among the light elements. Therefore, studies of the phase relation and physical properties at high pressure and temperature in the Fe-Ni-Si alloys are important in understanding the structure and chemistry of the Earth's core. In this study, the phase relations and the sound velocity of the Fe-Ni-Si alloys were investigated by using laser heated diamond anvil cells combined with in situ X-ray diffraction and inelastic X-ray scattering with synchrotron radiation at SPring-8. Our experiments revealed that existence of Ni modify the phase relation of the Fe-Si system reported by Fischer et al. [1, 2] significantly, and the stability field of two phase coexistence expands in the Fe-Ni and Fe-Ni-Si alloys, and the two phase mixture of hcp-phase and B2 phase is expected under the Earth's inner core conditions. The present result implies that inner core is composed of two phase mixture of hcp-Fe and B2 alloys. The fraction of B2 phase in the mixture increases with depth. Therefore, the heterogeneity of the inner core including change in intensity of anisotropy with depth [3] could be accounted for by the two phase mixture in the Fe-Ni-Si inner core.

References: [1] Fischer et al. (2013) Earth Planet. Sci. Lett., 373, 54-64. [2] Fischer et al. (2015) J. Geophys. Res. 119, 2810-2827. [3] Romanovich et al. (2016) Geophys. Res. Lett., 43 , 93 - 100, doi:10.1002/2015GL066734.