Iron-Silicon Alloy in Earth's Core?

Abstract

We have investigated the phase relations in the iron-rich portion of the iron-silicon (Fe-Si) alloys at high pressures and temperatures. Our study indicates that Si alloyed with Fe can stabilize the body-centered cubic (bcc) phase up to at least 84 gigapascals (compared to approximately 10 gigapascals for pure Fe) and 2400 kelvin. Earth's inner core may be composed of hexagonal close-packed (hcp) Fe with up to 4 weight percent Si, but it is also conceivable that the inner core could be a mixture of a Si-rich bcc phase and a Si-poor hcp phase.

The effect of silicon impurities on the phase diagram of iron and possible implications for the Earth's core structure

“…The parameters for pure iron [19][20][21][22] persists for about 20 GPa [4]. As can be seen in the figure, we calculate that 7.9% Si will increase the pressure of the pseudo-univariant transition by ~16 GPa, a value that falls within the experimental solid solution range.…”

Thermal expansion of iron-rich alloys and implications for the Earth's core

“…% (3 wt. %) (18,19,24,35). Assuming a linear relationship between density deficit and Si content, we estimated that, between 22.1 and 26.8 GPa, no more than 3.2 at.…”

“…Pairs of x-ray diffraction measurements were taken on both pure Fe and an Fe 0.864 Si 0.136 sample in the same cell assembly, within the pressure range of 21-27 GPa and the temperature range of 300 to Ͼ2,000 K. Within this pressure range and between 300 and 1,273 K, Fe adopts the hexagonal close-packed (hcp) structure, whereas Fe 0.864 Si 0.136 breaks down to a mixture of a Si-enriched bodycentered cubic phase and a Si-depleted hcp phase (24). These are proposed to be the stable forms of Fe and Fe-Si alloy in the Earth's inner core.…”

Ab initio theory of planetary materials

“…Recent experiments [165] have shown that at high pressures FeSi crystallises with the CsCl-structure (i.e. has identical atomic co-ordinates to bcc-Fe), and it has been found that at low concentrations Si favours the formation of bcc Fe over the hcp polymorph [166,167]. Vočadlo et al [160] therefore investigated the energetic effect of the substitution of S and Si in bcc and hcp Fe at representative core densities.…”