Static compression of iron-silicon alloys: Implications for silicon in the Earth's core

Abstract

Three iron-silicon alloys (Fe_85Si_15, Fe_71Si_29, and \epsilon-FeSi) have been studied in a diamond anvil cell at room temperature up to 55 GPa by in situ energy-dispersive X-ray diffraction techniques. A body centered cubic (bcc) to hexagonal close packed (hcp) phase transformation in Fe_85Si_15 began at 16 GPa and was completed by 36 GPa. No phase transformations were observed in either Fe_71Si_29 or \epsilon-FeSi at high pressures, even when laser-heated to about 2000 K. The isothermal bulk modulus (K_0T) of hcp-Fe_85Si_15 is 141 (+-10) GPa with K'_0T=5.70(+-0.60) and V_02=6.882(+-0.031) cm3/mol (per molar atom). The K_0T of Fe_71Si_29 is 199.0 (±5.3) GPa with K'_0T = 5.66(±0.61) and V_0 = 6.887(±0.014) cm3/mol, and the K_0T of \epsilon-FeSi is 184.7 (±3.9) GPa with K'_0T of 4.75 (±0.37) and V_0 = 6.790(±0.007) cm3/mol. Our study indicates that the substitution of Si into iron would lower the density of iron, but significantly changes its compressibility neither in the bcc phase, nor at high pressures in the hcp phase. Upon comparison with the Preliminary Reference Earth Model, the calculated equations of state (EOS) of hcp-Fe_85Si_15, using the Mie-Gru¨neisen EOS, indicate that an outer core containing about 8–10 wt.% Si and inner core containing about 4 wt.% Si in iron would satisfy the seismological constraints. Addition of silicon into iron increases the bulk sound velocity of iron, consistent with silicon being a light element in the Earth’s core.