HR: 0830h
AN: T11C-0399 [PDF]
TI: Equation of State of Iron-Silicon Alloys to Megabar Pressure: Implication for the Earth's
Core
AU: * Hirao, N
EM: hirao@ganko.tohoku.ac.jp
AF: Tohoku University, Institute of Mineralogy, Petrology and Economic Geology, Sendai, 980-8578
Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Tohoku University, Institute of Mineralogy, Petrology and Economic Geology, Sendai, 980-8578
Japan
AU: Kondo, T
EM: tdskondo@mail.tains.tohoku.ac.jp
AF: Tohoku University, Institute of Mineralogy, Petrology and Economic Geology, Sendai, 980-8578
Japan
AU: Kikegawa, T
EM: takumi.kikegawa@kek.jp
AF: High Energy Accelerator Research Organization (KEK), Photon Factory, Tsukuba, 305-0801
Japan
AB:
Iron is a dominant component in the Earth's core. Seismic data indicates that the core is less dense than pure iron at
pressures and temperatures of the core [Mao et al., 1990]. The density deficit of the Earth's core is explained by the
dissolution of one or more light elements, such as H, C, O, S, and/or Si, on the basis of geochemistry [Hillgren et al.,
2000]. Silicon has long been a favorite light element. A number of experimental and theoretical studies have been made in
order to estimate silicon solubility in iron under the core conditions, but these results are not consistent with each other
and give varying values of 0$\sim$20 wt.% of the silicon content in the core. Previous studies on Fe-Si system up to 270 GPa
have been conducted by the shock compression method [Balchan and Cowan, 1966] and there are no compressibility measurements
for the Fe-Si system by the static compression method under the core conditions. In order to understanding the stable phase
and equation of state relevant to the Earth's core, we have investigated the stable phase and pressure-volume equation of
state of iron-silicon alloys, Fe-8.7 wt.$%$ Si and Fe-17.8 wt.$%$ Si, using diamond-anvil cell (DAC) technique up to 196
GPa and 124 GPa, respectively. \\In situ high-pressure X-ray powder diffraction experiments were performed at the BL13A and
BL18C beamlines in the Photon Factory, National Laboratory for High Energy Physics (KEK). A body-centered cubic (bcc) Fe-8.7
wt.$%$ Si transformed to a hexagonal close-packed (hcp) structure at around 16$\sim$36 GPa, which is in good agreement with
previous work [Lin et al., 2002; 2003]. The high-pressure phase of hcp Fe-8.7 wt.$%$ Si was found to be stable up to 196 GPa
and no phase transition of bcc Fe-17.8 wt.$%$ Si was observed up to 124 GPa. The pressure-volume data was fitted to a
third-order Birch-Murnaghan equation of state with zero-pressure parameters: the zero-pressure volume \textit{V}$_{0}$ =
22.2(8) $\AA^{3}$, the isothermal bulk modulus \textit{K}$_{0}$ = 198(9) GPa, and the pressure derivative of bulk modulus
\textit{K'}$_{0}$ = 4.7(3) for hcp Fe-8.7 wt.$%$ Si and \textit{V}$_{0}$ = 179.41(45) $\AA^{3}$, \textit{K}$_{0}$ = 207(15)
GPa and \textit{K'}$_{0}$ = 5.1(6) for Fe-17.8 wt.$%$ Si. The bulk moduli of iron-silicon alloys obtained in the present
study are higher than those of the other iron compounds, such as FeS, FeO, and Fe$_{3}$C. The density and bulk sound velocity
of hcp Fe-8.7 wt.$%$ Si suggest that the iron inner core containing a few weight $%$ Si would satisfy the density and bulk
sound velocity of the seismological data (PREM). Lin et al. (2002) and Dubrovinsky et al. (2003) show that a Si-poor
hcp-structured phase appears at high pressure and high temperature. If the inner core contains silicon, it may be composed of
a hcp-structured phase with a few weight $%$ silicon and/or a Si-rich phase (bcc- and/or B2-structured).
DE: 1015 Composition of the core
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2003 Fall Meeting