Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

1Tetsuya Komabayashi, 1Giacomo Pesce, 2Guillaume Morard,2Daniele Antonangeli,3,4Ryosuke Sinmyo,5Mohamed Mezouar
American Mineralogist 104, 94-99 Link to Article [https://doi.org/10.2138/am-2019-6636]
1School of GeoSciences and Centre for Science at Extreme Conditions, University of Edinburgh, EH9 3FE, U.K.
2Sorbonne Université, Muséum National d’Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
3Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
4Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
5 Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France
Copyright: The Mineralogical Society of America

The phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated diamond-anvil cell (DAC) under high-pressure (P) and high-temperature (T) conditions to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. Complementary laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase transition boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phases region is observed over a temperature range increasing with pressure, going from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the melting curve same as in Fe is assumed. This supports the idea that the hcp phase is stable at Earth’s inner core conditions. The stable structures of the inner cores of the other terrestrial planets are also discussed based on their P-T conditions relative to the triple point. In view of the reduced P-T conditions of the core of Mercury (well below the triple point), an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth’s core (far beyond the triple point). Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.

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