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

  • Tetsuya Komabayashi EMAIL logo , Giacomo Pesce , Guillaume Morard , Daniele Antonangeli , Ryosuke Sinmyo and Mohamed Mezouar
Published/Copyright: January 2, 2019
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American Mineralogist
From the journal American Mineralogist Volume 104 Issue 1

Abstract

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.

Keywords: Earth’s core; high-pressure; diamond-anvil cell; internal resistive heating; Fe-Si alloy

Acknowledgments

The synchrotron experiments were performed at ID27 ESRF. Constructive comments by two anonymous reviewers improved the quality of the manuscript. This research was supported by the European Research Council (ERC) Consolidator Grant to T.K. (Earth core no. 647723).

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Received: 2018-05-16
Accepted: 2018-09-24
Published Online: 2019-01-02
Published in Print: 2019-01-28

© 2019 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2019-6636
Keywords for this article
Earth’s core; high-pressure; diamond-anvil cell; internal resistive heating; Fe-Si alloy

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. Iron carbide in the core
  3. Reconstruction of residual melts from the zeolitized explosive products of alkaline-mafic volcanoes
  4. Inefficient high-temperature metamorphism in orthogneiss
  5. Nitrogen incorporation in silicates and metals: Results from SIMS, EPMA, FTIR, and laser-extraction mass spectrometry
  6. Activation of [100](001) slip system by water incorporation in olivine and the cause of seismic anisotropy decrease with depth in the asthenosphere
  7. In-situ high-temperature vibrational spectra for synthetic and natural clinohumite: Implications for dense hydrous magnesium silicates in subduction zones
  8. Stability of the hydrous phases of Al-rich phase D and Al-rich phase H in deep subducted oceanic crust
  9. Minerals in cement chemistry: A single-crystal neutron diffraction study of ettringite, Ca6Al2(SO4)3(OH)12·27H2O
  10. Nature of hydrogen defects in clinopyroxenes from room temperature up to 1000 °C: Implication for the preservation of hydrogen in the upper mantle and impact on electrical conductivity
  11. Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores
  12. Cathodoluminescence features, trace elements, and oxygen isotopes of quartz in unidirectional solidification textures from the Sn-mineralized Heemskirk Granite, western Tasmania
  13. Controls on cassiterite (SnO2) crystallization: Evidence from cathodoluminescence, trace-element chemistry, and geochronology at the Gejiu Tin District
  14. Anomalous elastic behavior of phase egg, AlSiO3(OH), at high pressures
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  17. Letter
  18. Zinc transport in hydrothermal fluids: On the roles of pressure and sulfur vs. chlorine complexing
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  20. Book Review: Infrared and Raman Spectroscopies of Clay Minerals
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