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Title: Phase relations in the Fe-FeSi system at high pressures and temperatures

Abstract

The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9 wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200 GPa and ~145 GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42 GPa. The minimum temperature of an Fe–Si outer core is 4380 K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, onlymore » the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8 wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases.« less

Authors:
; ; ; ; ; ;  [1]
  1. Maryland
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
National Science Foundation (NSF)
OSTI Identifier:
1082535
Resource Type:
Journal Article
Journal Name:
Earth and Planetary Science Letters
Additional Journal Information:
Journal Volume: 373; Journal Issue: 07, 2013; Journal ID: ISSN 0012-821X
Country of Publication:
United States
Language:
ENGLISH

Citation Formats

Fischer, Rebecca A., Campbell, Andrew J., Reaman, Daniel M., Miller, Noah A., Heinz, Dion L., Dera, Przymyslaw, Prakapenka, Vitali B., and UC). Phase relations in the Fe-FeSi system at high pressures and temperatures. United States: N. p., 2016. Web. doi:10.1016/j.epsl.2013.04.035.
Fischer, Rebecca A., Campbell, Andrew J., Reaman, Daniel M., Miller, Noah A., Heinz, Dion L., Dera, Przymyslaw, Prakapenka, Vitali B., & UC). Phase relations in the Fe-FeSi system at high pressures and temperatures. United States. https://doi.org/10.1016/j.epsl.2013.04.035
Fischer, Rebecca A., Campbell, Andrew J., Reaman, Daniel M., Miller, Noah A., Heinz, Dion L., Dera, Przymyslaw, Prakapenka, Vitali B., and UC). 2016. "Phase relations in the Fe-FeSi system at high pressures and temperatures". United States. https://doi.org/10.1016/j.epsl.2013.04.035.
@article{osti_1082535,
title = {Phase relations in the Fe-FeSi system at high pressures and temperatures},
author = {Fischer, Rebecca A. and Campbell, Andrew J. and Reaman, Daniel M. and Miller, Noah A. and Heinz, Dion L. and Dera, Przymyslaw and Prakapenka, Vitali B. and UC)},
abstractNote = {The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9 wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200 GPa and ~145 GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42 GPa. The minimum temperature of an Fe–Si outer core is 4380 K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, only the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8 wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases.},
doi = {10.1016/j.epsl.2013.04.035},
url = {https://www.osti.gov/biblio/1082535}, journal = {Earth and Planetary Science Letters},
issn = {0012-821X},
number = 07, 2013,
volume = 373,
place = {United States},
year = {Fri Jul 29 00:00:00 EDT 2016},
month = {Fri Jul 29 00:00:00 EDT 2016}
}