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Origin and consequences of non-stoichiometry in iron carbide Fe (sub 7) C (sub 3)

Feng Zhu, Jie Li, David Walker, Jiachao Liu, Xiaojing Lai and Dongzhou Zhang
Origin and consequences of non-stoichiometry in iron carbide Fe (sub 7) C (sub 3)
American Mineralogist (March 2019) 104 (3): 325-332


The Eckstrom-Adcock iron carbide, nominally Fe (sub 7) C (sub 3) , is a potential host of reduced carbon in Earth's mantle and a candidate component of the inner core. Non-stoichiometry in Fe7C3 has been observed previously, but the crystal chemistry basis for its origin and influences on the physical properties were not known. Here we report chemical and structural analyses of synthetic Fe (sub 7) C (sub 3) that was grown through a diffusive reaction between iron and graphite and contained 31 to 35 at % carbon. We found that more carbon-rich Fe (sub 7) C (sub 3) has smaller unit-cell volume, suggesting that excess carbon atoms substituted for iron atoms instead of entering the interstitial sites of closed-packed iron lattice as in FeCx steel. Carbon may be the lightest alloying element to substitute for iron. The substitution leads to a larger reduction in the unit-cell mass than the volume so that the carbon-rich end-member may be as much as 5% less dense than stoichiometric Fe (sub 7) C (sub 3) . If Fe (sub 7) C (sub 3) solidifies from Earth's iron-rich liquid core, it is expected to have a nearly stoichiometric composition with a compositional expansion coefficient of approximately 1.0. However, laboratory experiments using carbon-rich synthetic Fe (sub 7) C (sub 3) to model the inner core may overestimate the amount of carbon that is needed to account for the core density deficit.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 104
Serial Issue: 3
Title: Origin and consequences of non-stoichiometry in iron carbide Fe (sub 7) C (sub 3)
Affiliation: University of Michigan, Department of Earth and Environmental Sciences, Ann Arbor, MI, United States
Pages: 325-332
Published: 201903
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 41
Accession Number: 2019-024143
Categories: Solid-earth geophysicsGeophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Annotation: From the special collection on Physics and chemistry of Earth's deep mantle and core
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Columbia University, USA, United StatesUniversity of Hawaii at Manoa, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201914
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