Edscottite, Fe5C2, a new iron carbide mineral from the Ni-rich Wedderburn IAB iron meteorite

Chi Ma 1  and Alan E. Rubin 2 , 3
  • 1 Division of Geological and Planetary Sciences, California Institute of Technology, 91125, Pasadena, U.S.A
  • 2 Department of Earth, Planetary, and Space Sciences, University of California, 90095-1567, Los Angeles, U.S.A
  • 3 Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Maine, U.S.A
Chi Ma
  • Corresponding author
  • Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, 91125, U.S.A
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and Alan E. Rubin
  • Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, 90095-1567, U.S.A
  • Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Maine, 04217, U.S.A
  • Search for other articles:
  • degruyter.comGoogle Scholar

Abstract

Edscottite (IMA 2018-086a), Fe5C2, is a new iron carbide mineral that occurs with low-Ni iron (kamacite), taenite, nickelphosphide (Ni-dominant schreibersite), and minor cohenite in the Wedderburn iron meteorite, a Ni-rich member of the group IAB complex. The mean chemical composition of edscottite determined by electron probe microanalysis, is (wt%) Fe 87.01, Ni 4.37, Co 0.82, C 7.90, total 100.10, yielding an empirical formula of (Fe4.73Ni0.23Co0.04)C2.00. The end-member formula is Fe5C2. Electron backscatter diffraction shows that edscottite has the C2/c Pd5B2-type structure of the synthetic phase called Hägg-carbide, c-Fe5C2, which has a = 11.57 Å, b = 4.57 Å, c = 5.06 Å, b = 97.7 °, V = 265.1 Å3, and Z = 4. The calculated density using the measured composition is 7.62 g/cm3. Like the other two carbides found in iron meteorites, cohenite (Fe3C) and haxonite (Fe23C6), edscottite forms in kamacite, but unlike these two carbides, it forms laths, possibly due to very rapid growth after supersaturation of carbon. Haxonite (which typically forms in carbide-bearing, Ni-rich members of the IAB complex) has not been observed in Wedderburn. Formation of edscottite rather than haxonite may have resulted from a lower C concentration in Wedderburn and hence a lower growth temperature. The new mineral is named in honor of Edward (Ed) R.D. Scott, a pioneering cosmochemist at the University of Hawai‘i at Manoa, for his seminal contributions to research on meteorites.

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