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.
Acknowledgments
We thank Ed Scott for discussions and insightful comments. We thank Alex Ruzicka, Cyrena Goodrich and Tim McCoy for their constructive reviews.
Funding
Optical microscopy was done at UCLA and Caltech. SEM, EBSD, and EPMA were carried out at the Geological and Planetary Science Division Analytical Facility, Caltech, which is supported in part by NSF grants EAR-0318518 and DMR-0080065. This work was also supported by NASA grants NNX15AH38G and NNG06GF95G.
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