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.
We thank Ed Scott for discussions and insightful comments. We thank Alex Ruzicka, Cyrena Goodrich and Tim McCoy for their constructive reviews.
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.
Armstrong, J.T. (1995) CITZAF: A package of correction programs for the quantitative electron beam X-ray analysis of thick polished materials, thin films, and particles. Microbeam Analysis, 4, 177–200.Search in Google Scholar
Buchwald, V.F. (1975) Handbook of Iron Meteorites. University of California Press, Berkeley. http://evols.library.manoa.hawaii.edu/handle/10524/33750Search in Google Scholar
Fang, C.M., Sluiter, M.H.F., van Huis, M.A., Ande, C.K., and Zandbergen, H.W. (2010) Origin of predominance of cementite among iron carbides in steel at elevated temperature. Physical Review Letters, 105, 055503.10.1103/PhysRevLett.105.055503Search in Google Scholar
Goldstein, J.I., Huss, G.R., and Scott, E.R.D. (2017) Ion microprobe analyses of carbon in Fe-Ni metal in iron meteorites and mesosiderites. Geochimica et Cosmochimica Acta, 200, 367–407.10.1016/j.gca.2016.12.027Search in Google Scholar
Goodrich, C.A., and Bird, J.M. (1985) Formation of iron-carbon alloys in basaltic magma at Iivfaq, Disko Island: The role of carbon in mafic magmas. Journal of Geology, 93, 475–492.10.1086/628967Search in Google Scholar
Goodrich, C.A., Ash, R.D., Van Orman, J.A., Domanik, K., and McDonough, W.F. (2013) Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis. Geochimica et Cosmochimica Acta, 112, 340–373.10.1016/j.gca.2012.06.022Search in Google Scholar
Goodrich, C.A., Harlow, G.E., Van Orman, J.A., Sutton, S.R., Jercinovic, M.J., and Mikouchi, T. (2014) Petrology of chromite in ureilites: Deconvolution of primary oxidation states and secondary reduction processes. Geochimica et Cosmochimica Acta, 135, 126–169.10.1016/j.gca.2014.02.028Search in Google Scholar
Keller, L.P. (1998) A transmission electron microscope study of iron-nickel carbides in the matrix of the Semarkona unequilibrated ordinary chondrite. Meteoritics & Planetary Science, 33, 913–919.10.1111/j.1945-5100.1998.tb01696.xSearch in Google Scholar
Krot, A.N., Zolensky, M.E., Wasson, J.T., Scott, E.R.D., Keil, K., and Ohsumi, K. (1997) Carbide-magnetite assemblages in type-3 ordinary chondrites. Geochimica et Cosmochimica Acta, 61, 219–237.10.1016/S0016-7037(96)00336-5Search in Google Scholar
Leineweber, A., Shang, S., Liu, Z., Widenmeyer, M., and Niewa, R. (2012) Crystal structure determination of Hägg carbide, c-Fe5C2 by first-principles calculations and Rietveld refinement. Zeitschrift für Kristallographie—Crystalline Materials, 227, 207–220.10.1524/zkri.2012.1490Search in Google Scholar
Ma, C., and Rubin, A. (2019) Edscottite, IMA 2018-086a. CNMNC Newsletter No. 47, February 2019: 204. European Journal of Mineralogy, 31, 199–204.Search in Google Scholar
Scott, E.R.D. (1972) Geochemistry, mineralogy and petrology of iron meteorites. Ph.D. thesis, University of Cambridge, U.K.Search in Google Scholar
Scott, E.R.D., and Agrell, S.O. (1971) The occurrence of carbides in iron meteorites (abstract). Meteoritics, 6, 312–313.Search in Google Scholar
Scott, E.R.D., and Jones, R.H. (1990) Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites. Geochimica et Cosmochimica Acta, 54, 2485–2502.10.1016/0016-7037(90)90235-DSearch in Google Scholar
Simon, S.B., Sutton, S.R., Brearley, A.J., Krot, A.N., and Nagashima, K. (2019) The effects of thermal metamorphism as recorded in CO3.0 through CO3.2 chondrites. Lunar and Planetary Science, 50, abstract 1444.Search in Google Scholar
Taylor, G.J., Okada, A., Scott, E.R.D., Rubin, A.E., Huss, G.R., and Keil, K. (1981) The occurrence and implications of carbide-magnetite assemblages in unequilibrated ordinary chondrites (abstract). Lunar and Planetary Science, 12, 1076–1078.Search in Google Scholar
Wasson, J.T., and Kallemeyn, G.W. (2002) The IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts. Geochimica et Cosmochimica Acta, 66, 2445–2473.10.1016/S0016-7037(02)00848-7Search in Google Scholar
Weerasinghe, G.L., Needs, R.J., and Pickard, C.J. (2011) Computational searches for iron carbide in the Earth’s inner core. Physical Review B, 84, 174110.10.1103/PhysRevB.84.174110Search in Google Scholar
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