Edscottite, Fe5C2, a New Iron Carbide Mineral from the Ni-rich Wedderburn IAB Iron Meteorite

Astrogeology, Asteroids, Robots, Space Flight & Extraterrestrial Matters

Ma and Rubin reported in 2019 that 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 (Fe_4.73Ni_0.23Co_0.04)C_2.00. 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, _xFe₅C₂, which has a = 11.57 Å, b = 4.57 Å, c = 5.06 Å, β= 97.7 °, V = 265.1 Å3, and Z = 4. The calculated density using the measured composition is 7.62 g/cm^3.

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 the Wedderburn iron meteorite. The formation of edscottite rather than haxonite may have resulted from a lower C concentration in the iron meteorite and hence a lower growth temperature.

The authors concluded that the Wedderburn iron meteorite is a slowly cooled, Ni-rich iron meteorite that appears to have reached carbon supersaturation at a lower temperature than other irons due in part to a lower bulk carbon concentration. These conditions facilitated the rapid growth of edscottite laths within low-Ni iron (kamacite). Iron carbides (cohenite and haxonite) in other iron meteorites do not form laths and appear to have grown more slowly and at somewhat higher
temperatures. Edscottite may be restricted to slowly cooled Ni-rich iron meteorites like the Wedderburn iron meteorite.

The new mineral is named in honor of Edward (Ed) R.D. Scott, a pioneering cosmochemist at the University of Hawaii at Manoa, for his seminal contributions to research on meteorites.