Iron, IIIE, octahedrite
standby for coopertown photo
Found 1860
36° 26′ N., 87° 0′ W. A single wedge-shaped iron meteorite weighing 16.8 kg was found by Mr. D. Crockett near Coopertown, Tennessee, and was sent to a professor for analysis (Buchwald, 1975). The fusion crust and heat-affected zone have been completely removed by terrestrial corrosion leaving an oxidized crust, while the interior is still well preserved.

Etched sections of Coopertown exhibit a coarse Thomson (Widmansttätten) structure composed of short, swollen kamacite lamellae having a bandwidth of 1.5 mm, 10–20% wider than would be expected based on its chemical composition (Buchwald, 1975). The Ni content is 8.47%, with P occurring throughout as schreibersite grains and rhabdites. Plessite fields constitute about 30% of the area and contain aggregates of the the very hard, white-colored carbide haxonite; haxonite has been partially or fully decomposed to graphite in those IIIE irons with more intense impact shock histories (Breen et al., 2015). Inclusions of troilite occur sparsely throughout, while hard, needle-shaped inclusions of the nitride carlsbergite are distributed in the matrix.

Resolution of this magmatic group from the similar IIIAB group is based on several factors including the swollen kamacite lamellae, the abundance of inclusions of carbide haxonite, the presence of carlsbergite normally associated with lower Ni levels, the unique Ga,Ge/Ni and Ga,Co/Au plots compared to other IIIAB members, and the cosmic-ray exposure age. However, strong similarities between these two iron groups, including N, C, and particularly Cu isotopic compositions (Bishop et al., 2012), indicate a genetic relationship exists between them indicating accretion in close proximity to each other in the early solar nebula.

Only fifteen iron meteorites have been classified to date as members of the IIIE group. Breen et al. (2015) initially assigned each of these irons to five separate categories based on degree of shock—weak, moderate, intermediate, strong, and severe, corresponding to the proximity of each meteorite from the point of impact. In a subsequent study involving all of the IIIE irons except two which have been artificially heated (Burlington and Paneth ’s Iron), Breen et al. (2016) distinguished only four shock categories—weak, moderate, strong, and severe—based on improved textural and compositional (sulfide, metal, and C-rich phases) parameters; Coopertown is assigned to the weakly shocked category. The photo above shows a 42.9 g etched partial slice of Coopertown.

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