NORTHWEST AFRICA 5804

Iron, IAB-ungrouped
standby for northwest africa 5804 photo
Purchased 2005
no coordinates recorded

An unusual looking 726 g iron mass lacking regmaglypts and any remnant fusion crust was purchased by A. Gren at the Munich Mineral Fair in 2005. A preliminary analysis was conducted at the University of Hamburg, and the finding of zoned nickel provided the first indication that this mass was not a meteorwrong. A sample was sent to Northern Arizona University (T. Bunch and J. Wittke) and an analysis was made utilizing scanning electron microscopy and energy dispersive spectrometry (SEM–EDS).

This iron exhibits an unusual texture, containing light-colored Ni-rich taenite grains together with darker kamacite grains. Graphite in a wide range of sizes is present throughout. One formation scenario under consideration is that this meteorite experienced an impact-induced melting event followed by slow cooling. Recrystallization is evident, and this is either the result of a separate heating event or a further consequence of the initial impact event. Shock and terrestrial weathering are minimal.

With equipment limitations constraining investigators to bulk Ni content, and with no Thomson (Widmanstätten) structure to measure, it was ascertained that this iron might be a member of the nonmagmatic IAB iron-meteorite complex or could possibly belong to the IIF chemical group. Further petrographic and geochemical analyses were subsequently conducted at the University of Alberta, Canada (C. Herd and J. Duke), utilizing analytical techniques which can better measure the trace levels of Ni, Ir, Ga, and Ge used to distinguish among chemical groups. Although the Ni, Ga and Ge concentrations were found to be similar to the IAB complex, sLM/sLH subgroup (formerly IIICD), the concentration of Au is significantly less than 1.3 µg/g, the minimum abundance required for inclusion into the IAB complex. Therefore, NWA 5804 has been classified as a IAB-complex ungrouped iron.

The photo of the Gren iron meteorite shown above is a 4.569 g slice exhibiting a recrystallized texture with abundant graphite grains and aggregates. The photos displayed below are the main mass as found, a full cut face, and a close-up of the unusual texture of this meteorite.

gren iron
click on photo for a magnified view
gren iron
click on photo for a magnified view
gren iron
click on photo for a magnified view
Photos courtesy of Andreas Gren