Northwest Africa 4861 (NWA 4861)

Northwest Africa

Purchased: July 2005

Classification: Iron meteorite (IAB complex)

History: An unclassified iron meteorite with a total mass of ~2.4 kg from the Rudis collection in Midelt, Morocco, was purchased by Mr. Edwin Thompson. One large slice and two smaller pieces including interior and exterior portions were donated to Cascadia for study.

Physical characteristics: (A. Ruzicka, Cascadia): The exterior surface is rounded and contains a dark, black to purple-tinted weathering patina with a sporadic occurrence of oxide minerals.

Petrography: (S. Kissin, LHU; A. Ruzicka, Cascadia): Cut and etched faces of the meteorite have a striking appearance, and show tapering kamacite spindles ranging from ~2-32 mm long and ~0.5-1.5 mm wide, set in a taenite-rich, fine-grained (ataxitic) groundmass. Clear-etching taenite zones surround the kamacite spindles. The spindles display three orientations on cut surfaces, as in an octahedrite, indicating that the meteorite a plessitic octahedrite. Coarse schreibersite is found, including acicular grains up to 25 mm long, which are surrounded by swathing kamacite. Minor troilite is present. No heat-affected zone is visible. Microhardness measurements yielded VHN = 276 mean (range 269-285) for the kamacite spindles; 209 mean (range 196-217) for the swathing kamacite; and 231 mean (range 178-259) for the microtextured matrix.

Geochemistry: (S. Kissin, LHU; A. Ruzicka, Cascadia): INAA was performed according to the methods of Wasson et al. (1998) at Activation Laboratories (Ancaster, Ontario), irradiating polished cubes of ~0.4 g and ~3.2 mm thick. This yielded the following elemental concentrations (all units μg/g except where noted): As (31.8), Au (3.03), Co (6.02 mg/g), Cr (9), Cu (145), Ga (76), Ge (350), Ir (0.458), Ni (119 mg/g), Pt (<0.1), Re (0.07), Sb (<20), W (<10). These data suggest assignment to the IAB complex, similar to subgroup sHL (high Au/Ni) of Wasson and Kallemeyn (2002), except that the new iron is significantly enriched in Ga and Ge (by ~4x and ~10x, respectively) compared to other IAB-sHL members. However, errors in the Ga and Ge values cannot be rules out.

Classification: Iron, plessitic octahedrite (IAB)

Specimens: Type specimens available at Cascadia consist of several pieces that collectively add to a total mass of 210.4 g. The main mass is held by Thompson.