Northwest Africa 12540 (NWA 12540)

(Northwest Africa)

Purchased: 2018 Nov

Classification: HED achondrite (Eucrite, anomalous)

History: Purchased by Aziz Habibi in November 2018 from a Mauritanian dealer.

Petrography: (A. Irving and S. Kuehner, UWS; P. Carpenter, WUSL) Recrystallized breccia containing ~8 vol% (estimated from BSE imagery) of Fe-Ni metal (kamacite with taenite). Precursor angular grains of exsolved pigeonite and calcic plagioclase are discernible, but much of the plagioclase has been recrystallized into microgranular aggregates. Accessory phases include silica polymorph, ilmenite and troilite. Magnetic susceptibility log χ (× 10^-9 m³/kg) = 4.25 is more than one order of magnitude higher than in the usual eucrites (log χ (× 10^-9 m³/kg) = 2.95±0.30, n=70, Rochette et al. 2009) and in agreement with the observed metal content.

Geochemistry: Orthopyroxene host (Fs_46.3-48.8Wo_4.6-1.6, FeO/MnO = 25-26, N = 3), clinopyroxene exsolution lamellae (Fs_19.5-22.9Wo_42.7-38.8, FeO/MnO = 23-25, N = 3), plagioclase (An_88.3-89.8Or_0.3), kamacite (Ni = 4.3 wt.%), taenite (Ni = 51.0 wt.%). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave δ¹⁷O 1.778, 1.909, 1.786; δ¹⁸O 3.911, 4.120, 3.936; Δ¹⁷O -0.287, -0.266, -0.292 per mil.

Classification: Eucrite (anomalous, annealed metal-rich breccia). This specimen has an unusually high amount of Ni-bearing metal and also exhibits extensive post-brecciation annealing textures.

Specimens: 26.8 g including one polished thin section at UWB; remainder with Mr. A. Habibi.