Northwest Africa 12951 (NWA 12951)

(Northwest Africa)

Purchased: 2019 Aug

Classification: Primitive achondrite (Winonaite)

History: Three stones of similar size were purchased by Dr. Albert Jambon in August 2019 from a dealer in Nouakchott, Mauritania.

Physical characteristics: About 80% of the surface of each stone is wind ablated with prominent metal veins and wedges separating depressed silicate regions, but remnant regmaglypts can be discerned. Large exposed metal areas exhibit a visible Widmanstätten pattern as thin, shiny taenite bands separating millimetric kamacite with a greenish oxidized surface. The bottom (formerly buried) sides of the stones are covered with remnants of weathered fusion crust. Fine grained olivine and green pyroxene are observed in places where the crust is missing.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) Silicate-rich regions of the specimen have a metamorphic, triple grain junction texture (mean grainsize ~150 µm) and are composed of olivine, orthopyroxene, sodic plagioclase, kamacite, taenite and troilite. Cross-cutting metal-rich veins and patches are composed predominantly of kamacite and taenite.

Geochemistry: Olivine (Fa_1.1-1.3, FeO/MnO = 3-5, N = 5), orthopyroxene (Fs_6.5-6.9Wo_1.8-2.1, FeO/MnO = 9-10, N = 4), plagioclase (An_14.8-15.4Or_3.1-3.4, N = 2). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave, respectively, δ¹⁷O 1.969, 2.049, 2.119; δ¹⁸O 4.730, 4.887, 5.035; Δ¹⁷O -0.528, -0.531, -0.540 per mil.

Classification: Winonaite (metal-veined).

Specimens: 33.3 g including one polished thin section at UWB; remainder with Dr. A. Jambon.