Northwest Africa 12542 (NWA 12542)

(Northwest Africa)

Purchased: 2019 Feb

Classification: Martian meteorite (Nakhlite)

History: Purchased by Ben Hoefnagels in February 2019 from Rachid Chaoui in partnership with a Mauritanian dealer.

Physical characteristics: A single fresh, black stone (1082 g) exhibiting oriented flow lines in remnant fusion crust on one side and portions of two other sides, and with small pits from desert wind ablation on other exterior surfaces.

Petrography: (A. Irving and S. Kuehner, UWS) Cumulate texture. Cumulus phases are larger grains of minimally zoned olivine (up to 2 mm) and chromian titanomagnetite (up to 1 mm) together with twinned, prismatic grains of augite (up to 2 mm long, with thin rims of ferroan augite). Intercumulus regions contain pyrrhotite, K-feldspar, intermediate plagioclase, ferrosilite, ferroan pigeonite, cruciform grains of titanomagnetite and very rare chlorapatite together with K-bearing glass. Secondary chlorophaeite-like material (orange in thin section) is present in some intercumulus regions and also filling fractures in olivine phenocrysts.

Geochemistry: Olivine (Fa_65.7-70.2, FeO/MnO = 50-55, N = 4), augite (cores Fs_22.8-28.9Wo_39.2-38.2, FeO/MnO = 34-37, N = 3; rim Fs_45.6Wo_40.2, FeO/MnO = 41), ferrosilite (Fs_54.9Wo_3.1, FeO/MnO = 36), ferroan pigeonite (Fs_65.2-66.1Wo_6.6-5.8, FeO/MnO = 23-28, N = 2), K-feldspar (Or_75.2Ab_23.6An_1.1), titanomagnetite (Al₂O₃ = 3.2 wt.%, Cr₂O₃ = 4.5 wt.%). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave δ¹⁷O 3.133, 2.927, 2.950; δ¹⁸O 5.463, 5.076, 5.096; Δ¹⁷O 0.249, 0.247, 0.259 per mil.

Classification: Nakhlite.

Specimens: 20.6 g including one polished thin section at UWB; remainder with Mr. B. Hoefnagels.