Northwest Africa 5400

Algeria

Find: June 2008

Achondrite (brachinite-like, ungrouped)

History: Purchased by Greg Hupé in June 2008 from a dealer in Er Rachidia, Morocco.

Physical characteristics: A single dense, dark brown, rounded stone (4818 g) is about half covered with weathered fusion crust and exhibits several narrow fractures. Dimensions are 19.5 cm × 16.0 cm × 9.7 cm. Petrography: (A. Irving and S. Kuehner, UWS) Coarse-grained rock (mostly 0.2–0.8 mm) with protogranular texture, composed predominantly of olivine (79 vol%) with subordinate orthopyroxene (10.5 vol%), clinopyroxene (8.9 vol%), chromite (1.4 vol%), Cl-rich apatite, troilite, kamacite (altered to iron hydroxides) and minor taenite. No plagioclase was observed despite a careful search.

Geochemistry: Olivine (Fa_30.1–30.4, FeO/MnO = 56.1–70.8), clinopyroxene (Fs_9.4Wo_45.1, FeO/MnO = 25.5, Cr₂O₃ = 0.74 wt%, Al₂O₃ = 0.74 wt%), orthopyroxene (Fs_24.4Wo_2.1, FeO/MnO = 34.2), chromite [Cr/(Cr+Al) = 0.821, Mg/ (Mg+Fe) = 0.236, TiO₂ = 1.4 wt%, ZnO = 0.30 wt%].

Oxygen isotopes (D. Rumble, CIW): clean, acid-washed silicate material from two separate portions of the type specimen analyzed in duplicate by laser fluorination at different times gave consistent results: respectively, δ¹⁸O = 5.061, 5.223, 4.486, 4.880; δ¹⁷O = 2.681, 2.741, 2.367, 2.525; Δ¹⁷O = 0.019, −0.006, 0.008, −0.042 per mil.

Classification: Achondrite (brachinite-like, ungrouped). In texture, mineral assemblage and mineral compositions, NWA 5400 is similar to brachinites, but the oxygen isotopic compositions plot essentially on the terrestrial fractionation line.

Specimens: A total of 22.4 g of sample and one polished thin section are on deposit at UWS. Mr. G. Hupé holds the main mass.