Northwest Africa 7605 (NWA 7605)

Northwest Africa

Purchased: 2012

Classification: Primitive achondrite (Brachinite)

History: Purchased by Stefan Ralew in 2012 from a dealer in Erfoud, Morocco.

Physical characteristics: Dark brown, rounded stone with small areas of remnant fusion crust. Interior slices show the rock to be composed of black, interlocking elongate grains with a preferred orientation, plus elongate, lighter gray-colored oikocrysts (up to 15 mm by 5 mm in size).

Petrography: (A. Irving and S. Kuehner, UWS) Composed mostly of olivine with subordinate clinopyroxene, Ti-V-bearing chromite, pyrrhotite and rare kamacite, plus large oikocrysts of orthopyroxene + clinopyroxene enclosing ovoid olivine chadacrysts. All mafic silicates exhibit a strongly-oriented grain shape fabric. Intergrowths of pure iron metal + orthopyroxene are present along olivine grain boundaries and along microfractures within olivine grains.

Geochemistry: Olivine (Fa_26.1-26.2, FeO/MnO = 43-49; chadacrysts in pyroxene Fa_24.8-24.9, FeO/MnO = 43-52), orthopyroxene (oikocryst Fs_25.8Wo_2.5, FeO/MnO = 39, Cr₂O₃ = 0.2 wt.%; with metal on olivine margins Fs_21.8-21.9Wo_1.0-2.1, FeO/MnO = 28-30), clinopyroxene (Fs_10.2-10.6Wo_44.0-47.8, FeO/MnO = 23-31, Al₂O₃ = 0.7-0.8 wt.%; Cr₂O₃ = 0.7-0.9.%; oikocryst Fs_10.2Wo_43.6, FeO/MnO = 32, Al₂O₃ = 0.7 wt.%, Cr₂O₃ = 0.7 wt.%). Oxygen isotopes (K. Ziegler, UNM): analysis of an acid-washed sample by laser fluorination gave δ¹⁷O = 2.370; δ¹⁸O = 4.822; Δ¹⁷O = -0.176 per mil.

Classification: Brachinite. The mineralogy and oxygen isotopic composition of this specimen are similar to those of other brachinites, but the oriented grain fabric and large pyroxene oikocrysts are unusual.

Specimens: A total of 20 g of material and one polished thin section are on deposit at UWB. The main mass is held by Ralew.