Northwest Africa 12774 (NWA 12774)

(Northwest Africa)

Purchased: 2019 Jun

Classification: Angrite

History: The stone was purchased by Rachid and Jaouad Chaoui in March 2019 from a Mauritanian dealer, and in June 2019 the main mass was acquired from them at the Ensisheim Show by Marc Jost and Karl Wimmer.

Physical characteristics: A single dark, dense ellipsoidal stone (454 g) lacking fusion crust. The fresh interior exhibits small green crystals within a black matrix.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) Porphyritic texture. Euhedral, skeletal compositionally-zoned phenocrysts of olivine and subhedral phenocrysts of Al-Ti augite are set within a much finer grained, rapidly-cooled groundmass composed of parallel-grown anorthite, olivine, kirschsteinite and Al-Ti augite together with minor troilite, ulvöspinel and rare spherical grains of kamacite. Subhedral microphenocrysts of chromite are associated with olivine phenocrysts.

Geochemistry: Olivine (cores Fa_16.5-18.9Ln0.7, FeO/MnO = 88-111, N = 3; mantles Fa_34.1-44.7Ln1.1-1.5, FeO/MnO = 89-100, N = 5; rim Fa_69.5Ln4.3, FeO/MnO = 76), olivine in groundmass (Fa_76.2Ln4.8, FeO/MnO = 76), Al-Ti augite phenocrysts (Fs_17.9-19.5Wo_56.4-52.9, FeO/MnO = 64-77, Al₂O₃ = 17.0-17.7 wt.%, TiO₂ = 1.1-1.5 wt.%, N = 3), Al-Ti augite in groundmass (Fs_30.3-42.1Wo_50.8-53.6, FeO/MnO = 75-120, Al₂O₃ = 6.3-7.8 wt.%, TiO₂ = 2.2-5.3 wt.%, N = 2), anorthite (An_99.4-99.7Or_0.6-0.3, N = 2), kirschsteinite (Fa_61.5-61.7Ln35.5-35.8, FeO/MnO = 68-71, N = 2). Chromite (cores mg 0.53-0.58, cr 0.18-0.23, N = 3; rims mg 0.51-0.54, cr 0.10-0.11, N = 3). Bulk composition (J. Tepper, UPugetSound): analysis of a representative whole rock powder by ICP-OES gave (in wt.%) SiO₂ 39.4, Al₂O₃ 10.9, CaO 10.9, Na₂O 0.32, Mg/(Mg+Fe) = 0.0598.

Classification: Angrite (olivine-phyric). Although texturally similar to NWA 7812, this specimen has different mineral and whole rock compositions, and thus pairing between these stones is ruled out.

Specimens: 22.1 g including one polished thin section and a polished endcut at UWB; the main mass is held jointly by M. Jost and K. Wimmer.