Northwest Africa 10784 (NWA 10784)

(Northwest Africa)

Purchased: 2015 May 1

Classification: HED achondrite (Eucrite, brecciated)

History: Found in the region of Er-Rich to the north of Errachidia.

Physical characteristics: One flat stone consisting of breccia, with fusion crust on the lower part protected from ablation by wind.

Petrography: (R. Hewins, S. Pont, B. Zanda, MNHNP) This is a breccia with lithic clasts clasts up to 4 mm in size. The main minerals are othopyroxene, augite and plagioclase, with a silica phase, chromite, ilmenite, zircon and rare kamacite both Ni-free and Ni-bearing. The grain size is up to 1.5 mm in the subophitic clast, with augite lamellae up to 80 μm in width. Plagioclase laths are more elongated in finer grained clasts, but there are also granoblastic clasts containing 50 μm grains with up to 2 μm exsolution lamellae. Exsolution lamellae in orthopyroxene are commonly bent and/or displaced up to 25 μm by fracturing. Large plagioclase grains have crystallographically controlled pyroxene inclusions, whereas much plagioclase in fine grained matrix is smooth and featureless except for vugs. The matrix varies from granoblastic to clasts embedded in smooth plagioclase. A vein of melt crosses the section and invades the matrix. It is a nanophyric intergrowth of 1 μm pyroxene and plagioclase crystals, though locally with plagioclase dendrites, containing clasts mainly of pyroxene, and blobs and schlieren of melted plagioclase. The meteorite contains traces of calcite and barite.

Geochemistry: Mineral compositions and Geochemistry: (R. Hewins and S. Pont, MNHNP) The pyroxene compositions are Fs_57.2Wo_2.2 to Fs_64.9Wo_4.9, and Fs_25.8Wo_41.7 to Fs_29.6Wo_44.1. Fe and Mn are equilibrated between the two pyroxenes, with atomic ratio 31.2±1.9 (n=13) plotting on the UNM eucrite line. Plagioclase is An_89.3-87.8Or_0.3-0.6. Chromite has ~49% Cr₂O₃.

Classification: (R. Hewins MNHN) Basaltic eucrite breccia. The meteorite contains anorthite, and pyroxene with Fe/Mn ratios matching eucrite, plus typical eucrite textures.

Specimens: The type specimen consists of 20.63 and 0.93 g at MNHNP. The main mass (937 g) is held by MHNN. Labenne retains 202.6 g and the rest (in the form of caliche crust, crumbs and saw dust) was discarded.