Northwest Africa 10785 (NWA 10785)

(Northwest Africa)

Purchased: 2015 May 1

Classification: Ungrouped achondrite

History: Purchased in Agadir, with no details on provenance.

Physical characteristics: A half-stone covered by fusion crust, with 2 cm yellow blocks in a whitish matrix. Recognized as similar to NWA 7835 by L. Labenne.

Petrography: (R. Hewins, S. Pont, B. Zanda, MNHNP) The dominant minerals are olivine and orthopyroxene, in a polished section that shows a blocky zone and a pseudo-porphyritic zone. Olivine grains in the blocky zone are at least 6 mm in maximum dimension, and are separated by patches and sparse veinlets of mesostasis. Olivine-orthopyroxene contacts are irregular, often with plagioclase (glass?) separating the two phases, with olivine-rich veinlets invading the pyroxene. Outside the blocky zone, angular grains of olivine and orthopyroxene are supported in a mesostasis with 10-20 μm roundish granules of orthopyroxene embedded in plagioclase probably glass. Minor minerals in the mesostasis are augite, chromite, taenite and troilite.

Geochemistry: Mineral compositions and Geochemistry: (R. Hewins, S. Pont, MNHNP and J.-A. Barrat, UBrest) The olivine is Fa_26.1±0.4, with FeO/MnO 53.8±3.4. The orthopyroxene composition is En_77.5±1.1Fs_21.0±0.7Wo_1.6±0.4, with Fe/Mn atomic ratio 38.9±3.3. "Plagioclase" is An_15.1±1.1Ab_80.6±0.6Or_4.3±0.7. These compositions are almost identical to those of NWA 7835 (Irving et al., 2014), though differences in the descriptions of the textures might mean different thermal history. Taenite contains ~37% Ni.

Classification: (R. Hewins MNHN, L. Labenne, Paris) (Harzburgitic) ungrouped achondrite. Possibly paired with NWA 7835.

Specimens: The main mass (57 g) is held by Labenne. The type specimen of 10.87 g and 3.74 g is at MNHNP.