Northwest Africa 2828

Mauritania or Algeria

Find: December 2005

Achondrite (aubrite)

History: Purchased in December 2005 by Greg Hupé from a Moroccan dealer in Tagounite.

Physical characteristics: Thirty-six pale gray to whitish stones lacking fusion crust with a total weight of 8672 g. Several stones have exterior light orange staining, and two stones contain one or two dark brown cross-cutting veins (1–2 mm wide) of magnetic, fine-grained iron oxide and hydroxide minerals.

Petrography: (A. Irving and S. Kuehner, UWS) Monomict microbreccia, which is mostly fairly uniform but contains sporadic small, rounded clasts (up to 4 mm across). Relatively fine but variable grain size (0.3–1.5 mm), and composed predominantly of bladed grains of essentially pure enstatite (exhibiting lamellar twinning) with ~15 vol% oligoclase, accessory altered troilite with fresh, subparallel exsolution blades of daubreelite, and sporadic rounded to ellipsoidal grains of graphite (up to 1.2 mm across). Small (<0.2 mm) lobate cavities partly filled with fine-grained calcite, silica, and an Fe-bearing mineral are present in the interior of even the freshest stones, and may represent former oldhamite grains. Small grains found as inclusions within enstatite are fresh Ti-free troilite, pure Mn-alabandite, daubreelite, fresh oldhamite (some Mn-bearing), schreibersite, and very rare specks of kamacite and taenite. Minor barite and calcite are present, probably the products of terrestrial weathering.

Mineral compositions and geochemistry: Pyroxene (En_99.8Wo_1.4; Al₂O₃ = 0.21 wt%), plagioclase (An_13.5–15.3 Or_3.0–4.4). Oxygen isotopes: (D. Rumble, CIW) Analyses of two whole-rock fragments by laser fluorination gave, respectively, δ¹⁸O = 5.50, 5.56; δ¹⁷O = 2.89, 2.90; Δ¹⁷O = +0.001, −0.026 (all ‰).

Classification: Achondrite (aubrite). Weathering effects in most stones are limited to alteration of interior troilite and probable oldhamite, and minor orange staining and dissolution on exterior surfaces. This aubrite appears to be completely different in appearance and texture from more metal-rich enstatite-rich meteorites NWA 002, NWA 1067, NWA 2736, and NWA 2965.

Type specimen: A total of 20.1 g and two polished thin sections are on deposit at UWS. GHupé; holds the main mass.