Northwest Africa 6704 (NWA 6704)

Algeria

Purchased: Jan-May 2011

Classification: Ungrouped achondrite

History: A single, dense, yellowish-green stone was found in pieces in Algeria, and was purchased in February 2011 at the Tucson Gem and Mineral Show and over subsequent weeks from Moroccan dealers by Greg Hupé.

Physical characteristics: The reassembled 42 pieces (plus some granular debris) fit together as a single ovoid mass (8387 g) with rounded, shiny exterior surfaces and small patches of remnant black fusion crust. The very fresh interior is mostly pale yellowish green with sporadic darker brownish grains and sparsely distributed tiny grains of opaque oxide and silvery metal.

Petrography: (A. Irving and S. Kuehner, UWS): Overall medium grained with an igneous cumulate texture. Small grains of olivine (0.5-0.8 mm) and chromite (0.1-0.6 mm) enclosed within large (to 4 mm) orthopyroxene oikocrysts, which are in turn surrounded by large optically continuous, intercumulus grains of untwinned albite and <0.1 mm awaruite. The silicates contain curvilinear trains of tiny rounded inclusions (2-20 μm), which appear on polished surfaces to be empty bubbles with smooth rounded walls.

Geochemistry: Olivine (Fa_51.6-53.2; FeO/MnO=120-140; NiO=0.9-1.0 wt.%), orthopyroxene (Fs_41.6-42.4Wo_2.8-3.6, FeO/MnO=81-82; Cr₂O₃=0.3 wt.%), plagioclase (Ab_92-93An_4-3Or₄). Oxygen isotopes (R. Tanaka, OkaU): replicate analyses of acid-washed material by laser fluorination gave: δ¹⁷O 1.015, 0.880; δ¹⁸O 3.922, 3.613; Δ¹⁷O -1.048, -1.020 per mil.

Classification: Achondrite (ungrouped). This specimen is unlike other achondrites in its combined features: relatively ferroan mafic silicate minerals with elevated FeO/MnO ratios and anomalous Ni contents, extremely sodic plagioclase, very Ni-rich metal, and oxygen isotopic composition that plots within the field for acapulcoites-lodranites. Weathering is low and limited to minor coatings of pale orange desert dust on broken surfaces. Unshocked.

Specimens: A total of 20.5 g of type material and two polished thin sections are on deposit at UWS. The remaining material is held by GHupé.