Northwest Africa 4882

Algeria

Find: July 2007

Achondrite (brachinite)

History: Purchased by Greg Hupé in July 2007 from a dealer in Tagounite, Morocco.

Physical characteristics: Two dense, dark brown, broken rounded stones (2891 g and 206 g) with weathered fusion crust on some original exterior surfaces and thin desert varnish coatings on hackly broken surfaces.

Petrography: (A. Irving and S. Kuehner, UWS) Coarse-grained rock (mostly 0.2-0.8 mm) with protogranular texture, composed predominantly of olivine with subordinate green, Cr-bearing diopside, K-poor plagioclase, chromite, iron sulfide, and kamacite (partially altered to iron hydroxides). Plagioclase is interstitial to mafic silicates and is heterogeneous in distribution. Very fine-grained (2-10 µm), polyphase assemblages composed mostly of orthopyroxene, Ni-bearing pyrrhotite and Ni-free metal with variable amounts of fayalite and chromite occur around larger pyrrhotite grains within olivine, and also as small, isolated apparent inclusions within olivine.

Geochemistry: Olivine (Fa_35.0-35.2, FeO/MnO = 70.9-71.3), clinopyroxene (Fs_9.3Wo_47.1, FeO/MnO = 38.6, Cr₂O₃ = 0.76wt%, Al₂O₃ = 1.05 wt%), plagioclase (An_{32.1- 37.6}Or_0.3-0.5), chromite [Cr/(Cr + Al) = 0.717, Mg/(Mg + Fe) = 0.239, TiO₂ = 0.71 wt%, ZnO = 0.30 wt%]. Oxygen isotopes: (D. Rumble, CIW) Replicate analyses of acid-washed silicate material by laser fluorination gave, respectively, δ¹⁸O = 2.064, 2.095; δ¹⁷O = 4.368, 4.455; Δ¹⁷O = -0.234, -0.248 per mil.

Classification: Achondrite (brachinite). This specimen is very similar in external appearance, texture, and mineral compositions to NWA 4969, with which it may be paired (Wittke et al. 2008). It also is very similar to NWA 3151 in texture, olivine, and plagioclase compositions, and patterns of staining around metal (Irving et al. 2005). However, the clinopyroxenes in these two specimens have somewhat different compositions, and NWA 3151 contains much less plagioclase and lacks the distinctive polyphase assemblages found in NWA 4882. Specimens: A total of 20.4 g of sample and one polished thin section are on deposit at UWS. GHupé holds the main mass.

Submitted by: A. Irving, UWS.