Northwest Africa 5517 (NWA 5517)

Northwest Africa

Purchased: 2004

Classification: Ungrouped achondrite

History: Purchased from a dealer in Erfoud, Morocco, in 2004 by David DGregory.

Physical characteristics: The dark brown stone (369 g) is fully crusted with light to moderate desert ablation and minimal surface chemical weathering.

Petrography: (T. Bunch and J. Wittke. NAU) Fine-grained (<0.3 mm) with poikiloblastic texture, and composed of essentially equigranular olivine grains and clinopyroxene (Cr-diopside) oikocrysts enclosing small (< 0.03 mm) olivine chadacrysts. Orthopyroxene is absent. Mineral mode (in vol.%): olivine 56; clinopyroxene 42, metal+FeS 2. Most metal is stained by terrestrial weathering.

Geochemistry: Olivine grains and chadacrysts (Fa_12.3-12.6, N = 4; Cr₂O₃ = 0.51-0.65 wt %; CaO = 0.28 wt %; FeO/MnO = 22), clinopyroxene (Fs_6.0-6.2Wo_39.0-39.3, N = 3; Cr₂O₃ = 1.35 wt %; Al₂O₃ = 1.25 wt %; FeO/MnO = 10), metal (Ni = 0.7 to 1.3 wt %). Oxygen isotopes (D. Rumble, CIW): an acid-washed and metal-free sample was analyzed in triplicate by laser fluorination and gave, respectively, δ¹⁷O = 2.570, 2.651, 3.117; δ¹⁸O = 6.618, 6.807, 7.681; Δ¹⁷O = -0.911, -0.929, -0.923 (all per mil).

Classification: Achondrite (ungrouped). Mineral and oxygen isotopic compositions indicate that this specimen has affinities to acapulcoites, but the poikiloblastic texture, complete lack of orthopyroxene and paucity of metal are anomalous features.

Specimens: 20.1 g including one polished thin section at UWB. The main mass is held by DGregory.