Northwest Africa 11917 (NWA 11917)

(Northwest Africa)

Purchased: 2012

Classification: Primitive achondrite (Winonaite)

Petrography: No chondrules are present, but there are irregularly shaped silicate clusters (50-700 µm in size) that may have been derived from recrystallized chondrule fragments. There is no clear igneous texture. Olivine exhibits mildy undulose extinction. Plagioclase also occurs. The rock contains ~20 vol.% metal plus sulfide. Opaque phases include kamacite, martensite, taenite, schreibersite, ferroan alabandite, and magnesiochromite. Metal and sulfide grains average ~100 µm in size. There are some grains of polycrystalline troilite and polycrystalline kamacite. There are rare 8x12-µm-size patches of daubreelite at the ede of some troilite grains. There are also rare ~40-µm-size metal grains with martensitic structure, rimmed by cloudy taenite. There is a single thick, elongated band of troilite at the edge of one thin section (~300 × 3000 µm).

Geochemistry: Also present are plagioclase (Ab_84.7±0.2Or_2.7±0.2; n=5); kamacite (in wt.%): 92.5 Fe, 0.48 Co, 0.06 P, 6.8 Ni, <0.03 Si; taenite: 73.5 Fe, 0.17 Co, <0.03 P, 23.5 Ni, <0.03 Si; troilite: 62.6 Fe, 37.0 S, 0.35 Cr; magnesiochromite: 67.3 Cr₂O₃, 5.8 MnO, 12.0 MgO, 6.2 FeO. The O-isotopic composition of replicate samples was analyzed by R. Greenwood (OU): δ¹⁷O = 1.876±0.006, δ¹⁸O = 4.541±0.011, Δ¹⁷O = -0.486±0.012 (per mil); this is in the winonaite range.

Classification: This is one of the most reduced winonaites, with very little FeO in the mafic silicates. The presence of some grains of polymict troilite and kamacite indicates moderately high shock, but the mildly undulose extinction in the olivine (characteristic of S2) suggests that the rock was shocked and then annealed, and perhaps (mildly) shocked again.