Northwest Africa 8186 (NWA 8186)

(Northwest Africa)

Purchased: 2013

Classification: Ungrouped achondrite

History: Purchased from a dealer in Morocco in 2013 by Steve Witt and Mr. Smara Addi.

Physical characteristics: Single complete stone, slightly weathered, covered in black fusion crust, saw-cut reveals a fresh, fine-grained mix of green, white, and shiny metallic-luster crystals, also a black shock melt veinlet observed, friable.

Petrography: (C. Agee, UNM) Microprobe examination of a polished mount shows a highly equilibrated texture with numerous triple junction grain boundaries, significant fracturing present suggests high shock level. Olivine (up to 1 mm) is the dominant phase ~80%, plagioclase (up to 200 μm) ~10%, oxides (magnetite, hercynite-spinel, ilmenite; up to 200 μm) ~5%, augite ~3%, Cl-rich apatite ~1%, small amounts of Fe,Ni sulfide (most likely pentlandite), fusion crust (~150 μm thick) shows fine heterogeneous quench crystals of silicates and oxides.

Geochemistry: (C. Agee and H. Miley, UNM) Olivine Fa_34.8±0.4, Fe/Mn=129±10, NiO=0.81±0.36 wt%, n=7; augite Fs_11.8±0.4Wo_48.0±0.5, Fe/Mn=179±72, n=7; plagioclase An_52.3±4.6Ab_45.8±4.4Or_1.9±0.2, n=15; magnetite with Cr₂O₃=5.83±0.74, Al₂O₃=3.00±0.36 (wt%), n=8; hercynite-spinel Al₂O₃=51.06±3.70, FeO=33.31±3.98, MgO=8.20±0.58, Cr₂O₃=8.39±0.48 (wt%), n=5; ilmenite TiO₂=30.89, Al₂O₃=4.08, Cr₂O₃=5.93, FeO=54.72, MgO=1.96 (wt%); fusion crust (proxy for bulk composition): SiO₂=40.9±6.1, A₂O₃=2.2±0.9, FeO=24.6±3.3, MgO=28.4±5.8, CaO=2.8±0.8, Na₂O=0.6±0.4 (wt%). Oxygen isotopes (Karen Ziegler, UNM) Oxygen isotope values of 3 acid-washed aliquots of bulk sample, 1.5, 1.6, 1.8 mg, gave δ¹⁷O = -4.119, -4.118, -4.378, δ¹⁸O = -0.069, -0.022, -0.176, Δ¹⁷O = -4.083, -4.106, -4.285 (linearized, all permil).

Classification: Achondrite (ungrouped). This is a unique achondrite with oxygen isotopic values identical to some CV/CK carbonaceous chondrites. Oxygen isotopic values are similar to the ungrouped achondrite NWA 7822, however this meteorite has values that are on the CCAM with significantly higher δ¹⁸O than NWA 7822. It does not appear to be paired with NWA 7822 in that it contains no Fe-Ni metal or troilite, but instead has magnetite, hercynite-spinel, ilmenite, Cl-rich apatite, and olivines and pyroxenes with higher Fe/Mn. The lack of Fe-Ni metal, the ubiquity of magnetite, and the high Ni-content of olivine indicates that this meteorite was formed at relatively high fO2 and suggests that it could have been derived from a CK carbonaceous chondrite-like precursor. Low weathering grade.

Specimens: 1.9 g including a probe mount on deposit at UNM, Steve Witt holds the main mass.