Northwest Africa 6698 (NWA 6698)

Morocco

Found: 2009

Classification: Ungrouped achondrite

History: A 38.4 g stone found October 2009 was purchased from a dealer in Erfoud, Morocco, in 2010.

Physical characteristics: No fusion crust. Surface is lightly weathered, but heavily desert wind-ablated.

Petrography: (T. Bunch and J. Wittke, NAU; A. Irving, UWS): Fine to medium grained (<2 mm) dioritic rock that contains 69 vol. % plagioclase, 27 vol. %, pyroxenes (augite and pigeonite), and 4 vol. % accessory phases (taenite, chromite, chlorapatite and FeS). Plagioclase displays simple albite and Carlsbad twinning, is mostly equigranular, and contains included pyroxene chadacrysts. Pyroxenes are dominantly augite and unexsolved pigeonite with highly variable grain size, and contain included plagioclase chadacrysts. Sparse pockets of brown, glassy late-stage residuum with microlites are present between the other phases. Shock level is low to moderate, with undulatory extinction in plagioclase and some pyroxenes. Interior weathering is low.

Geochemistry: Most plagioclase shows a range in composition An_22.4-36Or_2.2, with a few zoned grains with cores of An_52-56Or_2.1 to rims of An₃₆; residuum albite, An₁₀Or_5.7. Augite, Fs_18.5-27.0Wo_36.7-41.2 (FeO/MnO = 14-17), Al₂O₃ = 2.2 wt%, Na₂O = 1.2 wt%; TiO₂ = 1.23 wt%; Cr₂O₃ = 1.55 wt%. Pigeonite, Fs_34.4-39.7Wo_8.6-14.5 (FeO/MnO = 20), Al₂O₃ contents up to 3.2 wt %. Taenite Ni = 15.9-49.7, chromite cr# = 0.90, and FeS Ni = 1.58 wt %. Residuum glass (wt %): SiO₂, 70; Al₂O₃, 13.8; Na₂O, 9.4; K₂O, 2.9; CaO 2.1; FeO, 2.8. Oxygen isotopes (D. Rumble, CIW): Replicate analyses of acid-washed whole rock samples by laser fluorination gave δ¹⁸O = 4.80, 4.73; δ¹⁷O = 3.72, 3.75; Δ¹⁷O =1.199,1.265 (all per mil).

Classification: Achondrite (ungrouped). Mineralogy and mineral chemistry are unlike those of any other known achondrite, and the oxygen isotopic composition plots well above the TFL in the field for LL chondrites.

Specimens: A total of 7.7 g of material including one polished thin section are on deposit at NAU. Mr. A. Habibi holds the main mass.