Northwest Africa 13921 (NWA 13921)

(Northwest Africa)

Purchased: 2020 Jul

Classification: Ungrouped achondrite

History: Purchased by Didi Baidari in July 2020 from a dealer in Tindouf, Algeria and subsequently acquired by Luc Labenne.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) Complex specimen consisting of some polymineralic lithic clasts and irregularly-shaped mineral grains within a dominant quench-textured groundmass. Lithic clasts are relatively coarse grained gabbroic assemblages composed of olivine, low-Ca pyroxene (patchily-zoned orthopyroxene and pigeonite) and highly calcic plagioclase with accessory kamacite, taenite, Ti-chromite and troilite. Separate irregularly shaped (apparently resorbed) mineral grains are olivine, orthopyroxene, pigeonite, augite, exsolved pigeonite, silica polymorph and calcic plagioclase. The groundmass or matrix is composed mainly of radiating arrays of very thin, acicular, strongly-zoned pyroxene grains, some of which are oriented perpendicular to edges of the larger mineral grains and appear to have nucleated on them. Accessory phases in the matrix include kamacite (with taenite rims), ilmenite, troilite and pentlandite.

Geochemistry: Olivine (Fa_29.4-40.9, FeO/MnO = 53-63, N = 9), low-Ca pyroxene (Fs_17.5Wo_1.0, FeO/MnO = 27; Fs_25.9-27.7Wo_1.0-4.7, FeO/MnO = 38-47, N = 4), pigeonite (Fs_36.9-45.5Wo_5.9-10.0, FeO/MnO = 28-32, N = 2), subcalcic augite (Fs_40.4-48.6Wo_20.0-26.7, FeO/MnO = 28-31, N = 2; Fs_72.0Wo_24.4, FeO/MnO = 43), pigeonite host (Fs_46.3Wo_11.4, FeO/MnO = 32), augite exsolution lamella (Fs_29.1Wo_36.1, FeO/MnO = 31), calcic plagioclase (An_87.3-93.1Or_0.5-0.1, N = 7). Oxygen isotopes (K. Ziegler, UNM): analyses of 7 acid-washed subsamples by laser fluorination gave, respectively, δ¹⁷O 1.770, 1.963, 1.851, 1.875, 1.757, 1.900, 1.856; δ¹⁸O 3.691, 4.092, 3.885, 3.905, 3.706, 4.012, 3.913; Δ¹⁷O -0.179, -0.197, -0.200, -0.187, -0.199, -0.218, -0.210 per mil (all data linearized).

Classification: Achondrite (ungrouped, mafic, melt-textured with remnant lithic and mineral clasts). Although the mineralogy of some components in this specimen (except for the Ni-bearing metal) have some resemblance to those in typical diogenites and eucrites, oxygen isotopes do not not support such a relationship. In fact, oxygen isotopic compositions are displaced closer to the TFL than the fields for diogenites and main group eucrites, and plot near the results for anomalous eucrites Pasamonte, Moama, and Allan Hills 78132. This specimen may represent an anomalous mafic, olivine gabbroic achondrite which has been partially melted and rapidly quenched.

Specimens: 23.4 g including one polished thin section at UWB; remainder with Mr. D. Baidari.