Northwest Africa 4883

Northwest Africa

Find: July 2007

Achondrite (eucrite, polymict)

History: Purchased by G. Hupé in July 2007 from a dealer in Tagounite, Morocco.

Physical characteristics: A single stone (610 g) with thin, dark brown fusion crust, composed of abundant white, cream and dark gray clasts (up to 6 mm), and less abundant clear, pale bluish to colorless maskelynite crystals (up to 1.2 mm), in a finer grained, medium gray matrix.

Petrography: (A. Irving and S. Kuehner, UWS) Complex breccia composed of mineral fragments and lithic clasts in a matrix of the same phases. Mineral fragments include compositionally variable low-Ca pyroxene (mostly unexsolved pigeonite and some orthopyroxene), completely maskelynitized calcic plagioclase, exsolved pigeonite (orthopyroxene with augite lamellae), silica polymorph, ilmenite, chromite, Ni-free metal, and troilite. There also are clasts of both cumulate and basaltic eucrites, and rare Fe-rich clasts composed of ferrosilite ± hedenbergite rimmed by fayalite. Diogenitic orthopyroxene is not abundant and no polycrystalline diogenite clasts were found.

Geochemistry: Pigeonite (Fs_30.9-35.1Wo_5.8-6.6; FeO/MnO = 28.1-32.5). Pigeonite with rims of ferroan orthopyroxene- Fs_55.4Wo_2.8; FeO/MnO = 35.3. Ferroan pigeonite (Fs_62.6Wo_5.6; FeO/MnO = 34.7). Fe-rich, Ca-rich pigeonite (Fs_48.0-51.5Wo_10.3-15.4; FeO/MnO = 30.3-33.2). Diogenitic orthopyroxene (Fs_26.5Wo_3.5; FeO/MnO = 29.8). Augite (Fs_34.6Wo_41.6; FeO/MnO = 35.8) in intergrowth with fayalite (Fa_83.6; FeO/MnO = 45.7). Plagioclase (maskelynite, An_85.7-88.8Or_0.4-0.6).

Classification: Achondrite (polymict eucrite); extensive shock. This specimen is distinctive because the large plagioclase grains have been completely transformed to maskelynite, signifying a higher degree of shock than that experienced by most polymict eucrites and howardites.

Specimens: A total of 20.0 g of sample and one polished thin section are on deposit at UWS. GHupé holds the main mass.

Submitted by: A. Irving, UWS.