Northwest Africa 5298

Northwest Africa

Find: March 2008

Achondrite (Martian, basaltic shergottite)

History: Found near Bir Gandouz, northwest Africa, in March 2008 and purchased by an anonymous dealer.

Physical characteristics: A single 445 g stone lacking fusion crust with a brown exterior weathered surface (Fig. 3). The fresh interior is composed of yellow green grains with interstitial dark gray patches containing small vesicles.

Petrography: (A. Irving and S. Kuehner, UWS) The rock is composed mainly of intergrown prismatic grains of zoned pyroxene (up to 1.5 mm long) and lath-shaped regions of similar size composed of vesicular material with plagioclase composition. Some of this material is clear isotropic glass, but the majority consists of intersecting domains of subradiating, fibrous, birefringent plagioclase crystallites. Accessory minerals are titanomagnetite, ilmenite, silica polymorph, Na-Fe-merrillite, Cl-apatite, pyrrhotite, fayalite and baddeleyite. Silica is relatively abundant and forms subhedral grains (up to 0.4 mm across) within plagioclase composition material. Pyroxene grains have complex compositional zoning, with highly irregular and curvilinear zone boundaries between augite and pigeonite. Titanomagnetite and ilmenite commonly coexist in adjacent portions of composite grains.

Geochemistry: Pyroxene is zoned from subcalcic augite cores (Fs_23.8Wo_28.4, FeO/MnO = 26.3) to pigeonite mantles (Fs_30.2Wo_11.7; FeO/MnO = 30.7) to ferropigeonite rims (Fs_68.0Wo_15.3, FeO/MnO = 39.4). The average composition of the plagioclase is An_51.8Or_1.9. Titanomagnetite (TiO₂ 23.7– 24.5 wt%, FeO 68.7–67.6 wt%, Al₂O₃ 2.1–2.1 wt%); ilmenite (TiO₂ 48.2–49.0 wt%, FeO 48.4–47.7 wt%).

Classification: Achondrite (Martian, basaltic shergottite).

Type specimens: A total of 20 g and one polished thin section are on deposit at UWS. The main mass holder is anonymous.