Northwest Africa 7496 (NWA 7496)        22°56.8687’N, 13°23.1163’W

Ri­­o­­ de Oro, Western Sahara

Found: Jan 2012

Classification: HED achondrite (Eucrite, polymict)

History: Found in Western Sahara in a dune on the bank of an old river called Tiziualtin, January 2012, some 70 km south of Mijek region.

Physical characteristics: A single, 788.4 g, rounded rock, with a ~80% dark-brown fusion crust. The rest of the surface bears an erosion scar that bares a light-gray, fine-grained matrix that encloses angular rock clasts <1 cm in size.

Petrography: Set in a fine, fragmental groundmass, up to 5 mm size clasts have variably coarse, subophitic to protogranular textures of plagioclase and pyroxene with minor quartz and cristobalite domains. Symplectitic portions, 0.2 mm in size, of very fine grained, euhedral augite, ferrosilite, olivine and silica are also observed in these clasts. Aphanitic, clast-rich impact-melt fragments and the variable shock metamorphic deformation of the lithic clasts indicate a polymict nature. Ilmenite is a common minor component, chromite, troilite and baddeleyite are accessories. The stone appears fresh with very little alteration present outside a 0.1 mm thick iron-hydroxide bearing vein. The scarcity of metal or troilite in the thin section may inhibit an evaluation of the extent of alteration.

Geochemistry: (A. Wittmann, WUSL): Plagioclase is variable in composition (An_73-95, Or_0.2-1.7), orthopyroxene occurs as zoned grains with magnesian cores (Fs_29-33, Wo_1.2-4.9, FeO/MnO 25-35) and ferroan rims (Fs_47-59, Wo_1.2-4.9, FeO/MnO 30-37) as solitary fragments or in protogranular eucrite clasts. Such clasts also contain pigeonite grains (Fs_51-58, Wo_7-10, FeO/MnO 29-35) with ~5 μm thick augitic exsolution lamellae, and pigeonite (Fs_42-50, Wo_6-12) with finer exsolution lamellae. Commonly, these protogranular clasts also contain 0.1 to 0.2 mm size symplectitic domains that are composed of ~10 μm size augite (Fs_28-40, Wo_33-44, FeO/MnO 33-34), orthopyroxene (Fs₆₃, FeO/MnO 31-32), olivine (Fa_78-82, FeO/MnO 41-47), and quartz ± tridymite crystals (Raman spectroscopy resolved the presence of quartz, tridymite, and cristobalite). Pyroxene in subophitic clasts is compositionally variable (Fs_36-58, Wo_6-20, FeO/MnO 25-35).

Classification: Achondrite (polymict eucrite, components record variable degree of shock metamorphic overprint from unshocked (S0) to whole rock melting; very minor alteration).

Specimens: Type specimen, 21.7 g, ASU. The main mass is held by an anonymous person.