Northwest Africa 2907

Northwest Africa

Find: 2005

Achondrite (ungrouped)

History: A large number of pieces were collected within a small strewn field of ~15 m in diameter in Algeria and sold in Erfoud, Morocco.

Physical characteristics: Most of the fragments composing the 586 g mass are without fusion crust, dark brown in color, and show extensive surface weathering.

Petrography: (T. Bunch and J. Wittke, NAU) Partially recrystallized polymict breccia that contains at least two distinct lithologies: a) medium-grained clasts (<2 mm) of untwinned orthopyroxene without discernible exsolution lamellae and a roughly equal amount of complex, polysynthetically twinned plagioclase partially converted to maskelynite; and b) medium-grained, partially recrystallized pigeonite with minor development of exsolution lamellae and twinning with inclusions of simple, albite-twinned plagioclase together with a fine-grained assemblages (<0.1 mm) of tridymite, metal, and plagioclase at recrystallization boundaries. Both lithologies are set in a fine-grained, partially recrystallized matrix of pyroxenes and plagioclase compositionally similar to lithologies (a) and (b). Minor minerals include Al-rich chromite, a sulfide phase, tridymite, phosphate, and metal.

Mineral compositions and geochemistry: Lithology (a): orthopyroxene (cores = Fs_22.0–25.3Wo_2.5; FeO/MnO = 24–27 with grain margins of Fs_33.8Wo_7.2 that in turn have exsolved Ca-rich pyroxene), plagioclase (An_99.5Or_0.5). Lithology (b): pigeonite (Fs_38.1Wo_11.1), plagioclase (An_94.8Or_5.6).

Classification: Achondrite (ungrouped); S3, moderate weathering. Overall characteristics of this meteorite are similar to clast material in some mesosiderites and may represent a large, separated mesosiderite clast.

Type specimen: A total of 23 g is on deposit at NAU. Reed holds the main mass.