Northwest Africa 4286

Algeria

Find: 2005

Ordinary chondrite (L melt rock)

History: A stone was found in southern Algeria in 2005 and sold in Erfoud in November 2005.

Physical characteristics: A 71 g partial stone with no residual crust is reddish brown in color with raised, small (<1 mm) bumps of lightly oxidized metal and sulfide.

Petrography: (T. Bunch and J. Wittke, NAU) Very dark interior that is mostly shock-melted with three distinct grain sizes: 1) shocked and unmelted olivine and orthopyroxene fragments (<0.3 mm), 2) melt-crystallized olivine > orthopyroxene > plagioclase > augite (<0.05 mm), and 3) very fine grained matrix (<0.01 mm) that consists of olivine, orthopyroxene, SiO₂-rich feldspathic glass, and tiny metal melt spheres. Large oblate to rounded melt-formed taenite is commonly mantled by sulfide and is <3 mm in size. No metal-sulfide eutectic structures were noted. There is a strong directional flow orientation of the metal-sulfide complexes. No residual kamacite and taenite is partially oxidized.

Mineral compositions and geochemistry: Olivine megacrysts (Fa_23.6; FeO/MnO = 50), midsize olivine (Fa_20.2–27.6; FeO/MnO = 44–54). Orthopyroxene megacrysts (Fs_19.6Wo_2.6; FeO/MnO = 33), augite (Fs_15.6Wo_30.4). Taenite (Ni = 15.2 wt%) and chromite fragments (Cr/[Cr+Al] = 0.84).

Classification: Ordinary chondrite (L melt rock); S5, moderate weathering. This sample is similar to NWA 2085, but lacks metal-sulfide eutectic structures.

Type specimen: A total of 14.6 g and one thin section are on deposit at NAU. Thompson is holder of the main mass.