Northwest Africa 2646

Algeria or Morocco

Find: December 2004

Achondrite (Martian, plagioclase-olivine clinopyroxenite)

History: A 9.3 g, broken stone (allegedly part of a larger specimen found at an unknown site in Algeria or Morocco) was purchased from a Moroccan dealer in December 2004 for N. Oakes.

Petrography and Geochemistry: (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS) The specimen is relatively coarse-grained and has an overall gray color, but grain surfaces have a thin whitish coating. It is modally heterogeneous and is composed of ~40.7% pigeonite, ~24.3% augite, ~21.6% olivine, and ~11.4% maskelynite, with 2% chromite, ilmenite, merrillite, and pyrrhotite. Euhedral to subhedral chromite (Cr/(Cr+Al) = 0.869, Mg/(Mg+Fe) = 0.12) and augite (Fs_19–23Wo_26–36; FeO/MnO = 22–27; Cr₂O₃ = 0.6, Al₂O₃ = 2.2 [both wt%]) appear to have crystallized first, followed by subhedral to anhedral olivine (Fa_38–44; FeO/MnO = 35–56). These are poikilitically enclosed in large oikocrysts of pigeonite (Fs_24.4Wo_5.7 zoned to Fs_34.4Wo_12.1; FeO/MnO = 26–32); augite and olivine chadacrysts tend to be clustered. Laths of plagioclase (now maskelynite, An_58.4–60.7Or_0.9), some in clusters, occur interstitially to pigeonite grains. All maskelynite grains are rimmed by zones up to 40 µm in width consisting of a mixture of fine grained calcite, hydrous Al-rich silicate (possibly kaolinite), and very minor calcium chloride, which appear to be replacing the primary feldspar. This specimen has affinities to the six recognized “lherzolitic shergottites,” but differs from them in having a higher abundance of plagioclase and more ferroan mafic mineral compositions.

Classification: Achondrite (Martian, plagioclase-olivine clinopyroxenite); minimal weathering.

Specimens: A 3.4 g type specimen and one polished thin section are on deposit at NAU. One polished mount is on deposit at UWS. N. Oakes holds the main mass.