Northwest Africa 2975

Algeria

Find: 2005

Achondrite (Martian, basaltic shergottite)

History: A minimally weathered fully encrusted whole stone of 70.1 g was purchased in Erfoud, Morocco, by M. Farmer in November 2005.

Petrography: (T. Bunch and J. Wittke, NAU; A. Irving, UWS) A medium-grained (<3.1 mm greatest dimension) basaltic shergottite that consists of ~57.3 vol% augite and pigeonite pyroxenes, and 38.3 vol% plagioclase (present as shock-formed maskelynite and glasses) with minor opaques (2.7 vol%) and phosphates (1.7 vol%) arranged in a weakly foliated subophitic to granular texture. Accessory phases include ulvöspinel, ilmenite, chlorapatite, merrillite, pyrrhotite, Si-Al-Na-K-rich glasses, and baddeleyite. Vesicular black glass veins (<3 mm in width) and pockets (up to 6 mm) are prominent.

Geochemistry: Pigeonite (Fs_35.2–57.6Wo_12.6–16.5; FeO/MnO = 28–38) and augite (Fs_27.2–41.5Wo_30.8–35.2) show mottled compositional zoning. Fe-rich margins of pigeonite contain very thin (0.2–0.5 µm) orthopyroxene exsolution lamellae. Maskelynite is compositionally homogeneous An₅₅Or_1.8 in contrast to mesostasis maskelynite (An_48–60Or_9.2) and glasses. Melt inclusions (15–60 µm, longest dimension) in ulvöspinel have rims of Fe-rich pigeonite (Fs_73.5Wo_5.8), merrillite, and pyrrhotite; cores are Si-Al-K-Na-rich glass. Bulk composition: Calculated average bulk composition based on analyzed phases and their modes of three melt inclusions is: SiO₂ = 70.2, Al₂O₃ = 8.5, TiO₂ = 0.8, FeO = 2.1, MnO = 0.2, MgO = 2.35, CaO = 3.5, Na₂O = 0.95, K₂O = 2. 1, and P₂O₅ = 2.8 (all wt%) with trace amounts of NiO, CoO, and S. One melt inclusion contains only fayalite (Fa₈₄) mantled around a core of glass (K₂O = 6.4, SiO₂ = 78; both wt%).

Classification: Achondrite (Martian basaltic shergottite); moderate to high shock, minimal weathering.

Specimens: A 20.2 g type specimen and one thin section are on deposit at NAU. An anonymous owner holds the main mass.