Northwest Africa 5990

Morocco

Find (found July 2009, purchased September 2009)

Achondrite (Martian, shergottite)

History: A small stone was found on Hamada du Drâa, Morocco and purchased by Stefan Ralew and Martin Altmann.

Physical characteristics: A single 59 g stone coated on one side by fusion crust showing linear ablation lines. The interior is black, beige, and white with the overall appearance of a terrestrial diabase (Fig. 5).

Petrography: (A. Irving and S. Kuehner, UWS): Medium-grained (up to 1.1 mm) with a subophitic texture, and composed of pyroxene (mostly pigeonite with some subcalcic augite, 35 vol%), plagioclase (35 vol%), and olivine (25 vol%) with accessory chromite, ilmenite, pyrrhotite, Na-Mg-bearing merrillite, and rare magnetite. Shocked plagioclase is in part maskelynite, and in part spherulitic aggregates of birefringent plagioclase blades containing vesicles. None of the minerals exhibit much compositional zoning.

Geochemistry: Olivine (core Fa_40.7, rim Fa_46.9, FeO⁄MnO = 52.0–53.5), pigeonite (Fs_28.3Wo_7.8 to Fs_33.1Wo_12.5, FeO⁄MnO = 27.6–30.0), clinopyroxene (Fs_19.9Wo_29.9, FeO⁄MnO = 24.1), plagioclase (An_64.3-65.2Or_0.2–0.1). Oxygen isotopes (D. Rumble, CIW): acid-washed material analyzed by laser fluorination gave, respectively δ¹⁸O = 4.15, 4.21; δ¹⁷O = 2.41, 2.47; Δ¹⁷O = 0.226, 0.253 per mil.

Classification: Achondrite (Martian, shergottite). This specimen is unlike other shergottites in being coarser grained than most (diabasic), and in containing significant amounts of olivine that is not phenocrystic.

Specimens: A total of 11.8 g of type material, two polished thin sections and a polished mount are on deposit at UWS. The remaining material is held by anonymous collectors and institutions.

Fig. 5. Polished face of cut slice of NWA 5990. Scale cube is
1 cm.