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Northwest Africa 5990
Find (found July 2009, purchased September 2009)
Achondrite (Martian, shergottite)
History: A small stone was found in July 2009 on Hamada du Drâa, Morocco and purchased in September 2009 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.
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 (probably representing quenched plagioclase melt). None of the minerals exhibit much compositional zoning.
Geochemistry: Olivine (core Fa40.7, rim Fa46.9, FeO/MnO = 52.0-53.5), pigeonite (Fs28.3Wo7.8 to Fs33.1Wo12.5, FeO/MnO = 27.6-30.0), clinopyroxene (Fs19.9Wo29.9, FeO/MnO = 24.1), plagioclase (An64.3-65.2Or0.2-0.1). Oxygen Isotopes (D. Rumble, CIW): acid-washed material analyzed by laser fluorination gave, respectively d18O = 4.15, 4.21; d17O = 2.41, 2.47; D17O = 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.