Northwest Africa 13939 (NWA 13939)

(Northwest Africa)

Purchased: 2010 Jun

Classification: Martian meteorite (Shergottite)

History: Purchased from a Moroccan mineral dealer at the meteorite show in Ensisheim, France

Physical characteristics: Fragment without fusion crust.

Petrography: (K. Metzler, IfP). Ultramafic igneous rock, mainly consisting of various pyroxenes, olivine, and some plagioclase. It exhibits a cumulate texture with grain sizes in the mm range with pyroxene-rich domains up to ~8 mm. Point counting on BSE images (n=175; area 1.6 cm2) revealed 46 vol% olivine, 47 vol% pyroxenes with various compositions, and 7 vol% plagioclase. Three different pyroxene varieties were found. Olivine partially shows brownish staining in transmitted light. It occurs in paragenesis with pyroxene and plagioclase and as poikilitically enclosed grains in the pyroxene-rich domains. Plagioclase is completely transformed to maskelynite by shock metamorphism. Vesicular shock melt pockets and veins are present, showing quench textures. Accessories are Mg-bearing ilmenite, two different varieties of chromium spinel, merrillite, and small amounts of pyrrhotite with tiny pentlandite inclusions.

Geochemistry: Mineral compositions and geochemistry: Olivine: Fa_34.1±1.6 (Fa_29.5-36.4); mean Fe/Mn (at.): 49.9; n=16. Pyroxene #1: Fs_20.1±0.8 Wo_2.9±0.3 (Fs_19.0-21.0 Wo_2.5-3.1); mean Fe/Mn (at.): 29.8; n=4. Pyroxene #2: Fs_26.8±3.4 Wo_9.4±1.5 (Fs_21.9-30.8 Wo_6.7-11.7); mean Fe/Mn (at.): 28.9; n=17. Pyroxene #3: Fs_16.9±0.6 Wo_32.3±1.7 (Fs_16.0-18.0 Wo_30.5-36.0); mean Fe/Mn (at.): 24.3; n=12. Plagioclase (maskelynite) An_51.6±2.1 Or_1.3±0.4 (An_48.4-54.7 Or_0.5-1.9); n=14. Chromium spinel #1: Atomic Cr/(Cr+Al)=0.87-0.89; Mg/(Mg+Fe)=0.25-0.34; TiO₂=0.9-1.0 wt%; n=5. Chromium spinel #2: Atomic Cr/(Cr+Al)=0.79-0.82; Mg/(Mg+Fe)=0.10-0.16; TiO₂=11.7-16.1 wt%; n=5.

Classification: Achondrite (martian, ultramafic shergottite)