Northwest Africa 13790 (NWA 13790)

(Northwest Africa)

Purchased: 2020 Nov

Classification: Primitive achondrite (Winonaite)

Physical characteristics: Some fragments have fusion crust. The main mass is 131.4 g.

Petrography: (K. Metzler, IfP) Coarse-grained mafic rock, consisting of olivine (~45 vol%), low-Ca pyroxene (~10 vol%), Ca-rich pyroxene (~20 vol%), plagioclase (~10 vol%), Fe-Ni metal (~15 vol%), and small amounts of troilite. Olivine grains with reverse chemical zoning (Mg-rich reduction rims) occur. Metal occurs as isolated grains and as veins up to several cm in length. The rock shows a metamorphic texture with frequent 120° triple junctions at crystal boundaries and partly poikilitic intergrowths. Typical grain sizes vary between 100 and 500 µm. However, cm-sized areas of olivine and pyroxene, respectively, show identical optical orientation.

Geochemistry: Mineral compositions and geochemistry: The mean olivine composition is Fa_3.4±0.6 (Fa_2.0-4.1; n=15); Fe/Mn (at.): 15.8. The mean low-Ca pyroxene composition is Fs_4.4±0.3Wo_1.9±0.2 (Fs_3.9-5.0Wo_1.6-2.2; n=15); Fe/Mn (at.): 12.6. The mean composition of Ca-rich pyroxene is Fs_1.3±0.3Wo_43.8±0.6 (Fs_0.9-2.2Wo_42.9-45.1; n=15); Fe/Mn (at.): 10.7. The mean feldspar composition is An_20.9±0.7Or_3.0±0.5 (An_19.9-22.1Or_2.1-3.8; n=10).

Classification: Winonaite, based on the mineral chemistry and the high abundance and shape of Fe-Ni metal.

Specimens: Main mass with Ben Hoefnagels. 180 g (14 pieces) with Mohamed Ali Loud.