NWA 11756 Brachinite
Northwest Africa 11756 (NWA 11756)
Purchased: 2017 Sep
Classification: Primitive achondrite (Brachinite)
Petrography: (K. Metzler, IfP) Olivine-dominated ultramafic rock with metamorphic texture (~120° triple junctions), consisting of olivine, low-Ca pyroxene, and Ca-pyroxene grains up to ~0.8 mm. Metal is replaced by weathering products which are also concentrated along grain boundaries. Accessories are Cr-Al-spinel (up to 700 µm) and weathered FeS. No plagioclase was found.
Geochemistry: Mineral compositions and geochemistry: Olivine grains (n=12): Fa26.8±0.2, mean Fe/Mn (at.)= 58; Low-Ca pyroxene grains (n=12): Fs22.1±0.4Wo2.1±0.1. Ca-pyroxene grains (n=5): Fs9.1±0.2Wo43.6±0.5. The concentration of Cr2O3 in olivine and pyroxene is <0.1 wt% and 0.3 wt%, respectively. Composition of Cr-spinel (n=2; at.): Cr/(Cr+Al)=0.78; TiO2 1.4-1.5 wt%. SEM analyses of large thin section areas reveal that this sample is nearly devoid of aluminum.
Classification: Brachinite, based on metamorphic texture, grain size, modal composition, absence of plagioclase, and mineral chemistry.