Northwest Africa 8190 (NWA 8190)

(Northwest Africa)

Purchased: 2007

Classification: Carbonaceous chondrite (CK3)

Physical characteristics: Many pieces with a light brown weathered surface. This zone of severe weathering forms an up to 2 mm thick shell around each sample. Fractured surfaces are dark brown with blurred chondrules up to 3 mm in size. CAIs with sizes up to 8 mm have been observed. Very compact interior without any visible porosity.

Petrography: (K. Metzler, IfP) The investigated sample consists of two distinct lithologies with sharp contact. The groundmass of both lithologies shows a fine-grained recrystallized texture, consisting of olivine grains and fewer pyroxene and plagioclase grains with sizes on the order of 50-100 μm. Magnetite and weathered sulfides are concentrated in the interstices and around larger objects like chondrules. Lithology 1 contains only sparse chondrules while lithology 2 contains subequal amounts of matrix and chondrules+CAIs. Many chondrules show rims of weathered opaque phases. Several large chondrules show relict Mg-rich olivine (Fa_0.5-1) in the center and Fe-rich olivine at the margin.

Geochemistry: (K. Metzler, IfP) The mean composition of olivine (except chondrule cores) is Fa_32.8 (Fa_28-38). Olivine grains contain about 0.5 wt% NiO (0.2-07 wt%). Olivine in chondrule cores is Ni-free and retained its original Mg-rich composition (Fa_0.5-1). Low-Ca pyroxene is less abundant, with a range of compositions (Fs_2-27). It is rich in Al with a mean concentration of about 4 wt% Al₂O₃. Ca-rich pyroxene also occurs. The compositional range of feldspar is An_25.5-43.5. Magnetite is the most abundant opaque phase, with significant concentrations of elements other than Fe (n=2): MgO 1-1.5 wt%; Al₂O₃ 3-3.5 wt%; TiO₂ 1 wt%; Cr₂O₃ 3.5 wt%; NiO 0.5 wt%. The magnetite composition is characteristic for CK chondrites.

Classification: CK chondrite based on mineral chemistry. Petrologic type (3.9) based on olivine variability (PMD=6)