Northwest Africa 8631 (NWA 8631)

(Northwest Africa)

Purchased: 2012

Classification: Carbonaceous chondrite (CO3.0)

History: Purchased in Erfoud in 2012

Physical characteristics: A single stone partly covered with fusion crust. Cut surface reveals a dark brown interior with small brown-orange chondrules.

Petrography: Chondrules and mineral fragments in a fine grained Fe-rich matrix. Chondrules are mostly of type I, some armored by opaques. Sulfides and metal are found in chondrules and matrix including blebs in the matrix (to 200 μm for metal, to 50 μm for sulfides). Modal abundances (by point counting N=448): matrix 53%, chondrules and mineral fragments 40%, opaques and their weathering products 7% (including ~2% metal, in agreement with magnetic susceptibility). Average chondrule size 240±130 μm (N=67).

Geochemistry: Olivine Fa_13.2±17.0 (PMD=118%, Fa_0.4-46.9, N=22). Type I chondrules olivine Fa_1.0±0.4 (N=14), type II chondrules olivine Fa_34.5±9.0 (N=8). Cr₂O₃ in ferroan olivine is 0.35±0.06 (N=8). Orthopyroxene Fs_2.0±0.8Wo_1.6±0.9 (Fs_1.2-3.3, PMD-FeO=34%, N=5). Magnetic susceptibility log χ =4.65 (χ in 10^-9 m³/kg).

Classification: Carbonaceous chondrite (CO3.0). Sub-type from Cr₂O₃ content of ferroan olivine and olivine Fa range. Matrix abundance is higher and chondrules larger than the usual CO chondrite. Moderate weathering.

Specimens: 7.5 g and a polished section in CEREGE. Main mass with Jean Redelsperger