Northwest Africa 12957 (NWA 12957)

(Northwest Africa)

Purchased: 2018

Classification: Carbonaceous chondrite (C3.00, ungrouped)

History: Purchased in Morocco in 2018.

Physical characteristics: Black irregular fragments. Cut face reveals a dark interior with small light chondrules.

Petrography: (J. Gattacceca, CEREGE) Chondrules with average apparent diameter 300±200 µm (n=25) set in a abundant iron-rich fine-grained matrix. Most chondrules have fine-grained rims. Opaques are metal, sulfides and magnetite. No phyllosilicates were observed. Raman spectra of the fine-grained matrix reveal the presence of polyaromatic carbonaceous matter characterized by a structural order comparable to type 2 chondrites, thus reflecting a peak metamorphic temperature lower than in the least metamorphosed type 3 chondrites (e.g., Semarkona).

Geochemistry: Olivine Fa_24.3±20.8, range Fa1.0-Fa_58.7, Fa PMD 79% (n=21), Cr₂O₃ in ferroan olivine 0.32±0.10 (n=16). Low-Ca pyroxene Fs_8.8±12.7Wo_1.5±0.8 (n=10). Matrix microprobe over an area of 30 × 30 µm2: total 84.4%, FeO 36.1 wt% (n=2). Oxygen isotopic composition (J. Gattacceca, C. Sonzogni, CEREGE) from analysis of one acid-washed 1.5 mg aliquot of a powdered 148 mg bulk sample is δ¹⁷O=-3.82‰, δ¹⁸O=1.45‰, Δ¹⁷O=-4.59‰ (linearized, slope 0.5247, analytical uncertainties 0.08‰, 0.12‰, 0.03‰ respectively

Classification: Carbonaceous chondrite (C3.00-ungrouped). The type 3 derives from the unequilibrated nature, the absence of phyllosilicates and abundance of metal. The subtype 3.00 derives from Raman spectroscopy. The ungrouped designation derives from petrography. Possibly paired with Chwichiya 002 despite having distinct oxygen isotopic composition.

Specimens: Type specimen at CEREGE. Main mass with Isabelle Pothier.