Northwest Africa 15746 (NWA 15746)

(Northwest Africa)

Purchased: 2022 Oct

Classification: Carbonaceous chondrite (C3, ungrouped)

Physical characteristics: A single crusted stone.

Petrography: (J. Gattacceca, CEREGE) Chondrules with average apparent diameter 610±200 µm (n=12), chondrule fragments and mineral fragments set in an abundant fine-grained iron-rich matrix (73vol% by point counting, n=303). Opaque minerals are metal present in the matrix and in chondrules, sulfide and magnetite in the matrix. Metal and sulfides are more abundant than magnetite. Carbonate grains to 50 µm are present in the matrix. No CAI was observed. X-ray diffraction analyses over a ~2 cm² area on a polished section (D. Borschnek, CEREGE) does not reveal the presence of hydrous phases (serpentine, tochilinite) typically observed in type 2 chondrites using the same experimental setup.

Geochemistry: Olivine Fa_24.2±22.8, range Fa_0.8-74.5 (n=12), Cr₂O₃ is ferroan olivine is 0.41±0.12 wt% (n=9). Pyroxene Fs_1.9Wo_0.9, Fs_40.5Wo_0.6. Matrix defocused analyses: FeO 40.7 wt%, analytical total 89.0 wt%.

Classification: Carbonaceous chondrite (C3-ungrouped). Carbonaceous chondrite based on matrix abundance and overall petrography. Ungrouped designation from chondrule size that is larger than for CM and CO chondritest, and the absence of phyllosilicates from X-ray diffraction analyses. The Cr₂O₃ content of ferroan olivine, and the range of olivine composition suggest a low petrographic type of 3.00. The analytical total of defocused matrix analyses is higher than for most CM2 chondrites (see fig. 11 of Krämer-Ruggiu et al. 2022) and in the range of some C3.00-ung chondrites.

Specimens: Type specimen at CEREGE. Main mass with Kuntz.