Northwest Africa 16000 (NWA 16000)

(Northwest Africa)

Purchased: 2023 Jan

Classification: Carbonaceous chondrite (C3, ungrouped)

Physical characteristics: Partially crusted stone

Petrography: (J. Gattacceca, CEREGE) Chondrite with chondrules with average apparent diameter 390±240 µm (n=23) and CAIs to 100 µm set in very abudant iron-rich fine-grained matrix (81 vol% by point counting, n=454). Most chondrules have fine-grained dust rims about 40 µm thick, with radial cracks. Opaque minerals are abundant sulfides and magnetite, and rare metal. Magnetite is found under the form of 5-10 µm coalescent spheres. 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_23.8±19.5, range Fa_0.9-62.9, Fa PMD 68% (n=12). Cr₂O₃ in ferroan olivine 0.27±0.05 wt%. Defocused analyses of the matrix yield FeO=35.1 wt% and analytical totals 91.2 wt% (n=2). Oxygen isotopic composition (J. Gattacceca, C. Sonzogni, D. Au Yang, CEREGE) from analysis of one acid-washed 2.48 mg aliquot of a powdered 68 mg bulk sample is δ¹⁷O=7.70‰, δ¹⁸O=16.71‰ , Δ¹⁷O=-1.00‰ (slope 0.52, analytical uncertainties 0.115‰, 0.062‰, 0.005‰ respectively).

Classification: Carbonaceous chondrite (C3-ung). Carbonaceous chondrite based on matrix abundance and the presence of CAIs. Type 3 based on the uequilibrated silicate composition, and the absence of phyllosilicates as tested by X-ray diffraction. Based on chondrule size, matrix abundance, and the bulk O-isotopic composition the sample is not a CO3, CV3 or CK3.

Specimens: Type specimen at CEREGE, main mass with Kuntz