Northwest Africa 13689 (NWA 13689)

(Northwest Africa)

Purchased: 2020 Oct

Classification: Carbonaceous chondrite (C3, ungrouped)

History: Bought from a mineral dealer in Nouakchott, Mauritania.

Physical characteristics: Individual meteorites with fusion crust, showing a network of contraction cracks.

Petrography (K. Metzler, IfP): Carbonaceous chondrite, consisting of small chondrules, chondrule fragments, AOAs, and some CAIs (up to ~250 µm), embedded in a black, fine-grained matrix. The mean apparent chondrule size is 140±80 µm (50-510 µm; n=60). Mostly type I chondrules, but some type II chondrules and chondrule fragments occur. Many chondrules are rimmed by accretionary dust mantles. Point counting on BSE images revealed ~30 area% (vol%) fine-grained matrix, including dust mantles. Nearly all preexisting metal and sulfide appears to be replaced by pure magnetite (free of MgO, Al₂O₃, TiO₂, and Cr₂O₃). Some tiny relics of very Ni-rich metal (~Fe₄₀Ni₆₀) and sulfides (troilite and pentlandite) are present in the centers of magnetite grains. Some dust mantles are crosscut by magnetite veins. Magnetic properties (J. Gattacceca, CEREGE): magnetic susceptibility log χ (× 10^-9 m³/kg) = 4.98 ; saturation magnetization Ms=16.33 Am2/kg indicates a magnetite content of 17.8 wt%.

Geochemistry: Mineral compositions and geochemistry: Mean value of randomly chosen olivine grains: Fa_4.5±9.0 (0.4-39.5; n=30). Mean value of randomly chosen low-Ca pyroxene grains: Fs_2.3±2.6 Wo_1.9±1.4 (Fs_0.6-13.1 Wo_0.5-4.9; n=30). EPMA analysis (J. Gattacceca, CEREGE) of 29 ferroan olivine give 0.32±0.12 wt% Cr₂O₃. The analytical total in the matrix obtained by EPMA with a defocused beam is 84 wt%, with FeO = 31 wt%. Semi-quantitative X-ray diffractometry (P. Schmidt-Beurmann, Inst. Min, Univ. Münster) revealed ~15±5 wt% magnetite and no indications for phyllosilicates. Oxygen isotope analysis (J. Gattacceca, C. Sonzogni, CEREGE) of a 1.5 mg aliquot obtained from a bulk 15 mg sample yield δ¹⁸O=0.468‰ and δ¹⁷O=-4.084‰.

Classification: Carbonaceous chondrite based on the predominance of type I chondrules, CAI occurrence, and O isotopic composition. Petrologic type 3 based on the strong chemical variation of olivine and pyroxene and the absence of phyllosilicates. Petrologic subtype 3.00, due to the concentration and standard deviation of Cr₂O₃ in ferroan olivine. Ungrouped due to the oxygen isotopic composition which is in the CV-CK-CL range and the exceptionally high fraction of magnetite (~18 wt%).