Northwest Africa 12563 (NWA 12563)

(Northwest Africa)

Purchased: 2015 May

Classification: Carbonaceous chondrite (CM2, anomalous)

History: Purchased in Agadir, with no details on provenance.

Physical characteristics: A 13g stone with a very fractured surface and one face covered with a highly vesicular fusion crust.

Petrography: (R. Hewins, S. Pont, MNHNP; P.-M. Vanetta, Univ. Lille) The chondrite consists of 14% chondrules and chondrule fragments, and 86% fine-grained matrix. Chondrules are Types IA, IAB and IIA. Rare refractory inclusions are partly altered spinel-rich CAI, and AOA. Metal and glass are almost totally altered, and pyroxene is partly altered. The matrix contains amorphous silicate, serpentine and chlorite; magnetite occurs as spherules, framboids and plaquettes; troilite and pentlandite in rosettes. There are some carbonate patches, but TCI are absent.

Geochemistry: Mineral compositions and geochemistry: (R. Hewins, S. Pont, MNHNP, P.-M. Zanetta, Univ. Lille) The average olivine of Type I chondrules is Fo_99.0 with Fe correlating with Cr and Mn. Type IAB chondrule and isolated pyroxene is generally En_98-94Fs_1-4Wo_1-2. Type II olivine has a composition range of Fo_84-44. Phyllosilicates are Al-rich (chlorite) and Al-poor (serpentine). The bulk oxygen isotopic composition (J. Gattacceca, C. Sonzogni, CEREGE) from analysis of one acid-washed 1.5 mg aliquot of a powdered 20 mg bulk sample: δ¹⁷O=4.26‰, δ¹⁸O=12.61‰ , Δ¹⁷O=-1.33‰ (linearized, slope 0.5247, analytical uncertainties 0.08‰, 0.12‰, 0.03‰ respectively), close to Essebi. Magnetic properties (J. Gattacceca, CEREGE) : magnetic susceptiblity log χ (× 10^-9 m³/kg) = 4.67 is close to the values for Bells and Essebi, C2-ungrouped, at 4.82 and 4.85, respectively. Saturation magnetization indicates 6.8 wt % of magnetite.

Classification: CM2-anomalous. Though this meteorite has chondrule and matrix abundances like CMs, its alteration minerals are different: serpentine, abundant chlorite and abundant magnetite, not serpentine, cronstedtite and tochilinite, and it is possible that it is an ungrouped carbonaceous chondrite. Chlorite is found in CR alteration, but is very minor in CMs. The meteorite lacks TCI intergrowths, yet has fresh olivine. The magnetite content of 6.8 wt. % (derived from saturation magnetization Ms=6.26 Am²/kg) is 6 times higher than the average for CM chondrites that have Ms=1.14±0.56 Am²/kg. The magnetic susceptibility is close to the values for Bells and Essebi, and higher than all except 4 of 52 chondrites classified as CM chondrites (Rochette et al., 2008). The oxygen isotopic composition plots at the upper end of the CO-CM trend line very close to Essebi.

Specimens: The main mass is held by Labenne. The type specimen is at MNHNP.