Northwest Africa 12736 (NWA 12736)
Specimen in collection : main mass
Petrography: (J. Gattacceca, CEREGE) Blocky aggreagte of olivine and pyroxene with grain size 600 µm. Metal and troilite are present as veinlets, along silicate rims and as small aligned crystals within silicates. No graphite was observed; and no silicate zoning was observed.
Geochemistry: Olivine Fa12.2±0.6, FeO/MnO 23.4±4.5, CaO 0.26±0.04 wt%, Cr2O3 0.50±0.04 wt% (n=6). Pyroxene: low-Ca pyroxene Fs10.9±0.2Wo4.7±0.1 (n=5), high-Ca pyroxene Fs6.6±0.1Wo37.8±0.3 (n=5), FeO/MnO 12.8±2.7 (n=10). Oxygen isotopic composition (J. Gattacceca, C. Sonzogni, CEREGE) from analysis of one acid-washed 1.5 mg aliquot of a powdered 118 mg bulk sample is δ17O=0.05‰, δ18O=1.65‰ , Δ17O=-0.81‰ (linearized, slope 0.5247, analytical uncertainties 0.08‰, 0.12‰, 0.03‰ respectively).
Classification: Achondrite-ungrouped. The mineralogy is reminiscent of ureilites but significant differences are the absence of carbon material (graphite, diamond) and the absence of reduced rim in silicates. Owygen isotopic composition is also distinct from that of ureilites.