New carbonaceous chondrite Northwest Africa 11781 (CM2)

Research subject. The article presents the results of a study of a new meteorite Northwest Africa 11781.

Material and methods. The material for the study was a fragment of a meteorite weighing 15.56 g, from which 4 transparent polished sections with a total area of 10.5 cm² were produced. The mineralogy and structural features of the meteorite were studied using a scanning electron microscope JSM-6390LV by JEOL, an electron probe microanalyzer Cameca SX-100 and a quadrupole mass spectrometer with inductively coupled plasma ELAN 9000. All analyses were performed in core facilities “Geoanalytic” IGG UB RAS.

Results. The meteorite is CM2 carbonaceous chondrite comprising ≈ 20-30% of chon-drules and 60-70% of a fine-grained matrix. Refractory inclusions (CAI, AOAs, forsterite rich objects) occupy no more than 3-5%. The average chondrule size is 0.3 mm. Porphyritic olivine (PO) and olivine-pyroxene (POP) chondrules predominate. The meteorite matrix consists of layered silicates and iron hydroxides. CAIs are small in size (0.05-0.3 mm). The main minerals of CAIs are spinel, Ca-pyroxene, hibonite and perovskite. Olivone in AOAs mainly contains spinel-diopside inclusions. The forsterite-rich object presents a low-Fe forsterite with a low-Fe enstatite margin. The meteorite also features an unusually large (1 mm) forsterite-rich object. This object has a transitional position from refractory forsterite to magnesian chondrules. Unusual isolate crystals of high-Fe (FeO 15.35-38.89 wt %) olivine were identified in the meteorite matrix. The mechanism of their formation remains controversial.

Conclusion. It is established that the discovered fragment is a carbonaceous chondrite, therefore being a meteorite that has not been studied before. The meteorite has been recorded as a new carbonaceous chondrite called Northwest Africa 11781 (NWA 11781). The meteorite shows no signs of a shock impact and demonstrates a high grade of terrestrial weathering.

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