CHERGACH Meteorite - FALL - IMB - Individual 28.01 g

CHERGACH
Individual 70% primary crusted, 20% secondary crusted - 28.01 g
Approx. dimensions: 39 x 22 x 20 mm

Impact Melt Breccia.

Meteoritical Bulletin #94

Chergach                   23° 41’47’’N, 5° 00’53’W

SW El Mokhtar, Erg Chech, Timbuktu district, Mali

Fall: 2 or 3 July 2007, daytime

Ordinary chondrite (H5)

History: In fall and winter, 2007 ~100 kg of meteorites were collected in the Erg Chech, north of Taoudenni. Nomads reported the stones fell after a smoke cloud was seen and several detonations were heard over a wide area during daytime in July 2007. The finder of the first meteorites was Mr Ouled Bleila, who died in a car accident on his way back from the trip to the Chergach strewn field in October 2007. According to the Tuareg people from Algeria who visited the fall site in September 2007, the elliptical strewn field stretches for more than 20 km in a northeasterly direction. No fireball was reported.

Physical characteristics: A large number of fusion-crusted stones have been recovered, the largest ones are 17.8 and 13.9 kg, the smallest ones about 1.5 g. Total known weight ~100 kg.

Petrography: (E. Gnos, MHNGE; B. Hofmann, NMBE, M. Eggimann, UBE/NMBE) Mean chondrule size is 0.38 mm (n = 61). Mean size of plagioclase grains is ~20 μm. Troilite is polycrystalline, rich in silicate inclusions, and shows diffuse boundaries to metal. Some metal is rich in silicate- and troilite inclusions. No Cu metal observed. Shock stage is S3, some shock veins are visible, no weathering (W0). In addition to this dominant lithology, a significant number of stones consist of chondritic clasts (H5 S3-4 W0, identical to the homogeneous lithology) set in a fine-grained, black, silicate impact melt matrix with abundant droplets of metal and metal-troilite. Shock stage of small clasts is up to S4.

Mineral compositions and geochemistry: Olivine Fa_18.2 (chondrite fragments in impact melt are Fa_18.4), pyroxene Fs_15.5 Wo_1.2 (fragments in impact melt are Fs_16.0, Wo_1.4).

Cosmogenic radionuclides: (Patrick Weber, Particle Physics Group, Institute of Physics, University of Neuchâtel) Gamma-spectroscopy performed in November, 2007 showed the presence of the following short-lived radionuclides: ⁴⁶Sc, ⁵⁶Co, ⁵⁴Mn, ⁵⁸Co, ⁷Be, ⁵⁷Co, ²²Na, ⁶⁰Co, ²⁶Al. Recalculated to July, 2007, ²²Na was 44.0 ± 1.1 dpm/kg and ²⁶Al 20.8 ± 0.8 dpm/kg. The ²²Na/²⁶Al activity ratio of 2.1 is consistent with a fall in July, 2007 and demonstrates that this material is not identical with Bassikounou.

Classification: Ordinary chondrite (H5), some stones suggest an H impact melt breccia. S3, W0.

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