Tafassasset
Chondrite CR ou achondrite primitive
trouvée le 14 février 2000 au Niger
masse totale : 110 kg
- Complète de 982 grammes
- Tranche de 11,7 grammes
Extrait du Meteoritical Bulletin n°86
A 3.61 kg stone, labeled « Te-1″, was found in 2000 March, probably on the same expedition noted above, and is reported by J. Otto (Frei) to have been found in the Tenere Desert at 20°45.8′ N, 10°26.5′ E. Classification (J. Otto and A. Ruh, Frei): a primitive achondrite; partly covered with black fusion crust; shows a recrystallization texture with abundant 120° triple junctions dominated by olivine (56 vol%, 100–700 µm, Fa28.7, 0.06 wt% CaO) and poikilitic orthopyroxene (23 vol%, up to 3 mm, Fs25.4Wo3.6) with exsolved clinopyroxene (Fs12.7Wo39.7); Fe-Ni metal is irregulary distributed (~10 vol%, up to 5 mm); poikilitic plagioclase occurs in interstices (6.5 vol%, An38.5Or3.7); troilite (~3.5 vol%, 0.01 wt% Ni); chromite (~1 vol%, Fe/(Fe + Mg) = 0.817, Cr/(Cr + Al) = 0.818). Oxygen isotopes (R. Clayton and T. Mayeda, UChi): δ17O = –0.85‰, δ18O = +1.70‰, different from other achondrites. Noble gases (L. Schultz, MPI): data compatible with those of brachinites; exposure age is ~45 Ma. Shock stage, S1/2; weathering grade, W0. Specimens: main mass with Christian Stehlin, Basel; type specimen, 30.2 g and thin section, Frei.
Tafassasset
undisclosed location until 2005 February 14
Tenere desert, Niger
Found 2000 February 14
Carbonaceous chondrite (equilibrated CR-like meteorite) or primitive achondrite
Twenty-six pieces with a total weight of ~110 kg were found by Bernard Dejonghe on an ancient alluvial plain. The two largest weigh ~30 kg each. Twenty stones were found on a trip in early 2000, and six more in 2001 March. The geographic coordinates of these meteorites are being withheld by the finder for three years. Classification (M. Bourot-Denise, MNHNP): Fa29.3, pyroxene Fs24.3; ~30 vol% millimeter-sized relic chondrules are clearly visible in backscattered electron images. Textures and relative abundances of the phases vary considerably. The dominant lithology has a coarse grain size. It consists of olivine (poikilitically enclosed within pyroxenes in the relic chondrules), a small amount of plagioclase, 10% metal in rounded or oval-shaped grains often edged by chromite, and minor sulphides in small grains. The interchondrule matrix consists of subhedral crystals of olivine embedded in plagioclase and within anhedral crystals of chromite and phosphate that are up to 1 mm in size. Two distinct plagioclase compositions are present: An46.4Or2.2 in the matrix and An26.6Or5.2 in chondrule relics. A second lithology is finer grained and porous, without large metal grains but with fairly abundant sulphide grains. The large metal grains are composed of kamacite (6% Ni, 0.6% Co) and martensite (12% Ni, 0.4% Co). Some smaller metal grains associated with sulphides are made up of taenite (up to 36% Ni, 0.2% Co) with martensite cores. The sulphide is troilite (Cr 0.02–0.08%), with some grains containing inclusions of Cu-bearing pentlandite. See also Bourot-Denise et al. (2002). Bulk chemistry (J. Zipfel and B. Spettel, MPI; H. Palme, Köln): refractory lithophiles fractionated compared to a typical CR composition, leading to possible classification as a primitive achondrite (Zipfel et al., 2002). Oxygen isotopes (M. Javoy, IPGP): δ17O = +0.18 ± 0.08‰, δ18O = +2.94 ± 0.2‰. Weathering grade, W0/1; shock stage, unknown. Specimens: type specimen, 396 g plus five polished sections, MNHNP; main mass partly left in Niger, partly with the finder.