Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.421r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 421
Other
Bawku, Electron Microprobe, Ll Chondrites, Plagioclase
Scientific paper
Introduction: A 59-g piece of stony meteorite with black fusion crust and medium grey interior was recovered in 1992 from Northern Ghana by Dr. S. Abudulai of the Centre for African Studies, University of Cambridge. This meteorite was reported to be part of a fall that occurred in the vicinity of Bawku (11 degrees 05'N, 0 degrees 11'W) at 1630 hours (local time) on December 29, 1989; the meteorite was found between the villages of Naarango-Anisi and Kpukparigu, east of Bawku. A further mass of 1.5 kg was recovered by Dr. Abudulai during a visit in January 1993. The name "Bawku" is proposed for this meteorite. Description: The Bawku chondrite is a monomict breccia. Some well-preserved chondrules with a variety of textures are present. Olivine exhibits undulose extinction and extensive irregular cracking. Other shock features include cracks infilled with metal/troilite. Electron microprobe analyses of olivines gave a mean Fe/Fe + Mg ratio of 26.8%, with a standard deviation of 1.0% (relative) and an average CaO content of 0.03% (by weight). The mean Fe/Fe + Mg content of orthopyroxene was found to be 22.6% (standard deviation 1.2% relative). Small (10-micrometer) grains of plagioclase (75-85% Ab) were found. Automated point counting (700 points) gave volume fractions of 4.6% troilite and 1.3% metal. Compositions of 200 randomly selected points on metal grains fell into three groups, corresponding to kamacite (4-7% Ni, 1.4% Co), taenite (34-38% Ni, 0.35% Co), and tetrataenite (48-56% Ni, 0.2% Co). The inferred volume fractions of these phases were 52%, 45%, and 3% respectively. The average Ni content of the metal was 21%, with 0.9% Co. On etching, Neumann lamellae became visible in kamacite, while the taenite took on a cloudy appearance. Tetrataenite was observed as narrow rims adjacent to cloudy taenite, but also occurred sometimes in larger areas. Minor phases noted included chromite, ilmenite, apatite, and whitlockite. Classification: The Fe/Fe + Mg ratios of olivine and pyroxene place this chondrite at the lower limit of the range for the LL group, adjacent to the hiatus between LL and L [1]. The low metal content is consistent with this classification. The relatively high proportion of kamacite compared to other LL chondrites is in accordance with Prior's law, which applies within this group, unlike others [2]. The homogeneity of olivine and orthopyroxene, the existence of moderately abundant preserved chondrules, and the presence of feldspar grains comparable in size to those observed in LL5 chondrites [3] point to Bawku being a member of this class. The undulose extinction and irregular fracturing of olivine are indicative of shock class S2 [4]. References: [1] Fredriksson K. et al. (1978) In Origin and Distribution of the Elements (L. Ahrens, ed.), 457-466, Pergamon, Oxford. [2] Sears D. W. and Axon H. J. (1976) Meteoritics, 11, 97-100. [3] Heyse J. V. (1978) EPSL, 40, 365-381. [4] Stoffler D. et al. (1991) GCA, 55, 3845-3867.
Chinner G. A.
Reed J. B. S.
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