Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.443b&link_type=abstract
Meteoritics, vol. 29, no. 4, page 443
Computer Science
Goethite, Iab Iron Meteorites, Maghemite, Magnetite, Mossbauer Spectroscopy, Weathering
Scientific paper
Monturaqui is a meteorite crater, 370 m diameter, in the Andes Mountains, Chile. The impact apparently took place in upper tertiary or pleistocene, judging from the topography of the drainage gullies in the crater area [1]. The surface topography and the state of the corroded meteorite suggest, however, that the climate some time ago must have been more humid. Small fragments of severely corroded iron meteorites have been collected in the vicinity, 50-200 m from the crater rim. Fourteen fragments of iron shale have been examined in order to determine the corrosion products of an entirely disintegrated iron meteorite. Polished sections display some vugs and microporosities and are >99.9% oxidic. The non-oxidic parts are minute particles of taenite (38-42%wt. Ni), schreibersite, rabdite (42%wt. Ni, 0.13%wt. Co, 12.5%wt. P), and cohenite (microhardness 1100 +- 50). The overall morphology of the oxide shales, which sometimes are of breadcrust type, is that of a coarse octahedrite, probably of Group IA similar to the Canyon Diablo meteorite [2]. X-ray diffractometry on powder samples disclose goethite and maghemite, and occasionally peaks from lepidocrocite and reevesite were identified. Peaks from magnetite, akaganeite, and hematite were not identified. The mineralogy and oxidation state of Fe was also investigated by ^57Fe Mossbauer spectroscopy. Spectra of a representative powder sample were obtained between 16 and 350 K. No indication of purely Fe(II)-containing compounds was present. Based on the identification of a component due to octahedrally coordinated Fe in magnetite (a mixed Fe(II)- Fe(III) site) a maximum of approximately 5 relative % of Fe(II) can be estimated. Consequently the determination of Fe in oxides by microprobe is reported as Fe(III). The dominating components in the Mossbauer spectra are due to maghemite and goethite, the latter exhibiting a wide variation in crystallite size as evidenced from the observed variation in blocking temperatures between 50 and 300 K. Based on fits of the spectrum obtained at 16 K the Fe is partitioned between maghemite and goethite in the ratio 55:45. The magnetic hyperfine fields are reduced in comparison to the pure oxides probably due to substitution in the lattice. The chemistry of the oxides was investigated by microprobe analysis including Fe2O3, NiO, CoO, SiO2, Al2O3, SO3, and Cl in the scheme. Based on 47 analyses a mean Fe2O3 content of 81.8%wt. with minimum and maximum contents of 73.8 and 97.0%wt., respectively. The NiO content varied from 1.7 to 5.3%wt. with a mean of 3.9%wt., and the CoO content varied from 0.2 to 0.8%wt. with a mean of 0.5%wt. Of the non-meteoritic derived elements small amounts of Al2O3 (0-0.21%wt.), SiO2 (0-4.0%wt.), Cl (0-0.2%wt.), and SO3 (0-0.3%wt.) were detected. The total of the analyses varied from 80.3 to 100.9%wt. with a mean of 87.9%wt. Despite the relative simple mineralogy it is not simple to relate the analyses to different mineralogies (or mixtures) because of the unknown porosity of the analysed points and the content of structural hydroxyls in goethite. However, point analyses having totals close to 100 may be considered to be dominated by relatively bulky aggregates of maghemite. These points are characterised by quite variable Fe/Ni weight ratios (between 15 and 25) being distinctly higher than the estimated average for the unweathered meteorite having an average value of 13. The values are also not significantly different from those calculated from all the analyses with a mean value of approximately 19. Thus a relative loss in Ni in all the oxide phases is evidenced. References: [1] Sanches J. and Cassidy W.(1966) JGR, 71, 4891-4895. [2] Buchwald V. F. (1975) Handbook of Iron Meteorites, 1-3.
Bender Koch Christian
Buchwald Vagn F.
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