Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.493b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 493
Computer Science
Akaganeite, Hibbingite, Lawrencite, Meteorites, Carbo, Iron, Jerslev, Odessa, Sardis, Toluca, Waverly, Willamette, Yamato 791694, Weathering
Scientific paper
In a continuing survey over the corrosion of meteorites[1-3], the role of chlorine has in particular been examined. It has been shown that even under pristine antarctic conditions, akaganeite, the chlorine containing Beta-FeOOH will form[1,2], and that akaganeite is a common mineral in terrestrial corrosion also under temperate and sub-tropic conditions. Especially when an (iron) meteorite is transported from its soil environment to the laboratory or the museum, akaganeite formation may be violent and lead to destruction of the material. One way of improving the resistance to deterioration is to remove chlorine by exposing the sample to a hydrogen plasma[4]. Chlorine is present at a higher content, 18 wt%, in the compound Beta-Fe(sub)2(OH)(sub)3Cl. This compound was identified in the corroded parts of the iron meteorites Jerslev, Carbo, Odessa, Sardis, Toluca, Waverly, Willamette and Yamato 791694. The research was mainly carried out in The National Museum of Natural History, Washington, in 1988. The mineral is remarkable as being the only corrosion product that contains iron solely in the oxidation state II[1]. The mineral has later been identified in terrestrial rocks and it has been named hibbingite[5]. Hibbingite in iron meteorites occurs as thin films and as up to 1 x 0.1 mm veins and void-fillings. The mineral appears greenish under the microscope. It is softer than the iron matrix and adjacent goethite and magnetite, but it withstands routine preparation of sections and wet polishing operations. It is often associated with akaganeite, but is greenish as compared to the orangered color of akaganeite. Hibbingite belongs to a small group of hexagonal metalhydroxyhalogenides, Me(sub)2(OH)(sub)3X, where Me may be Cu(II), Co(II), Ni(II) and Fe(II), and X may be Cl, Br and I. In meteorites hibbingite is surprisingly pure -compared to e.g. akaganeite- since the lattice only accepts nickel (0-6 wt%) in substitution for iron. Hibbingite has also been identified by microscopy in ancient corroded iron objects, e.g. in iron scrap from 1500-year old bloomery furnace sites, Sorte Muld[1]. Several samples of the weathering crust were obtained from this specimen with the object of further characterization of hibbingite. However, the content of the mineral was too small in all samples for positive identification by x-ray powder diffraction, which was dominated by magnetite (Fe(sub)3O(sub)4) and goethite (alpha-FeOOH). M"ssbauer spectra obtained between room temperature and 60 K show, in addition to components due to magnetite and goethite, the presence of a distinct paramagnetic high-spin Fe(II) component. Between 50 and 60 K magnetic ordering of this component occurs. The results indicate that even very small amounts of hibbingite in weathering crusts may be detected. For generations lawrencite, FeCl(sub)2, has been reported as an original mineral in weathered iron meteorites. However, it has been repeatedly shown[6], that lawrencite is a ghost and does not occur as a cosmic mineral. Perhaps what was observed in the weathered iron meteorites was the terrestrial corrosion product: hibbingite. References: [1]Buchwald V. F. (1989) Yearbook, Danish Metallurgical Society, 41-72. [2] Buchwald V. F. and Clarke R. S. (1989) Am. Mineral., 74, 656-667. [3] Post J. E. and Buchwald V. F. (1991) Am. Mineral., 76, 272-277. [4] Sj"gren A. and Buchwald V. F. (1991) Studies in Conservation, 36, 161-171. [5] Saini-Eidukat B. et al. (1994) Am. Mineral., 79, 555-561. [6] Buchwald V. F. (1975) Handbook of Iron Meteorites, Univ. of California, Vol. 1-3.
Buchwald Vagn F.
Koch Bender Chr.
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