Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.486b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 486
Physics
Chondrites, Ordinary, Spectroscopy, Mossbauer, Weathering
Scientific paper
Introduction: We conducted experiments to induce weathering in the Allegan H5 fall by immersion in aerated water, using: (i) de-ionized ice water - experiment A (ii) de-ionized water at room temperature - experiment B (iii) water at room temperature with a 0.2 molar solution of NaCl - experiment C. Water was changed at regular intervals. In each case, oxidation as a function of time was monitored by removing the sample and measuring the production of ferric iron components using 57Fe Mossbauer spectroscopy. In an earlier study [1], weathering was induced in the Noblesville H4/H6 fall by aerated ice water, with the unexpected result that Fe-Ni metal was relatively resistant to weathering. It was to test this finding, and hypotheses resulting from our own earlier studies [2], that these experiments were initiated. Results: In experiment A, after an initial period of rapid oxidation lasting 15-20 days, production of ferric species proceeded in a linear fashion. The bulk of the weathering products were observed as paramagnetic Fe^3+, which at 77K was resolved as principally akaganeite. Magnetically ordered iron oxides/oxyhydroxides (maghemite, magnetite and goethite) were observed after 93 days. The primary Fe2+/-containing phases (olivine, pyroxene and troilite) weathered at similar rates, whilst Fe-Ni was initially preferentially weathered. For both the B and C experiments, magnetically ordered iron oxides were observed within the first 12 days of weathering, however, a higher proportion of paramagnetic Fe^3+ was seen in C than B. Discussion: The observation of increased akaganeite in C vs. B is consistent with a situation in which increased supply of Cl-ions enhances the thermodynamic range over which akaganeite is stable. The lower proportion of paramagnetic Fe^3+ observed in the room temperature experiments, compared to that seen at 0 degrees C (experiment A), suggests that higher temperatures enhance the degradation of akaganeite to maghemite and goethite. This agrees with observations of [3] and [4]. The fact that Fe-Ni was not preferentially weathered in these experiments may be because the water was changed at regular intervals: Fisher and Bums (1992) proposed that acifification of porewaters by dissolution of FeS led to accelerated chemical weathering of olivine and pyroxene. The presence of akaganeite in samples weathered in de-ionized water is surprising as the accepted wisdom is that Cl- in tunnel sites is essential for the stability of this phase [5], [6], [7]. Akaganeite may contain 3.2-6.9 wt% chlorine [6], indicating around 1 wt% chlorine in our sample. Allegan has a chlorine content of only 9-14 ppm [8] and the water in which the sample was weathered was chlorine-free. Therefore, either chlorine was introduced by contamination; or akaganeite may be stable without Cl- in tunnel sites, possibly substituting OH- In this second hypothesis, it is suggested that Cl- has an affinity to akaganeite, but is not mandatory for stability. It indicates that akaganeite may be a true iron oxyhydroxide and that a review of its chemical formula may be required. References: [1] Fisher D. S. and Burns R. G. (1992) LPS XXIII 369-370. [2] Bland P. A. et al. (1995) LPI Tech. Rpt., in press. [3] Buchwald V. F. (1989) LPI Tech. Rpt. 90-01, 24-26. [4] Buchwald V. F. and Clarke R. S. Jr. (1989) Am. Mineral., 74, 656-667. [5] Chambaere D. G. and DeGrave E. (1984) Phys. Stat. Sol., 83, 93-102. [6] Johnston J. H. and Logan N. E. (1979) J. Chem. Soc., 13-16. [7] Post J. E. and Buchwald V. F. (1991) Am. Mineral., 76, 272-277. [8] Tarter J. G. et al. (1980) Meteoritics, 15, 373-374.
Berry Frank J.
Bland Philip A.
Pillinger Colin T.
No associations
LandOfFree
Artificial Weathering of the H5 Ordinary Chondrite Fall, Allegan does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Artificial Weathering of the H5 Ordinary Chondrite Fall, Allegan, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Artificial Weathering of the H5 Ordinary Chondrite Fall, Allegan will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830407