Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..334c&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 334
Mathematics
Logic
Cm Chondrites, Phyllosilicates, Spectral Reflectance
Scientific paper
The composition of the carbonaceous chondrites is dominated by a fine-grained opaque mineral mixture called matrix. In the lowest petrologic type C-chondrites significant alteration of matrix minerals has occured, resulting in compositions dominated by aqueous alteration products such as phyllosilicates, sulfates, oxides, hydroxides, and carbonates. The phyllosilicates top the list of abundant phases, and in the CM chondrites in particular, Fe-rich serpentines are the most important phases [e.g., 1]. King and Clark [2] have characterized the Mg-serpentines and chlorites, noting certain spectral similarities between chlorites and CI1 and CM2 chondrites. However, they found no exact spectral matches. We present here the results of an examination of the reflectance spectra of Fe-serpentines and two varieties of chamosite (chlorite group). We find these minerals can provide a reasonable spectral match to features seen in certain CM chondrites and by extension, the dark asteroids. We have measured the reflectance spectra of several different high-iron phyllosilicates. Samples were primarily obtained from the National Museum of Natural History (NMNH) with one extremely high iron chamosite provided by the University of Munster. Samples were hand picked and ground and measured in bidirectional reflectance from 0.3 to 25 micrometers. Samples were also characterized by x-ray. In the serpentine group we have measured samples of greenalite (Fe^2+,Fe^3+)(sub)2- 3(Si)2O(sub)5(OH)(sub)4, berthierine (Fe^2+,Fe^3+,Mg)(sub)2- 3(Si,Al)2O(sub)5(OH)4, and cronstedtite Fe^2+2Fe^3+(Si,Fe^3+)O5(OH)4. In the chlorite group we have measured two different samples of chamosite (Fe^2+,Mg,Fe^3+)(sub)5Al(Si(sub)3,Al)O(sub)10(OH,O)(sub)s. We have measured an additional Mg-serpentine amesite Mg(sub)2Al(Si,Al)O(sub)5(OH)(su)4, not examined by [2]. (Chemical formulas cited reflect the ideal given in [3].) For comparison to spectra of CM-type chondrites, samples of Murchison and Murray were available to us and these were also measured in reflectance from 0.3 to 25 micrometers. There are a number of spectral differences between the Fe- and Mg- serpentines, most notably that the Fe-bearing minerals lack the strong, narrow feature at 1.4 micrometers. They also lack the strong Mg-OH features between 2.2 and 2.4 micrometers. In addition several of the samples exhibit absorptions near 0.7 and 0.9 micrometers. The absence of the near-infrared features coupled with the presence of absorptions at the long end of the visible allows the Fe endmembers to provide a much better spectral match to near-infrared characterisitics of CM chondrites like Murchison and Nogoya. Additionally, the general slope characteristics below 0.58 micrometers in CM2 chondrites are also well matched by those observed in the Fe-serpentines, particularly the berthierine that we measured. Vilas and Gaffey [4] compared the absorptions near 0.7 and 0.9 micrometers in several CM chondrites with those observed in main- and outer-belt asteroids. They argued for a similar origin for the spectral features so Fe-phyllosilicates may contribute to the observed spectral characteristics of certain asteroids as well. In the infrared the spectra of CM chondrites Murray and Murchison are quite similar with broad absorptions at 3 micrometers, and from 8-12 micrometers, with a narrower feature centered on 6.2 micrometers. The Mg-serpentine, amesite, has abundant spectral features beyond 13 micrometers, which are not seen in the CM chondrite spectra. The Fe-serpentines have absorptions that can contribute to those seen in the CM chondrites, but lacks the large absorptions beyond 13 micrometers, again providing a better spectral match than the Mg-serpentines. In the future we hope to compare the spectra of these Fe- serpentines with a wider variety of CM chondrites. Additionally a theoretical modeling study is planned, which will attempt to match meteorite spectra using their mineralogy and grain size distribution as the initial input to the models. Acknowledgements: This work was begun while W. M. Calvin was a Humboldt Research Fellow at the Inst. fur Planetologie at the University of Muenster. References: [1] Zolensky M. and McSween H. Y. (1988) Meteorites and the Early Solar System (Kerridge and Matthews, eds.), 114- 143. [2] King T. V. V. and Clark R. N. (1989) JGR, 94, 13997- 14008. [3] Fleischer M. and Mandarino J. A. (1991) Glossary of Mineral Species. [4] Vilas F. and Gaffey M. J. (1989) Science, 246, 790-792.
Calvin Wendy M.
King Trude V. V.
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