Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.553m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 553
Other
1
Graphite, Iron-Nickel Metal, Meteorites, Bishunpur, Raman Spectrometry
Scientific paper
In the course of a study of Fe-Ni metal composition in chondrites, we found abundant graphite inclusions in the metal of Bishunpur (L3.1) and other primitive ordinary chondrites [1]. In addition, the metal composition in Bishunpur showed evidence that these graphite inclusions are remains of carbon which served to reduce Cr, P and Si into Fe-Ni during chondrule formation. To help determine the origin of this graphite, we tried to characterize it via Raman microprobe analysis, at LASIR-CNRS, Thiais. In Bishunpur, the generally small sizes of graphite inclusions (<=1 micrometers, which we call tiny graphite) make it difficult to obtain a detailed structural description using optical microscopy. When sufficiently large, we identified them as generally spherulitic [2]. Graphite can fill veins along grain boundaries, at kamacite-taenite or kamacite-kamacite interfaces. Other morphologies, which we did not classify, are also more rarely present. A large graphite-patch (so-called carbon-rich aggregate [3]) was also found in the matrix of Bishunpur. The Raman spectra show that all the analysed graphite in Bishunpur is poorly crystalline, as attested by the prominent peak at 1355 cm^-1 [4](Fig.1). To check the effect of polishing, we compared spectra from polished sections with spectra from non-polished graphite (obtained by dissolution of metal grains). This showed the effect of polishing is important for well crystallised graphite as in Canyon Diablo, but negligible for poorly crystallised graphite as in Bishunpur. The crystallite sizes L(sub)a, calculated from the intensity ratio of the peaks at 1680 and 1355 cm^-1 according to the calibration of [4], range from <=20 Angstrom for vein graphite and the carbon-rich aggregate, to ~50 Angstrom for the less disordered spherulitic graphite. Only the spherulitic and the unclassified graphites have mean crystallite sizes larger than 30 Angstrom, a value indicative of an early stage of crystalline growth [5]. Preliminary data obtained for 5 other type 3 chondrites (Semarkona, Tieschitz, Kohar, Mezo-Madaras, Massenya) are similar to those for Bishunpur. The graphite inclusions in Bishunpur may be formed, like Cr-, P-, and Si- based inclusions [6] by exsolution during metamorphism; either by direct precipitation or by decomposition of carbides. This appears a likely process in particular for the tiny and the vein graphite. However, Raman spectra of vein graphite are different from those of the other inclusions but very similar to that of the carbon-rich aggregate. It may thus be formed by conversion of pre- existing carbonaceous matter during metamorphism [3]. The presence of different graphite structures with different crystalline stages within the same metal grain indicates that metamorphism may not be solely responsible for the formation of graphite in metal. Ion probe isotopic analysis is necessary to obtain more information. It is presently under way. References: [1] Mostefaoui S. and Perron C. (1994) LPS XXV, 945-946. [2] El-Goresy A. (1965) GCA, 29, 1131-1151. [3] Brearley A. (1990) GCA, 54, 831-850. [4] Tuinstra F. and Koenig J. L. (1970) J. Chem. Phys., 53, 1126-1130; Wopenka B. and Pasteris J. D. (1993) Am. Mineral., 78, 533-557. [5] Rouzaud J. N. et al. (1983) Thin Solid Films, 105, 75-96. [6] Zanda B. et al. (1994) Science, 265, 1846-1849.
Mostefaoui Smail
Perron Claude
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