Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.224g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 224
Astronomy and Astrophysics
Astronomy
7
Scientific paper
In the early 1980s, it was speculated that various linear polytypes of carbon consisting of alternating single and triple bonds, and having the generic name carbyne, might be carriers of anomalous noble gases (Whittaker et al., 1980). Doubts were, however, expressed (Smith and Buseck, 1982) concerning the identification of carbynes within the meteorite residues studied by Whittaker et al. (1980). In fact, the very existence of a new allotropic form of carbon was questioned: was it indeed a mineral called chaoite present in ejecta from the Ries Crater, as suggested by El Goresy and Donnay (1968), and had it been made in the laboratory by condensation of a carbon vapor (Kasatochkin et al., 1967)? Since 1981 it has become apparent that acid residues prepared for the purpose of isolating the carrier of the noble gas component Xe(HL) are largely nm-sized diamond together with some amorphous diamond-like material (Lewis et al., 1987). While attempting to study the latter by high- resolution transmission, electron microscopy (HRTEM) using a residue (MIL) prepared from Murchison by HF/HCl/Cr2O7^2-/HClO4 treatment, we found crystalline regions with interplanar spacings higher than diamond (0.2 nm), silicon carbide (0.26 nm), and graphite (0.2 nm, 0.34 nm). We have acquired HRTEM images, SAED patterns, and EDX spectra from several of these particles, which tend to be very small, typically 10 nm, and therefore not likely to be airborne contaminants. Lattice images that match the {110} and {203} spacings of chaoite have been found for single crystals. The interplanar spacings calculated from the hexagonal electron diffraction data (using diamond as internal standard) can be compared favorably with chaoite (El Goresy and Donnay, 1968). EDX spectra for most grains show no evidence of elements with atomic number greater than 11 (Na), although in one or two cases peaks for Ca from an unknown source were observed. Diffraction data from Ca- contamined areas were not used for comparison with chaoite. An attempt has been made to obtain electron energy loss spectra (EELS) from individual crystals, so far without success because of their small size and fragility. EELS data with a characteristic carbon K edge, distinct from amorphous carbon, graphite, and diamond, have been obtained, however, from unspecified areas on the sample; the absence of any Si edges rules out the possibility of silicate contamination. Single crystals and microcrystalline material, having similar SAED patterns to the component found in meteorite residues, can be found in abundance in white carbon films deposited on a variety of substrates using plasma and arc techniques. Such materials are very low in Si abundance but contain some Fe. The existence of carbynes at the few percent level in acid residues from primitive meteorites in association with diamond, a material believed to be of presolar origin on the basis of isotopic characteristics, strongly suggests an analogous provenance to the linear carbon allotropes. Verchovsky et al. (this volume) have discussed the existence of an additional anomalous xenon host phase intimately coexisting with meteorite diamond. Given that if diamond is the carrier of Xe(HL), only one crystal in 3 x 10^6 actually contains a noble gas atom, carbynes may still have a role to play in understanding the first recognized noble gas isotope anomaly. A substantial body of astronomical observational data has been acquired documenting the presence of alternating single and triple carbon bonds in space: suggestions (Webster, 1980) that carbynes are a component of interstellar dust need to be further considered. The occurrence of triple- bonded species as grain material is particularly appealing, because cyano groups are common in interstellar molecules, and the exact location of istopically light nitrogen in primitive meteorite acid residues is still under active investigation (Russell et al., 1991). References: El Goresy A. and Donnay G. (1968) Science 216, 984-986. Kasatochkin V. I., Sladkov A. M., Kudryaytsev, Popov N. M., and Korshak V. V., Dokl. Acad. Nauk. SSSR, 177, 358-360. Lewis R. S., Ming T., Walker J. F., Anders E., and Steel E. (1987) Nature 326, 160-164. Russell S. S., Arden J. W., and Pillinger C. T. (1991) Science 254, 1188-1191. Smith P. P. K. and Buseck P. R. (1982) Science 216, 984-986. Webster A. S. (1980) Mon. Not. R. Astron. Soc. 192, 7P-9P. Whittaker A. G., Watts E. J., Lewis R. S., and Anders E. (1980) Science 209, 1512-1514.
Arden John W.
Gaskell P. H.
Gilkes W. R. K.
Pillinger Colin T.
Russell Stuart
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