Micro-Raman spectroscopic investigations of graphite in the carbonaceous meteorites Allende, Axtell and Murchison

Abstract

The graphite content of the carbonaceous meteorites Allende, Axtell and Murchison was investigated by Raman microscopy. Laser excitation wavelengths at 514.5 (2.43 eV), 785 (1.59 eV) and 1064 nm (1.17 eV) were used for Allende and Axtell. Owing to severe background problems, Raman spectra of Murchison were only recorded at 785 nm. The wavenumber of the order G-band was independent of the excitation laser wavelength. A linear dependence on the laser excitation energy of the wavenumber for the disorder D-band was found with an increasing wavenumber for increasing laser energy. The intensity ratios of the D-band and G-band for Allende at 514.5 nm were used to find the graphite order diameter (La) for the graphite sheets. This value was used to find correlation factors between the D/G intensity ratios and La in the NIR region at 785 and 1064 nm. La values for Axtell and Murchison were calculated from D/G-band intensity ratios. The C3-type meteorites Allende and Axtell show very similar La values around 45 Å, whereas the C2 meteorite Murchison has La around 70 Å. This difference is consistent with different environments during the formation process of the graphite. Copyright © 2006 John Wiley & Sons, Ltd.