Midinfrared (2.5–13.5 μm) reflectance spectra of powdered stony meteorites

Abstract

Reflectance spectra (2.5 to 13.5 μm) of 60 powdered meteorite samples representing 50 different meteorites are presented for comparison with spectral measurements of asteroids. Powdered samples were used as analogues of asteroidal regoliths. These spectra show that most powdered meteorite samples have undergone alteration, even if only exposed to water vapor in the air, and many have been contaminated by volatile hydrocarbons characterized by absorption bands near 3.45 μm. In contrast, primary macromolecular hydrocarbons do not display the 3.45-μm bands, or, in fact, any other detectable spectral features in these reflectance spectra. However, powdered meteorites display a wide variety of other spectral features that can be used for their identification. These include residual reststrahlen features, which occur as reflectance peaks, absorption bands due to overtone/combination tone bands, which occur as reflectance troughs, and the Christiansen feature, which also occurs as a trough in reflectance. The most prominent reststrahlen peaks are those of olivine and pyroxene, and the wavelength position of the olivine features is sensitive to the Mg/Fe ratio. Olivine and pyroxene also show strong overtone/combination tone absorption bands that shift position with the Mg/Fe ratio, and calcite can be identified in the spectra of some carbonaceous chondrites from such overtone/combination tone bands. These spectral features can be used independently to help determine mineralogy and meteorite type, but using the entire spectrum in a digital search library is preferred for successfull identification. The spectral effect of the vacuum environment should be relatively small for meteorites compared to that found in previous work for granites, but confident interpretation of the spectra of asteroids will require a better understanding of the effects of the space environment on meteorite spectra and of the spectral mixing model for meteorite minerals.