THE MIDINFRARED TRANSMISSION SPECTRA OF ANTARCTIC UREILITES

The mid-infrared (4000-450 cm-1; 2.5-22.2 mum) transmission spectra of seven Antarctic ureilites and 10 Antarctic H-5 ordinary chondrites ar e presented. The ureilite spectra show a number of absorption bands, t he strongest of which is a wide, complex feature centered near 1000 cm -1 (10 mum) due to Si-O stretching vibrations in silicates. The profil es and positions of the substructure in this feature indicate that Mg- rich olivines and pyroxenes are the main silicates responsible. The re lative abundances of these two minerals, as inferred from the spectra, show substantial variation from meteorite to meteorite, but generally indicate olivine is the most abundant (olivine:pyroxene = 60:40 to 95 :5). Both the predominance of olivine and the variable olivine-to-pyro xene ratio are consistent with the known composition and heterogeneity of ureilites. The H-5 ordinary chondrites spanned a range of weatheri ng classes and were used to provide a means of addressing the extent t o which the ureilite spectra may have been altered by weathering proce sses. It was found that, while weathering of these meteorites produces some weak bands due to the formation of small amounts of carbonates a nd hydrates, the profile of the main silicate feature has been little affected by Antarctic exposure in the meteorites studied here. The mid -infrared ureilite spectra provide an additional means of testing pote ntial asteroidal parent bodies for the ureilites. At present, the best candidates include the subset of S-type asteroids having low albedos and weak absorption features in the near infrared.