The mid-infrared transmission spectra of multiple stones from the Almahata Sitta meteorite


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Abstract– On October 7, 2008, the asteroid 2008 TC3 entered Earth’s atmosphere, exploded at 37 km altitude, and created a strewn field of stones, the Almahata Sitta meteorite, in Sudan. A preliminary analysis of one of these stones (#7) showed it to be a unique polymict ureilite (Jenniskens et al. 2009). Here we report 39 mid-infrared (mid-IR) (4000–450 cm−1; 2.5–22.2 μm) transmission spectra taken from 26 different stones collected from the strewn field. The ureilite spectra show a number of absorption bands including a complex feature centered near 1000 cm−1 (10 μm) due to Si-O stretching vibrations. The profiles of the silicate features fall along a mixing line with endmembers represented by Mg-rich olivines and pyroxenes, and no evidence is seen for the presence of phyllosilicates. The relative abundances of olivine and pyroxene show substantial variation from sample to sample and sometimes differ between multiple samples taken from the same stone. Analysis of a mass normalized coaddition of all our ureilite spectra yields an olivine-to-pyroxene ratio of 74:26, a value that falls in the middle of the range inferred from the infrared spectra of other ureilites. Both the predominance of olivine and the variable olivine-to-pyroxene ratio are consistent with the known composition and heterogeneity of other ureilites. Variations in the colors of the samples and the intensities of the silicate feature relative to the mass of the samples indicate a significant contribution from additional materials having no strong absorption bands, most likely graphitized carbon, diamonds, and/or metals.