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

Abstract

Abstract-On October 7, 2008, the asteroid 2008 TC 3 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 ). Here we report 39 mid-infrared (mid-IR) (4000-450 cm )1 ; 2.5-22.2 lm) 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 lm) due to Si-O stretching vibrations. The profiles of the silicate features fall along a mixing line with endmembers represented by Mgrich 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.

The elemental composition of Almahata Sitta

“…While fragments #4 and #15 were of the compact pyroxene‐rich lithology (Zolensky et al. 2010), #47 seems comprised of the olivine‐dominated compact lithology (Sandford et al. 2010), and fragment #7 is finer grained (Zolensky et al.…”

“…Although the mineralogy and petrography were not investigated in detail (Zolensky et al. 2010), analyses of sample #47 by Sandford et al. (2010) suggest that fragment #47 is representative of the olivine‐dominated compact lithology (99%, versus 1% pyroxene).…”

The recovery of asteroid 2008 TC3

“…Subsequent studies of the crumbs showed these to be ureilites (Rumble et al. 2010; Sandford et al. 2010).…”

The oxygen isotope composition of Almahata Sitta

“…A foliation of planar pores defines surfaces of low strength along which the asteroid would easily break upon impact with the atmosphere. Measured infrared spectra of some 40 fragments show a range of variation in the relative abundances of olivine versus pyroxene from all olivine to all pyroxene (Sandford et al. 2010).…”