Fe-bearing phases in a ureilite fragment from the asteroid 2008 TC3(= Almahata Sitta meteorites): A combined Mössbauer spectroscopy and X-ray diffraction study

Abstract

The iron-bearing phases in a ureilite fragment (AS#051) from the Almahata Sitta meteorite are studied using Mossbauer spectroscopy, X-ray diffraction (XRD), and electron microprobe analysis (EMPA). AS#051 has a typical ureilite texture of medium- to coarse-grained silicates (olivine, orthopyroxene, and pigeonite) with minor opaques (Fe-Ni metal, troilite, and graphite). The silicate compositions, determined by EMPA, are homogeneous: olivine (Fo90.2), orthopyroxene (En86.3Fs8.6Wo5.1), and pigeonite (En81.6Fs8.9Wo9.5), and are similar to those of magnesian ureilites. The modal abundance of mineral phases was determined by Rietveld refinement of the powder XRD data. The Mossbauer spectra at 295 K and 78 K are composed of two sharp well-defined paramagnetic doublets superimposed on a well-resolved magnetic sextet and other weak absorption features. The two paramagnetic doublets are assigned to olivine and pyroxene (orthopyroxene and pigeonite), and the ferromagnetic sextet to kamacite (magnetic hyperfine field ≈ 33.2 T), in agreement with the XRD characterization. The Mossbauer results also show the presence of small amounts of troilite (FeS) and cohenite ([Fe,Ni,Co]3C). Using the Mossbauer data, the relative abundance of each Fe-bearing phase is determined and compared with the results obtained by XRD.