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

The iron-bearing phases in a ureilite fragment (AS#051) from the Almahata Sitta meteorite are studied using Mössbauer 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 (Fo_90.2), orthopyroxene (En_86.3Fs_8.6Wo_5.1), and pigeonite (En_81.6Fs_8.9Wo_9.5), 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 Mössbauer 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 Mössbauer results also show the presence of small amounts of troilite (FeS) and cohenite ([Fe,Ni,Co]₃C). Using the Mössbauer data, the relative abundance of each Fe-bearing phase is determined and compared with the results obtained by XRD.