DETERMINING THE BONDING AND D-OCCUPANCY OF PRECIPITATES FROM IRON METEORITES BY ELECTRON ENERGY-LOSS SPECTROSCOPY (EELS)

A signature of the bonding and electronic structure of the Fe-rich materials in iron meteorites can be probed by measuring the occupancy of the 3d states (n_d) from the Fe L_2,3 edge (also called “white lines”) acquired by electron energy-loss spectroscopy (EELS). Here EELS is used to investigate the effects of bonding by probing the density of empty Fe states above the Fermi level. A series of EELS Fe L_2,3 spectra were acquired from Fe- bearing phases common in iron meteorites, including Fe, Fe₃C, FeS, Fe₃P, and Fe₂P, and compared with spectra from fayalite, pyrite, arsenopyrite, and lollingite. All spectra show the same basic shape of two main peaks that arise from transition from the 2p_3/2 (L₃ edge) and 2p_1/2 (L₂ edge) levels. The detailed shape differences between the spectra reveal changes in their bonding character such as ionicity/degree of covalent Fe d – X p (X = S, C, and P) bonding and the structure of the Fe d band. The measured 3d occupancies (n_d) are – aegerine, 5.2; fayalite, 6.1; troilite, 5.5; iron, 6.9; pyrite, 6.5; Fe₂P, 7.0; schreibersite, 7.3; and, Fe₃P, 7.6. The measured 3d-occupancy of Fe is consistent with the expected 3d⁶4s¹ valence configuration for the native metal. In comparison, the low 3d-occupancy for troilite, which is similar to that measured for aegerine, reflects its Fe²⁺ character, consistent with published band structure calculations. The lower occupancies for Fe₂P, Fe₃P, and Fe₃C are consistent with either filling of the 3d band and/or hybridization between the Fe d – X p states. These data demonstrate the utility of this methodology to provide detailed bonding information for extraterrestrial materials at the nanoscale. It is planned that this method and analysis will be used to measure the d-occupancy, and hence structure of the phases across the kamacite – tetrataenite (anti-taenite) boundary common in iron meteorites.