Characterization of iron meteorites by scanning electron microscopy, X‐ray diffraction, magnetization measurements, and Mössbauer spectroscopy: Gibeon IVA

Abstract

AbstractGibeon IVA iron meteorite fragment was characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of α‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases and plessite structures. There are no troilite inclusions observed in the studied section. EDS studies indicate some variations in the Ni concentrations: (i) within the α‐Fe(Ni, Co) phase in the range ~5.0 ± 0.1 – ~7.5 ± 0.1 at% and (ii) within the γ‐Fe(Ni, Co) phase in the range ~26.0 ± 0.2 – ~36.1 ± 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic γ‐phase in addition to the ferromagnetic γ‐phase. EDS also shows that Ni content in two plessite structures is varying in the range ~16–37 at%, which can indicate the presence of only the α2‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the γ‐phase decomposition with the incomplete martensitic transformation: γ → α2 + γ due to a faster cooling rate. XRD indicates the presence of ~1.3 wt% of the γ‐Fe(Ni, Co) phase in Gibeon VIA. The saturation magnetization moment of 185(2) emu g−1 obtained also confirms the presence of phases with low and high Ni concentrations. The most appropriate fit of the Gibeon IVA Mössbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic α2‐Fe(Ni, Co), α‐Fe(Ni, Co), γ‐Fe(Ni, Co), and paramagnetic γ‐Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the α2‐Fe(Ni, Co) phase, 78.3% in the α‐Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic γ‐Fe(Ni, Co) phases.