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

Gibeon 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 Mossbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of alpha-Fe(Ni, Co) and gamma-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 alpha-Fe(Ni, Co) phase in the range similar to 5.0 +/- 0.1 - similar to 7.5 +/- 0.1 at% and (ii) within the gamma-Fe(Ni, Co) phase in the range similar to 26.0 +/- 0.2 - similar to 36.1 +/- 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic gamma-phase in addition to the ferromagnetic gamma-phase. EDS also shows that Ni content in two plessite structures is varying in the range similar to 16-37 at%, which can indicate the presence of only the alpha(2)-Fe(Ni, Co) and gamma-Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the gamma-phase decomposition with the incomplete martensitic transformation: gamma -> alpha(2) + gamma due to a faster cooling rate. XRD indicates the presence of similar to 1.3 wt% of the gamma-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 Mossbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic alpha(2)-Fe(Ni, Co), alpha-Fe(Ni, Co), gamma-Fe(Ni, Co), and paramagnetic gamma-Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the alpha(2)-Fe(Ni, Co) phase, 78.3% in the alpha-Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic gamma-Fe(Ni, Co) phases.