Characterization of iron meteorites by scanning electron microscopy, X-ray diffraction, magnetization measurements, and Mössbauer spectroscopy: Kayakent IIIAB

Abstract

A fragment of the Kayakent IIIAB iron meteorite was analyzed using optical microscopy, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM show the presence of (i) the pure α₂-Fe(Ni, Co) grains, (ii) the γ-Fe(Ni, Co) phase grains, (iii) the γ-Fe(Ni, Co) rims around the α₂-Fe(Ni, Co) phase areas, (iv) the cloudy zone (a mixture of the γ-FeNi(Co) and α₂-Fe(Ni, Co) phases), (v) plessite structures, and (vi) schreibersite inclusions in the α-Fe(Ni, Co) phase. The α-Fe(Ni, Co) phase demonstrates the ε-structure α_ε-Fe(Ni, Co) with the presence of at least three different orientations of the α_ε-Fe(Ni, Co) microcrystals, as shown by EBSD. EDS indicates variations in the Ni concentrations in the following ranges: (i) ~5.4–7.2 atom% in the α-Fe(Ni, Co) phase, (ii) ~15–18 atom% in the α₂-Fe(Ni, Co) phase, and (iii) ~29–47 atom% in the γ-Fe(Ni, Co) phase grains. Schreibersite inclusions contain ~23.5–23.6 atom% of P, ~45.1–46.5 atom% of Fe, and ~28.8–31.4 atom% of Ni. The presence of ~98.1 wt% of the α-Fe(Ni, Co) phase and ~1.9 wt% of the γ-Fe(Ni, Co) phase is found by XRD in the powdered sample, while schreibersite is detected by XRD in the surface of the section only. Magnetization measurements show ferromagnetic multiphase material and a magnetic saturation moment of 175 emu g⁻¹. The room temperature Mössbauer spectrum of the powdered Kayakent IIIAB sample demonstrates six magnetic sextets related to the ferromagnetic α₂-Fe(Ni, Co), α-Fe(Ni, Co), and γ-Fe(Ni, Co) phases and one singlet assigned to the paramagnetic γ-Fe(Ni, Co) phase. In addition, the Mössbauer spectrum shows six minor magnetic sextets associated with ⁵⁷Fe in the M1, M2, and M3 sites in schreibersite and one minor doublet shape assigned to the superparamagnetic rhabdite microcrystals. The iron fractions in the detected phases can be roughly estimated as follows: (i) ~11.9% in the α₂-Fe(Ni, Co) phase, (ii) ~75.6% in the α-Fe(Ni, Co) phase, (iii) ~5.7% in the disordered γ-Fe(Ni, Co) phase with Ni content of ~34–40 atom%, (iv) ~1.5% in the more ordered γ-Fe(Ni, Co) phase with a higher Ni content (~46–47 atom%), (v) ~0.5% in the paramagnetic γ-Fe(Ni, Co) phase (~29–33 atom% of Ni), (vi) ~3% in schreibersite, and (vii) ~2% in rhabdite.