Characterization of iron meteorites by scanning electron microscopy, x-ray diffraction, magnetization measurements, and Mossbauer spectroscopy: Mundrabilla IAB-ung

A fragment of Mundrabilla IAB-ung iron meteorite was analyzed using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mossbauer spectroscopy. The polished section of meteorite fragment characterization by optical microscopy and SEM shows the presence of the gamma-Fe(Ni, Co) phase lamellae, plessite structures and schreibersite inclusions in the alpha-Fe(Ni, Co) phase. EDS indicates variations in the Ni concentrations in the following ranges: (i) similar to 6.3-6.5 atom% in the alpha-Fe(Ni, Co) phase and (ii) similar to 22 to similar to 45 atom% in the gamma-Fe(Ni, Co) phase lamellae including the range of similar to 29-33 atom% of Ni leading to the paramagnetic state of the gamma-Fe(Ni, Co) phase. Schreibersite inclusions contain similar to 23 atom% of P, similar to 33 atom% of Fe, similar to 43 atom% of Ni, and similar to 0.7 atom% of Co. Plessite structure contains the average Ni concentration of similar to 17 atom% while detailed EDS analysis shows: (i) the lowest Ni concentrations of similar to 5 to similar to 8 atom%, (ii) the intermediate Ni concentrations of similar to 9 to similar to 19 atom%, and (iii) the highest Ni concentration up similar to 38 atom% (some individual micro-grains demonstrate up to similar to 47 and similar to 59 atom% of Ni) that may indicate the presence of the (i) alpha-Fe(Ni, Co), (ii) alpha(2)-Fe(Ni, Co), and (iii) gamma-Fe(Ni, Co) phases. These may be a result of the c-phase decomposition with mechanism gamma -> alpha + alpha(2) + gamma that indicates a slow cooling rate for Mundrabilla IAB-ung iron meteorite. The presence of similar to 98.6 wt% of the alpha-Fe(Ni, Co) phase and similar to 1.4 wt% of the gamma-Fe(Ni, Co) phase is found by XRD while schreibersite is not detected. Magnetization measurements show the saturation magnetization moment of Mundrabilla IAB-ung of 188(2) emu g(-1) indicating a low average Ni concentration in Fe-Ni-Co alloy. Mossbauer spectrum of the bulk Mundrabilla powder demonstrates five magnetic sextets related to the ferromagnetic alpha(2)-Fe(Ni, Co), alpha-Fe(Ni, Co), and gamma-Fe(Ni, Co) phases and one singlet associated with the paramagnetic gamma-Fe(Ni, Co) phase, however, there are no spectral components corresponding to schreibersite. Basing on relatively larger and smaller values of the magnetic hyperfine field, two magnetic sextets associated with gamma-Fe(Ni, Co) phase can be related to the disordered and more ordered gamma-phases. The iron fractions in the detected phases can be roughly estimated as follows: (i) similar to 17.6% in the alpha(2)-Fe(Ni, Co) phase, (ii) similar to 68.5% in the alpha-Fe(Ni, Co) phase, (iii) similar to 11.5% in the disordered gamma-Fe(Ni, Co) phase, (iv) similar to 2.0% in the more ordered gamma-Fe(Ni, Co) phase, and (v) similar to 0.4% in the paramagnetic gamma-Fe(Ni, Co) phase.