Characterization of bulk interior and fusion crust of Calama 009 L6 ordinary chondrite

Abstract

Fragment of Calama 009 L6 ordinary chondrite recovered in the Atacama Desert was chosen for a complex study of the bulk interior and the fusion crust by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. SEM demonstrated the presence of Fe-Ni-Co grains, troilite and chromite inclusions in both the bulk interior and the fusion crust as well as many veins with ferric compound. EDS showed variations in the Ni concentration within the metal grains and within one metal phase in the grain. XRD revealed some differences in the contents of various phases in the bulk interior and in the fusion crust. XRD indicated the presence of magnesioferrite in the fusion crust as well as the formation of goethite nanoparticles with the mean size of 9 nm in both the bulk interior and the fusion crust. Magnetization measurements demonstrated the ferrimagnetic–paramagnetic phase transition in chromite at 44 K and low values of the saturation magnetization moments (6.46 and 3.26 emu g⁻¹ at 100 K) for the bulk interior and the fusion crust, respectively, due to the lack of Fe-Ni-Co alloy as a result of weathering. The Mössbauer spectra of the bulk interior and the fusion crust showed some differences in the number and relative areas of spectral components. The revealing of the Mössbauer spectral components related to ⁵⁷Fe in the M1 and M2 sites in olivine and orthopyroxene as well as determining the Fe²⁺ occupations of these sites from XRD permitted us to estimate the temperature of equilibrium cation distribution for these silicates which are (i) 662 K (XRD) and 706 K (Mössbauer spectroscopy) for olivine and (ii) 893 K (XRD) and 910 K (Mössbauer spectroscopy) for orthopyroxene.