Ultra-low background gamma-ray spectrometry, SEM-EDX and XRD investigation of a fragment of the Mundrabilla (Australia) iron meteorite. Rare cosmogenic 26Al and 60Co radioisotopes evidenced

The Mundrabilla meteorite can be classified as a medium octahedrite nickel-iron type, the kamacite being the dominant mineral. The meteorite was discovered in 1911 in Mundrabilla (Australia), the most important fragments weighing between 3.5 kg and 24 tons.

To get more information concerning the structure and composition of a 1.5 kg fragment of the Mundrabilla meteorite existing in the collection of the National Geological Museum, Bucharest, a small fragment was extracted using a water jet cutter. More analytic techniques, such as XRD, SEM-EDX, and ultra-low background gamma ray spectrometry, were used to analyse it.

A detailed investigation performed by XRD evidenced the presence of the α-FeNi phase, identified as kamacite. Its crystal chemical formula, calculated based on SEM-EDX analysis, was Fe_0.937Ni_0.063. The cell parameters of kamacite, as determined by least squares refinement of the X-ray powder data, are: a = 2.8717(7) Å and V = 23.68 ų. On the diffraction pattern, minor peaks were observed, which could be attributed to γ-FeNi taenite.

The geochemical composition determined by SEM-EDX investigation is typical of iron-bearing meteorites. XRD indicates as main phase kamacite, but traces of other elements reflect the presence of other minor mineral phases. The presence of quite abundant C and minor Si fits with the presence as minor phases of moissanite (SiC) and cohenite (Fe,Ni)₃C. The S content could be related to traces of troilite (FeS) or pyrrhotite (Fe_1-xS), while the presence of minor P could be attributed to rhabdite (Fe, Ni)P.

The gamma-ray spectroscopy performed in the ultra-low background laboratory at the Slanic (Prahova) salt mine evidenced the presence of ²⁶Al and ⁶⁰Co, two cosmogenic radionuclides produced by cosmic neutrons through the spallation of ²⁸Si or resulting from the β-decay of ⁶⁰Fe, which is also generated by the neutron activation of the stable ²⁸Fe. Both ²⁶Al and ⁶⁰Fe are long-lived isotopes with half-life times of 0.747 and 2.62 My, respectively, which explain their presence in meteorites.