Correlative microstructural analysis of a weathered nantan meteorite fragment

The weathering of iron-rich phases within meteorites is a multi-stage process that significantly alters the microstructure and chemical composition based both on the environmental condition at the location of landing and the exposure time since the fall. This work investigates the resulting phases of this process in a correlative and comparative manner using a naturally weathered Nantan meteorite fragment. Techniques including X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Fluorescence Spectroscopy (XRF) were used for chemical and compositional determination and X-ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD) for phase determination and microstructural analysis. The combination of these techniques revealed the meteorite matrix to be predominantly composed of magnetite, with goethite and maghemite present as secondary phases. The meteorite had two pairs of distinct regions, one separated by Ni content and the other by grain size and Cl distribution. The Ni content regions are believed to have formed from a former Widmanstätten pattern, with the high ( $\ge$ 2.6 at%) and low ( $\le$ 0.9 at%) Ni regions from plessite and kamacite, respectively. The kamacite and taenite within the plessite are suggested to have weathered through distinct mechanisms. The grain size regions were separated by a visible boundary 100-200 $\mu m$ wide where grains transitioned rapidly from fine to elongated grains and the Cl content changed from low and dispersed to concentrated spots. Lastly, a brecciated phase was found within the sample and was identified as a large cohenite grain which exhibited signs of weathering, including vein-like extensions of the matrix composed of NiO and magnetite, and deposits of iron and nickel carbonates.