Correlative Microstructural Analysis of a Weathered Nantan Meteorite Fragment

TL;DR

This study analyzes the microstructural and compositional changes in a weathered Nantan meteorite fragment using various spectroscopic and diffraction techniques, revealing weathering processes and phase formations.

Contribution

It provides a detailed correlative analysis of weathering effects on meteorite microstructure and composition, highlighting the formation of specific mineral phases and alteration pathways.

Findings

Meteorite matrix is mainly magnetite with high Ni regions.

Grain size varies with Ni content, about 5 μm in high Ni areas.

Presence of NiO, magnetite, and iron/nickel carbonates in brecciated phases.

Abstract

The weathering of iron-rich phases within meteorites is a process that significantly alters the microstructure and chemical composition based on the environmental conditions at the location of landing and exposure time since fall. This work investigates the resulting phases in a correlative and comparative manner using a Nantan meteorite fragment. Techniques including X-ray Photoelectron Spectroscopy, Energy Dispersive X-ray Spectroscopy, and X-ray Fluorescence Spectroscopy were used for compositional determination and X-ray Diffraction and Electron Backscatter Diffraction for phase determination and microstructural analysis. These techniques revealed the meteorite matrix to be predominantly composed of magnetite, with distinct regions of high Ni content. The grain size was found to be approximately 5 $\mu$m in $\geq$ 2.6 at$\%$ Ni content regions with a visible boundary of 100-200 $\mu$m extending into $\leq$ 0.9 at$\%$ Ni regions, wherein the grain size averaged 10s of $\mu$m. Additionally, a brecciated cohenite phase was found with a vein-line structure, composed of NiO, magnetite, and deposits of iron and nickel carbonates. This indicates that the matrix regions formed through the weathering of discrete primary phases, with the high Ni regions forming from aqueous alteration of kamacite and the low Ni regions forming from direct dissolution and oxidation of the source Fe-Ni metal.