Electron–optical observations of ordered FeNi in the Estherville meteorite

Abstract

Scott and Clarke¹, using optical techniques, have recently reported the presence of ordered FeNi (taenite) with the tetragonal Ll_o structure (AuCu) in over 40 meteorites. The ordered phase occurs in 10–60 µm diameter regions and in 1–5 µm wide rims of clear taenite containing 48–57% Ni (refs 1, 2). In contrast with disordered γ Fe-Ni, the ordered taenite is optically anisotropic. The X-ray diffraction pattern and Mössbauer spectrum of ordered FeNi ^3–5 are also similar to that of tetragonal FeNi synthesised using high flux neutron irradiation at 300 ° C of a disordered FeNi alloy in the presence of a magnetic field. Although Mössbauer and X-ray diffraction studies^6,7 have been used to detect superstructure in bulk samples, these techniques cannot spatially locate the ordered phase within the microstructure. The present study uses various electron–optical techniques, including transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and electron probe microanalysis (EPMA) to characterise and to resolve the ordered FeNi phase spatially. In addition, the first TEM evidence (both electron diffraction patterns and centred dark field (CDF) images) for superstructure in clear taenite, is presented.