On the Fine Structure of Meteoritical Taenite/Tetrataenite and its Interpretation

Taenite fields from several meteorites have been investigated by TEM, Electron microprobe and Mössbauer spectroscopy. The investigations have disclosed a delicate pattern of antiphase domains in the tetrataenite and the presence of low-Ni taenite at the antiphase boundaries, in what was hitherto believed to be pure tetrataenite. The observations indicate that the "cloudy taenite" (cloudy zone II) was formed by a magnetically induced spinodal decomposition of the metastable taenite during slow cooling below 400°C. As recently demonstrated [18, 20], such a decomposition is likely to occur when the Curie-temperature of the alloy changes rapidly with composition, as it does in f.c.c. iron-nickel alloys containing approximately 28-43 % Ni. (All compositions in weight %). The large contribution to Gibbs free energy from magnetic ordering gives rise to inflections in the Gibbs free energy curve, making the alloy unstable towards decomposition, here into a magnetically and atomically ordered Ni-rich alloy plus a magnetically and atomically disordered Ni-poor alloy. The model accounts apparently well for the structure and composition of the two phases in the cloudy taenite, and also for the peculiar response of f.c.c. iron-nickel alloys to irradiation with energetic neutrons or electrons at temperatures below 400°C.