Identification of clear taenite in meteorites as ordered FeNi

Abstract

In the Fe–Ni system, the face-centred cubic (f.c.c.) phase (called taenite in meteorites) is stable for all compositions above 1,185 K. Below this temperature, the body-centred cubic phase (kamacite) is stable and has a maximum Ni concentration of about 7% around 720 K (ref. 1). Metal in meteorites is commonly found to contain kamacite with 4–7% Ni and taenite with 20–45% Ni. Taenite grains have M-shaped Ni profiles which formed on slow cooling during diffusion-controlled growth of kamacite and preserve a record of the cooling rates. Another Fe–Ni phase with 49–57% Ni was found in ordinary chondrites by Taylor and Heymann² and identified as a variety of taenite. They called it clear taenite to distinguish it from the zoned variety, which develops cloudy borders on etching with acid, and proposed that it nucleated at 660–610 K on the edge of isolated kamacite grains. We report here studies of meteoritic metal with reflected-light microscopy and electron-probe analysis, in which we discovered that clear taenite is optically anisotropic and we have found it in all meteorite groups. We use the term clear taenite as originally defined to include only alloys with ∼48–57% Ni, not those with ∼25–30% Ni, which Lin et al.³ called clear taenite II. Except for cloudy taenite which contains ∼30–40% Ni and looks cloudy because it is a submicroscopic two-phase mixture, taenite remains clear on etching. We conclude that clear taenite is the ordered, tetragonal FeNi phase found by Albertsen et al.^4–6 in two iron meteorites using Mössbauer and X-ray techniques, and that it formed from taenite, not kamacite. This tetragonal phase has been synthesised by irradiating FeNi alloys with neutrons or electrons below 593 K (refs 7,8).