Abstract
Tetrataenite is a new meteoritic mineral with ideal formula FeNi, which forms by ordering of Fe and Ni atoms in taenite. Albertsen et al. (1978a), using Mössbauer spectroscopy and X-ray diffraction, identified ordered FeNi in two iron meteorites. They found it has the Ll0 (or CuAu) structure like synthetic FeNi: space group P4/mmm with a = 2.533±0.002, c = 3.582±0.002Å, Z = 1. Our electron-probe analyses give (range and mean wt. percent) Ni 48–57, 51; Fe 44–52, 49; Cu 0.11–0.36, 0.20; Co <0.02–2.0, 0.08; P <0.01. Tetrataenite, like taenite, is cream-colored in reflected light, but under crossed polars it can be distinguished by its anisotropy and characteristic polarization colors, which are clearly discernible in well-polished sections.
Tetrataenite has been identified optically in over 50 chondrites and mesosiderites, where it is commonly distributed as 10–50-μm-sized grains in contact with kamacite, troilite, taenite, and silicate. This occurrence was previously described as ‘clear taenite’ by Taylor and Heymann (1971). It also forms in two other locations identified by Albertsen et al.: rims 1–20 µm wide on taenite grains in chondrites, mesosiderites, irons, and pallasites, and grains ≤1 μm in size in adjacent cloudy taenite intergrowths. Massive and rim occurrences of tetrataenite are composed of three sets of irregular oriented crystals, which show intricate lamellar magnetic domains under crossed polars. Tetrataenite forms below 350°C in slow-cooled meteorites that contain taenite, and is most abundant in mesosiderites and some chondrites which have cooled the slowest.
