Low temperature phase transformations in the metallic phases of iron and stony - iron meteorites

Analytical electron microscopY techniques were used to determine the Ni content and structure of kamacite and decomposed taenile (clear taenite I (tetrataenite). the cloudy zone. and clear taenite 2) in the metallic phases of meteorites. Clear taenite 1 contains 51.4—45.6 ± 1.3 wt% Ni and is ordered FeNi with three variants of the L l superstructure. The structure formed by an order/disorder transformation below 320°C. The cloudy zone formed through spinodal decomposition resulting in a two-phase structure consisting of a globular phase and a surrounding honeycomb phase. The globular phase contains 50.9 ± 1.4 wt% Ni and is ordered FeNi; the honeycomb contains II.? ± 0.5 wt% Ni and is martensite. Clear taenite 2 contains 25.8 to 28.1 wt% Ni and may be ordered Fe3Ni with the LI2 superstructure. Clear taenite 2 was found in all of the iron meteorite groups studied and in the pallasites, but not in the mesosiderites. Using this chemical and structural information, and a newly determined Fe-Ni phase diagram, a better understanding of the low temperature transformations in the metallic regions of meteorites has been obtained.