An Improved Model to Determine the Cooling Rates of Mesosiderites and Iron

Introduction: Data on the Ni concentration and distribution in kamacite (α) and taenite (γ) of meteoritic metal can be used to determine the cooling rates of meteorites. Several metallographic cooling rate methods have been used to determine the cooling rates of meteorites and have been recently reviewed [1]. The reliability of these models depends on the accuracy of the phase diagram and diffusion coefficients in Fe-Ni alloy systems. This paper reports a major revision to the Hopfe and Goldstein computer model [1] for kamacite growth in meteoritic Fe-Ni metal using the latest Fe-Ni phase diagram [2] and interdiffusion coefficient data [3]. The improved model is used to determine the cooling rates of the mesosiderites and the Toluca iron meteorite. Revised Model Parameters: Phase Diagram. The latest Fe-Ni phase diagram has been assessed experimentally by Yang et al. [2] as shown in Fig. 1. The major revision in the phase diagram is in the region below 400 C. At around 400 C, the monotectoid reaction, γ1→ α+γ2, occurs, where γ2 is a high Ni ferromagnetic fcc phase which transforms to ordered FeNi γ" at around 320 C. In the equilibrium diagram, around 345 C, a eutectoid reaction, γ2→ α+γ', occurs, where γ' is ordered Ni3Fe.