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Fe Ni Phase Diagram

Fe and Ni are the two most abundant siderophile elements. The relative, solar abundances of Fe and Ni are at first order about 10:1. Meteoritic metal therefore contains about 90 wt% Fe and 10 wt% Ni. Metal in meteorites occurs as one of the phases: Ni-poor kamacite and Ni-rich taenite. This can be directly understood from the Fe-Ni phase diagram: all metal is liquid above ca. 1500ºC. A solid solution crystallises below this temperature – gamma-Fe. This gamma-Fe decomposes below ca. 500-700ºC and depending on Ni-concentration into alpha-Fe (kamacite) and gamma-Fe (taenite). Below ca. 360ºC gamma-Fe is replaced by a new phase, FeNi3. This transformation is at this low temperature, however, normally to slow, and only kamacite and taenite is found. The decomposition structure of kamacite and taenite is used to determine the cooling times of iron meteorites.