Tetragonal Lattice of Tetrataenite (Ordered Fe-Ni, 50-50) from 4 Meteorites

Since 1977 it has been known that an ordered alloy occurs in the taenite phase (f.c.c. iron-nickel alloy) of the majority of the meteorites [1-3]. Ideally the ordered alloy is Fe-Ni, 50-50 with the L10 structure, and this naturally occurring compound has been given the mineral name Tetrataenite. It is identical to the artificially ordered FeNi alloy which has been preparated and studied since 1962 [4, 5]. The ordering of taenite leads to a change of symmetry from f.c.c. to tetragonal, but no tetragonal distortion of the lattice has previously been observed. In the present paper we briefly review the mineral composition of the taenite fields in meteorites and report the lattice parameters of well ordered, stoichiometric tetrataenite from 3 iron meteorites and one mesosiderite. The lattice is found to show tetragonal symmetry, and the lattice parameters associated with the f.c.t. unit cell containing 4 atoms are: a = 3.5761 ± 0.0005 A; c = 3.5890 ± 0.0005 A; c/a = 1.0036 ± 0.0002.