An ammonia synthesis catalyst, fabricated by reducing magnetite (Fe3O4) containing small amounts of Al2O3, CaO and K2O, was studied using transmission electron microscopy. It is observed that the Fe3O4crystal grains are transformed into porous or spongy single crystals of body centred cubic iron (α-Fe) extending over large distances (more than l0 μm). Within the original magnetite grain three orientations of the porous α-Fe crystals are observed consistent with a Bain transformation during reduction. The three orientations of the cubic α-Fe crystal grains within the original magnetite crystal grain result from a 45° rotation of the α-Fe crystal unit cell around the three equivalent (100) axes of magnetite.