Depending on the conditions under which it occurs, a distinction is made between terrestrial, or telluric, native iron (nickel-iron) and meteoric (cosmic) native iron, which always contains nickel (kamacite and taenite).
Terrestrial iron is a rare mineral, a modification of α-iron, with a body-centered cubic structure, which crystallizes in the cubic system. It is found in the form of separate scales, grains, wirelike shapes, or spongy masses and aggregates, weighing up to several tons. Chemical composition is basically restricted to Fe and Ni, which form solid solutions with discontinuous miscibility; a distinction is made between the so-called ferrites, which contain up to 3 percent Ni, and native nickel-iron (awaruite, catarinite, octibennite, josephinite, and other varieties), which contains 30–80 percent Ni.
The hardness of native iron on the mineralogical scale is 4–5 (for nickel-iron); the density of ferrites is 7,300–7,800 kg/m3; of nickel-iron, 7,800–8,200 kg/m3. The color and reflex are the same as that of metallic iron; some varieties of nickel-iron have a silver-white color. It is strongly magnetic. It is rarely formed and preserved in the earth’s crust. It is known in the form of grains and spongy aggregates in basalt rocks (Disko Island, near Greenland; Kassel, Federal Republic of Germany). It is rarely found in peridotites and serpentines; very rarely, in granites. It is found in platinum-bearing placers and also forms in siderite sediments, coals, and bog iron ores. It is very unstable, passing readily into iron hydroxide. Meteoric native iron forms during the processes of formation of cosmic bodies, and it falls to earth as meteorites.
REFERENCEMineraly: Spravochnik, vol. 1. Moscow, 1960.
G. P. BARSANOV