Iron Meteorite
METEORITE composed of iron metal, generally with between 5 and 20% by weight nickel. Iron meteorites account for approximately 5% of all observed meteorite falls. The mineralogy of iron meteorites is dominantly an intergrowth of the two iron–nickel alloys kamacite and taenite. Kamacite (_Fe,Ni) has a bodycentred cubic structure and a nickel content less than 7% by weight. Taenite (_Fe,Ni) is face-centred cubic and c.20–50% by weight nickel.
Iron meteorites are highly differentiated materials, the products of extensive melting processes on their parent bodies. They can be divided into magmatic irons and non-magmatic irons. Magmatic irons are those that have solidified by fractional crystallization from a melt. Nonmagmatic irons are those that seem not to have completely melted; they may have formed during impact processes. The iron meteorites are subdivided into 13 different groups on the basis of nickel and trace element chemistries (gallium, germanium and iridum contents). Each separate group of magmatic irons has a fairly restricted range of nickel contents but a wide range of trace element abundances; these trends are consistent with fractional crystallization from a melt. In contrast, the nonmagmatic irons show a wide range in nickel contents but less variation in trace element composition; these trends can be better explained by formation by partial melting. Many irons defy chemical classification and simply remain 'ungrouped'. It is thought that each chemical group derives from its own parent asteroid.
Prior to classification on the basis of trace-element chemistry, iron meteorites were classified in terms of their metallographic structure. Laths of kamacite intergrown with nickel-rich phases form the 'Widmanstätten pattern' revealed in polished and etched iron meteorites. The width of the kamacite lamellae allows classification of iron meteorites into five structural groups: the coarsest, coarse, medium, fine and finest octahedrites. Plessitic octahedrites are transitional between octahedrites and ataxites. Ataxites are nickel-rich, with more than 20% by weight nickel, and are mainly taenite. Hexahedrites have nickel less than 6% by weight and comprise kamacite only. Neither hexahedrites nor ataxites display a classic Widmanstätten pattern. Meteorites from an individual chemical group can display a range of structural types.
