Introduction to Meteorics

Iron meteorites

Iron meteorites are composed of iron metal, generally with between 5 - 20 wt. % nickel, and account for approx. 5% of all observed meteorites falls. The mineralogy of iron meteorites is dominantly an intergrowth of two phases, the iron-nickel alloys kamacite and taenite. Kamacite, or alpha Fe,Ni metal, has a body-centred cubic structure and a Ni content < 7wt.%, whilst taenite, or gamma Fe,Ni metal, is face-centred cubic and approx. 20-50 wt.% Ni.

A polished and etched slice of the Henbury iron meteorite, found in Australia in 1931 Iron meteorites are highly differentiated materials, products of extensive melting processes on their parents. One major division of iron meteorites is into magmatic irons (those that have solidified by fractional crystallisation from a melt; an example is Henbury) and non-magmatic irons (those that seem not have completely melted, and possibly formed during impact processes on their parent asteroids; an example is Canyon Diablo). The irons are sub-divided into 13 different groups on the basis of nickel and trace element chemistries (Ga, Ge and Ir contents). The non-magmatic irons are groups IAB and IIICD, and show a wide range in nickel contents. Many irons defy chemical classification, and simply remain Ungrouped. The chemical classification of iron meteorites has mainly been undertaken by J.T. Wasson and co-workers at UCLA. 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 Ni-rich phases form the Widmansttten pattern revealed in polished and etched iron meteorites. The width of the kamacite lamellae allows classification of iron meteorites into 5 structural groups: the coarsest, coarse, medium, fine and finest octahedrites. Plessitic octahedrites are transitional between octahedrites and ataxites. Ataxites are Ni-rich, with > 20 wt.% Ni, and are mainly taenite. Hexahedrites have Ni < 6 wt.% and are comprised of kamacite only; neither hexahedrites nor ataxites display a classic Widmansttten pattern.