Kamacite
The nickel-poor form of iron-nickel metal in meteorites, forming the broad bands of the Widmanstätten pattern.
Kamacite is one of the two main metal minerals in meteorites. It is an alloy of iron with only a little nickel, usually below about 7.5 per cent, and its atoms sit in a body-centred cubic arrangement, the same structure as ordinary iron at room temperature. Its partner, taenite, is richer in nickel and has a different crystal structure.
As an iron meteorite cools very slowly, kamacite grows as plates inside taenite, drawing in iron and pushing nickel away. The width of those plates depends on how much nickel the metal holds and how fast it cooled, so kamacite is a natural cooling-rate gauge. In hexahedrites, such as Braunau and Coahuila, the whole meteorite is essentially one big kamacite crystal. In octahedrites, such as Gibeon or Canyon Diablo, kamacite forms the broad bands seen when the metal is etched. Kamacite also makes up most of the metal flakes in ordinary chondrites.
On an etched slice, kamacite appears as slightly darker, frosted bands, sometimes crossed by fine Neumann lines from shock. It is strongly magnetic and rusts more readily than taenite. Etching involves acids, so it is a task for experienced adults only.
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