Tetrataenite
An ordered iron-nickel alloy, close to half iron and half nickel, that forms only in very slowly cooled metal.
Tetrataenite is an atomically ordered form of iron-nickel with roughly equal amounts of each element. Instead of the random arrangement in taenite, iron and nickel atoms occupy alternating layers, giving a tetragonal structure. It was formally recognised as a mineral in 1980. Ordering happens only below about 320 °C, where atoms move so slowly that the process takes far longer than laboratory timescales; slow cooling over millions of years makes it possible in meteorites.
It is significant in two ways. First, it is magnetically very hard, so tiny tetrataenite islands in the "cloudy zone" at the edges of taenite can record ancient magnetic fields. Studies of the cloudy zone in the pallasites Imilac and Esquel have been interpreted as evidence of a magnetic dynamo in their parent body's core. Second, materials scientists are interested in it as a possible rare-earth-free permanent magnet, if a quick way to make it can be found.
Tetrataenite occurs in the taenite rims of slowly cooled irons, pallasites and mesosiderites, and in the metal of some chondrites. It cannot be seen by eye; it is identified with electron microscopy, X-ray methods or magnetic techniques.
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