Martensite
Martensite is a hard, needle-like form of iron-nickel metal produced when taenite is cooled very quickly.
Martensite is a form of iron-nickel metal with a distorted crystal structure that forms when hot taenite is cooled so quickly that its atoms cannot rearrange into kamacite in the normal way. Instead, the crystal shears suddenly into a needle-like pattern. The same process hardens quenched steel.
In meteorites, martensite appears where metal was reheated and rapidly cooled. In iron meteorites, this can happen in the heat-affected zone just under the fusion crust, a few millimetres deep, or throughout specimens reheated by impacts. It is also seen in the metal of shocked chondrites and in some taenite grains that cooled through critical temperatures faster than normal.
On etched surfaces martensite shows as very fine, featureless or finely needled areas. Its presence is a clue to rapid thermal events. Collectors should know that heating an iron meteorite, for example with a torch, can create artificial heat-alteration zones and destroy scientific information.
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