Cohenite
An iron-nickel carbide mineral, (Fe,Ni)₃C, found mainly in carbon-rich iron meteorites.
Cohenite is an iron carbide with small amounts of nickel and cobalt, the natural equivalent of the cementite found in steel. It is named after Emil Cohen, a 19th-century German mineralogist who worked extensively on meteorites. It occurs mainly in low-nickel irons that are rich in carbon, especially members of the IAB complex, and is also known in some enstatite chondrites. On Earth it is found in rare native iron, such as the basalt-hosted iron of Disko Island in Greenland.
Cohenite is metastable at low pressure: given enough time at elevated temperature, it breaks down into kamacite and graphite. Its survival, or partial decomposition, therefore offers clues to the thermal history of the meteorite, including reheating by impacts or by atmospheric entry. Its presence also requires sufficient carbon and low nickel in the parent metal, since higher nickel suppresses carbide formation. Canyon Diablo and Magura are classic cohenite-bearing irons.
On polished and etched surfaces, cohenite forms elongated, tin-white crystals, often along kamacite bands and associated with schreibersite. It is hard and brittle. Positive identification requires reflected-light microscopy, microprobe analysis or X-ray diffraction to distinguish it from schreibersite.
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