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Decomposing Cohenite |
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| Meteorite, Nantan, IAB iron |
| Cohenite is a naturally
occurring iron carbide mineral (Fe?C) that forms as a primary phase in
iron-nickel alloys, specifically from taenite (a high-nickel Fe-Ni phase) and
austenite, during cooling. It crystallizes over a temperature range of
approximately 675°C to 610°C, provided the alloy contains 6-8%
nickel. Above 8% nickel, cohenite formation is inhibited. The carbon required
for its formation is sourced from the surrounding matrix, reacting with taenite
to produce this mineral. The temperature window for cohenite formation-spanning 675°C to 610°C-varies slightly depending on the Ni/Fe ratio of the alloy. Below 610°C, cohenite ceases to form, and graphite begins to take its place. By this point, phases such as taenite, cohenite, and kamacite (a low-nickel Fe-Ni phase) have stabilized, while other minerals, like schreibersite ((Fe,Ni)?P), start to nucleate. Schreibersite can begin forming at temperatures as high as 850°C and continues down to around 400°C, when nucleation processes generally stop. Temperature is a critical factor in both the formation and breakdown of cohenite. As the alloy cools below 610°C, cohenite becomes unstable and decomposes into its constituent elements-iron (Fe), nickel (Ni), and carbon (C)-with the carbon often precipitating as graphite within the matrix. This decomposition occurs through a process of resolution and graphite nucleation, with its rate influenced by the nickel content, the temperature, and the time available before the material cools past 610°C. In iron meteorites, where cohenite is almost exclusively found, this decomposition continues below 610°C. The unique conditions of slow cooling in space allow cohenite to form and persist, though its instability at lower temperatures leads to its eventual breakdown into Fe, Ni, and graphite. |
Figure 1. Scale bar 1.2 mm. |
| Meteorite, Nantan, IAB |
| Decomposing cohenite surrounding kamacite. |
Figure 1. Scale bar 1.2 mm. |
| Meteorite, Nantan, IAB |
| Decomposing cohenite |
| Listing of Structures |