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Decomposing Cohenite |
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| Meteorite, Nantan, IAB iron |
Cohenite is a naturally occurring iron carbide mineral. It is primary, forming in cooling iron – austenite /taenite (Fe/Ni) over the temperature range of 675° C to 610° C. Below 610° C, cohenite no longer forms, and graphite replaces it. It is found almost exclusively in meteorites containing 6 to 8 percent nickel and cannot form in alloys containing more than 8 percent nickel. Cohenite formation results from a chemical reaction involving taenite and carbon (the carbon comes from the surrounding matrix). The temperature range for cohenite formation is sensitive to the bulk alloy Ni/Fe composition, varying from about 675° C to 610° C. Below 610° C, taenite, cohenite, and kamacite have formed, while others, i.e., schreibersite, are forming until around 400° C when nucleation ceases. Note: schreibersite (massive) can form in temperatures as high as 850° C. Temperature plays a crucial role in the formation and decomposition of cohenite. During cooling, cohenite decomposes through resolution and nucleation of graphite within the surrounding matrix. The decomposition rate of cohenite is dependent upon nickel content, temperature, and the time available before the cooling mass reaches 610° C. However, cohenite, in iron meteorites continues to decompose below 610° C. At this temperature cohenite becomes unstable and begins to break down into its constituent elements - Fe, Ni, and C with the carbon often forming 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 |
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