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Elongated Cohenite |
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| Meteorite: Uruacu, IAB iron |
| Figure 1. Scale bar 1.4 mm. |
| Meteorite: Uruacu, IAB Iron |
| Elongated cohenite. |
| Elongated bodies of cohenite (tarnished bronze
color), an iron carbide mineral, form due to directional growth during primary
cooling processes. Directional growth is a process where crystals grow preferentially in specific directions. This process is influenced by: 1. Temperature Gradient (cooling rates): Different parts may cool at different rates as the metal cools, creating a temperature gradient. Crystals tend to grow in the direction where the temperature decreases most rapidly. Slow cooling rates allow atoms to migrate and arrange themselves into elongated structures. 2. Chemical Composition: The chemical composition of the mass plays a crucial role in directional growth. Variations in the composition can lead to the migration and concentration of elements and compounds in specific areas, promoting growth in certain directions. 3. Pressure and Temperature Conditions: The specific conditions during the cooling phase, particularly the pressure and temperature, can significantly influence the formation of elongated crystals. 4. Inclusions: The presence of micron-sized inclusions - troilite, graphite, schreibersite, and daubreelite, in the meteoritic mass, can act as nucleation sites, guiding the growth of crystals in particular directions. Troilite, graphite, and some silicates can be nuclei for cohenite precipitation from solid solution. Elongated cohenite bodies aligned in kamacite lamellae are seen in many iron meteorites, specifically the octahedrites. |
| Figure 3. Scale bar 1.4 mm. |
| Meteorite: Uruacu, IAB Iron |
| Elongated cohenite. |
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