Crystallisation of carbon to form diamonds takes place when _____ metal expands during solidification.

Answer

To answer the question "Crystallisation of carbon to form diamonds takes place when _____ metal expands during solidification," we need to identify the metal that allows this process to occur.

### Step-by-Step Solution:

1. **Understanding Crystallization**: Crystallization is a process where a solid forms from a solution or melt, and in this context, we are discussing how carbon can crystallize to form diamonds.

2. **Identifying the Metal**: The question implies that a specific metal must expand during its solidification for carbon to crystallize into diamonds.

3. **Properties of Metals During Solidification**: Generally, most metals contract when they solidify. However, there are exceptions. One such metal is iron.

4. **Iron's Behavior**: When iron is heated and then allowed to cool, it can expand slightly before solidifying. This expansion can create conditions favorable for the crystallization of carbon.

5. **Conclusion**: Therefore, the metal that expands during solidification, allowing for the crystallization of carbon to form diamonds, is **iron**.

### Final Answer:
Crystallisation of carbon to form diamonds takes place when **iron** metal expands during solidification.

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