______ is a crystalline form of carbon which is soft and can write.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a crystalline form of carbon that is soft and can be used for writing.

2. **Identifying Crystalline Forms of Carbon**: There are two primary crystalline forms of carbon: graphite and diamond.

3. **Analyzing the Properties**:
- **Graphite**: This form of carbon is known for its softness and is commonly used in pencils, which allows it to write.
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