The formula of mineral borax is:

Explanation

To determine the formula of the mineral borax, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the mineral borax**: The mineral borax is commonly known as sodium borate.

2. **Determine the chemical composition**: Borax is composed of sodium (Na), boron (B), oxygen (O), and water (H2O).

3. **Write the formula**: The complete formula for borax includes the water of crystallization. It is represented as:
\[ \text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \]
This indicates that for every two sodium ions, there are four boron atoms, seven oxygen atoms, and ten water molecules associated with the borax structure.

4. **Summarize the findings**: Therefore, the formula of mineral borax is:
\[ \text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \]

### Final Answer:
The formula of mineral borax is \( \text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \).

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