Name the chief forms of the occurrence of the following in the earth's crust :
(a) Aluminium
(b) Calcium
( c) Sodium
(d) Lead.

Explanation

To answer the question regarding the chief forms of occurrence of the specified metals in the Earth's crust, we will identify the primary ores for each metal mentioned: Aluminium, Calcium, Sodium, and Lead.

### Step-by-Step Solution:

1. **Aluminium**:
- The chief ore of aluminium is **bauxite**.
- Chemical formula: **Al₂O₃·2H₂O** (Aluminium oxide with water).
- Another ore of aluminium is **cryolite**.
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