The naturally occurring aluminum oxide mineral that commonly forms hexagonal barrel-shaped prisms is corundum. Here's a breakdown:   

• Corundum:
  • It's a naturally occurring aluminum oxide mineral with the chemical formula Al₂O₃.   
  • It crystallizes in the hexagonal crystal system, which often results in hexagonal barrel-shaped prisms.   
  • Corundum is known for its hardness (9 on the Mohs scale), making it one of the hardest naturally occurring minerals, second only to diamond.   
  • Varieties of corundum include:
    • Ruby: Red corundum, colored by chromium.   
    • Sapphire: Any corundum that is not red, though most commonly blue, colored by iron and titanium.
  • It is found in a wide variety of rock types.
  • It has many industrial uses, due to its hardness.   

Therefore, the correct answer is Corundum.

✅ Correct Answer: Corundum

🔍 Explanation:

Step 1: Understand the Question

We need to identify a naturally occurring aluminum oxide mineral that typically forms hexagonal barrel-shaped crystals.

Step 2: Analyze the Options

Step 3: Confirm the Characteristics of Corundum

Chemical Formula: Al₂O₃

Crystal System: Trigonal (hexagonal close-packed structure)

Crystal Habit: Forms hexagonal barrel-shaped prisms, often terminated by flat or rhombohedral faces.

Hardness: 9 on the Mohs scale (second only to diamond)

Occurrence: Found in metamorphic rocks (like schist and marble) and igneous rocks.

Varieties:

Ruby (red corundum due to chromium)

Sapphire (other colors, mainly due to iron and titanium)

✅ Final Answer:

Corundum is the naturally occurring aluminum oxide mineral that usually forms hexagonal barrel-shaped prisms.

Solution to Identify the Naturally Occurring Aluminum Oxide Mineral

Step 1: Understand the Key Clues in the Question

The question specifies:

Naturally occurring aluminum oxide mineral → Must be a mineral composed of Al₂O₃.

Forms hexagonal barrel-shaped prisms → Indicates a hexagonal crystal system with a characteristic barrel-like shape.

Step 2: Analyze the Given Options

Let’s evaluate each mineral based on composition and crystal habit:

Corundum (Al₂O₃)

Composition: Pure aluminum oxide (Al₂O₃).

Crystal System: Hexagonal.

Habit: Often forms hexagonal barrel-shaped prisms or tabular crystals.

Examples: Ruby (red) and sapphire (blue) are gem varieties of corundum.

Dolomite (CaMg(CO₃)₂)

Composition: Calcium magnesium carbonate (not aluminum oxide).

Crystal System: Trigonal (rhombohedral).

Habit: Curved saddle-shaped crystals, not barrel-shaped.

Olivine ((Mg,Fe)₂SiO₄)

Composition: Magnesium iron silicate (no aluminum).

Crystal System: Orthorhombic.

Habit: Granular or stubby crystals, not hexagonal barrels.

Chalcopyrite (CuFeS₂)

Composition: Copper iron sulfide (irrelevant to Al₂O₃).

Crystal System: Tetragonal.

Habit: Tetrahedral or massive, not hexagonal.

Step 3: Match the Clues to the Correct Mineral

Only corundum fits both criteria:

It is an aluminum oxide (Al₂O₃).

It crystallizes in hexagonal barrel-shaped prisms.

Step 4: Additional Key Facts About Corundum

Hardness: 9 on the Mohs scale (second hardest natural mineral after diamond).

Occurrence: Found in metamorphic rocks (e.g., marble) and igneous rocks (e.g., syenite).

Uses: Abrasives (emery), gemstones (ruby/sapphire), and industrial applications.

Final Answer:

✅ Corundum (Option 1) is the naturally occurring aluminum oxide mineral that forms hexagonal barrel-shaped prisms.

Why Not Others?

Dolomite, Olivine, Chalcopyrite are not aluminum oxides and lack the hexagonal barrel-shaped crystal habit.

Summary Table for Quick Reference: