Alpha-alumina

Alpha-alumina (α-Al₂O₃) is the most stable crystalline form of aluminum oxide. It is widely known as corundum in its natural mineral form and is a cornerstone material in both geology and industry.

🌍 Origins and Naming

• Mineral Equivalent: Corundum (includes gem varieties ruby and sapphire).
• Synthetic Form: Produced industrially from bauxite via the Bayer process and high-temperature calcination.
• Stability: α-Al₂O₃ is the thermodynamically stable polymorph of alumina at all temperatures.

🔬 Chemical and Structural Properties

• Formula: Al₂O₃
• Crystal System: Trigonal (hexagonal close-packed oxygen lattice with Al³⁺ in octahedral sites).
• Color: Colorless to white in pure form; colored varieties due to trace impurities (Cr → ruby, Fe/Ti → sapphire).
• Hardness: 9 on Mohs scale (second only to diamond).
• Density: ~3.97–4.0 g/cm³.
• Melting Point: ~2050 °C.
• Luster: Vitreous.
• Transparency: Transparent to opaque depending on purity and grain size.

⚙️ Industrial and Technological Uses

• Abrasives: Grinding wheels, sandpapers, blasting media.
• Refractories: Furnace linings, crucibles, kiln furniture.
• Ceramics: High-performance ceramics, spark plugs, substrates for electronics.
• Gemstones: Ruby and sapphire are gem-quality α-Al₂O₃ colored by impurities.
• Catalyst Supports: Used in chemical industries due to high surface stability.
• Biomedical: Bioceramics for implants due to biocompatibility and wear resistance.

📖 Scientific Significance

• Polymorphism: Alumina has several metastable forms (γ, δ, θ, κ), but α is the stable end product.
• Materials Science: Serves as a model system for studying ionic bonding and crystal defects.
• Geology: Corundum is a common accessory mineral in metamorphic rocks and placer deposits.

✨ Conclusion

Alpha-alumina (α-Al₂O₃) is the stable crystalline form of alumina, naturally occurring as corundum and industrially vital for abrasives, refractories, ceramics, and gemstones. Its extreme hardness and stability make it one of the most important oxide materials in science and technology.

In short: α-Alumina = stable Al₂O₃ polymorph, corundum, extremely hard, industrially and gemologically vital.