Alunite

Alunite is a hydrated aluminum potassium sulfate mineral, KAl₃(SO₄)₂(OH)₆. It is a classic sulfate mineral, often forming in oxidized zones of sulfide deposits and in volcanic environments.

🌍 Origins and Naming

• Name Origin: From alum, due to its chemical relationship to alum salts.
• Discovery: Known since antiquity; formally described in the 19th century.
• Type Locality: Alunite was first identified near Tolfa, Italy.

🔬 Chemical and Structural Properties

• Formula: KAl₃(SO₄)₂(OH)₆
• Mineral Group: Alunite supergroup (includes jarosite and related minerals).
• Crystal System: Trigonal.
• Color: White, gray, pink, reddish, or yellowish.
• Habit: Tabular crystals, granular masses, or earthy aggregates.
• Hardness: ~3.5–4 on Mohs scale.
• Specific Gravity: ~2.6–2.8.
• Luster: Vitreous to pearly.
• Streak: White.

⚙️ Geological Occurrence

• Formation:
  • In oxidized zones of sulfide ore deposits.
  • In volcanic fumaroles and altered volcanic rocks.
  • By hydrothermal alteration of feldspar-rich rocks.
• Associated Minerals: Jarosite, kaolinite, pyrite, gypsum.
• Localities:
  • Tolfa, Italy (classic locality).
  • Goldfield, Nevada, USA.
  • Chile, Australia, and many volcanic districts worldwide.

📖 Scientific and Industrial Significance

• Petrology: Indicator of hydrothermal alteration and acid-sulfate environments.
• Industrial Uses:
  • Historically used as a source of alum (KAl(SO₄)₂·12H₂O).
  • Minor use in fertilizers and chemical industries.
• Collectors: Attractive specimens can show pinkish tabular crystals, though massive forms are more common.

✨ Conclusion

Alunite is a hydrated aluminum potassium sulfate, forming in oxidized and volcanic environments. It is scientifically important as an alteration indicator and historically significant as a source of alum, though today it is mainly of academic and collector interest.

In short: Alunite = KAl₃(SO₄)₂(OH)₆, trigonal sulfate, alteration mineral, historically used for alum production.