Alunite
A potassium-aluminum sulfate mineral historically mined for alum.
Alunite is a hydroxylated aluminium potassium sulfate mineral with the formula KAl3(SO4)2(OH)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum. Initially called aluminilite by J.C.
- formula
- KAl3(SO4)2(OH)6
- first_observed
- 15th century at Tolfa, near Rome
- crystal_symmetry
- trigonal
- hardness_mohs
- 4
- specific_gravity
- 2.6–2.8
- color
- white to yellow gray
- notable_variety
- natroalunite (sodium-rich)
Lore & Background
Alunite occurs as veins and replacement masses in trachyte, rhyolite, and similar potassium-rich volcanic rocks. It is formed by the action of sulfuric acid-bearing solutions on these rocks during the oxidation and leaching of metal sulfide deposits, and is also found near volcanic fumaroles. The white, finely granular masses closely resemble finely granular limestone, dolomite, anhydrite, and magnesite in appearance. More compact kinds from Hungary have been used for millstones. Historically extensive deposits were mined in Tuscany and Hungary, and at Bulahdelah, New South Wales, Australia. The Tolfa mines in Italy, once a major source, are no longer active for commercial mining. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; the ghost town of Alunite, Utah near Marysvale; and Red Mountain near Patagonia, Arizona. Alunite is mined as an ore of both potassium and aluminium at Marysvale. Some ore deposits were located by airborne and satellite multispectral imaging. In one jar, a mixture of alunite and potassium nitrate was found, thought to be a 'mixture of immortality' mentioned in ancient Chinese texts.
Reader's Guide
Alunite holds significance as a historical source of alum, used since the 15th century at Tolfa, Italy, for manufacturing alum—a critical mordant in dyeing and leather processing. The mineral's crystal morphology, rhombohedra with interfacial angles of 90° 50', causes it to resemble cubes, and its trigonal symmetry distinguishes it. Alunite's formation through sulfuric acid action on volcanic rocks links it to acid mine drainage and secondary mineral deposits. Its economic importance continues: it is mined for potassium and aluminium at Marysvale, Utah, with deposits located via airborne and satellite multispectral imaging. Alunite is also an analog of jarosite, with aluminium replacing Fe³⁺, and sodium can substitute for potassium, forming natroalunite. Its hardness of 4 and specific gravity of 2.6–2.8, along with insolubility in water or weak acids but solubility in sulfuric acid, define its physical and chemical properties.
Did You Know?
- Alunite was first observed in the 15th century at Tolfa, near Rome, where it was mined for alum manufacture.
- Alunite crystals are rhombohedra with interfacial angles of 90° 50', causing them to resemble cubes.
- In the United States, alunite is found at the ghost town of Alunite, Utah near Marysvale.
Frequently Asked Questions
Who is Alunite?
Alunite is a hydroxylated aluminium potassium sulfate mineral with the formula KAl3(SO4)2(OH)6. It crystallizes in the trigonal system and typically presents in pale white to yellowish-gray tones.
What is Alunite's role in the mineral world?
Alunite has been valued primarily as a practical industrial mineral, historically extracted to produce alum for tanning, dyeing, and water-treatment applications. It never gained a reputation as a gemstone, but its utility in alum manufacture defined its place in mining history.
What are Alunite's physical stats?
Alunite registers a Mohs hardness of 4 and a specific gravity between 2.6 and 2.8, placing it in the soft-to-moderate range. Its trigonal crystal symmetry gives specimens a recognizable geometric habit distinct from many other sulfate minerals.
Why is Alunite important to mineral enthusiasts?
Alunite stands out as one of the earliest sulfate minerals exploited on an industrial scale, with documented mining in central Italy stretching back centuries. Its two-stage naming history across three decades of French mineralogy also makes it a small case study in how scientific nomenclature evolves.
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