THE MINERAL ALUNITE

  • Chemistry: KAl3(SO4)2(OH)6, Potassium Aluminum Sulfate Hydroxide.
  • Class: Sulfates
  • Group: Alunite
  • Uses: As a minor source of alum, potassium and aluminum and as mineral specimens.
  • Specimens

Alunite is also known as alumstone and is a source of the chemical known as alum, KAl(SO4)2 - 12H2O. Some alunite has also been used to recover potassium and aluminum.

Alunite forms from the action of sulfuric acids upon potassium rich feldspars in a process called "alunitization". The sulfuric acids accompany hydrothermal solutions, usually rich in certain ore metals. These solutions can result in large bodies of alunite, making alunite a rock forming mineral. As such, alunite can easily be mistaken for massive rock forming dolomite or limestone (calcite). An acid test should prove adequate in identification as alunite does not bubble even when powdered. Alunite also forms at volcanic fumaroles.

The symmetry of alunite is the same as the members of the Tourmaline Group. Crystals of alunite however do not form prismatic crystals like those of the typical tourmaline mineral. Alunite's crystals are more flattened and resemble nearly cubic rhombohedrons. The "rhombohedrons" are actually a combination of two trigonal pyramids.

PHYSICAL CHARACTERISTICS:

  • Color is white or gray to reddish.
  • Luster is vitreous to pearly.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is trigonal; 3 m
  • Crystal Habits include tabular to flattened rhombohedral looking crystals. The "rhombohedrons" are actually a combination of two trigonal pyramids. Crystals are somewhat scarce, small and usually line the fissures in alunite rocks, more commonly as earthy masses, films or crusts, botryoidal and granular.
  • Cleavage is fair in one direction (basal), but only seen in the larger crystals.
  • Fracture is conchoidal to uneven.
  • Hardness is 3.5 - 4.
  • Specific Gravity is approximately 2.7 - 2.8+ (average for translucent minerals)
  • Streak is white.
  • Other Characteristics: Some specimens fluoresce a nice orange color under longwave UV light. Does not react to acids except to slowly dissolve, unlike dolomite and calcite. Also pyroelectric and piezoelectric.
  • Notable Occurrences include Marysvale, Utah; Red Mountain, Custer County, Colorado and Goldfield district, Nevada, USA and Tolfa, Italy.
  • Best Field Indicators are crystal habit, color, non-reaction to acids and hardness.

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