Mineral Information and Data:

Alunite
KAl3(SO4)2(OH)6 [CNMNC approved formula]

Named in 1824 as a contraction of the earlier name aluminilite.

IMA status : Redefined IMA Approved; 1999 ()
CLASSIFICATION 
Dana
(8th edition) :
30.02.04.01  
  (30) Anhydrous Sulfates Containing Hydroxyl or Halogen
  (30.02) where (A B)2 XO4 Zq
  (30.02.04) Alunite Group (Alunite Subgroup)
  
Nickel-Strunz
(10th edition) :
07.BC.10
  (07) Sulfates (selenates, tellurates, chromates, molybdates, wolframates)
  (07.B) Sulfates (selenates, etc.) with Additional Anions, without H2O
  (07.BC) With medium-sized and large cations
  
Crystal system: Trigonal System
Point group (H-M): 3m — ditrigonal-pyramidal
Unit cell: a = 6.97 Å, c = 17.38 Å
Crystal Habit: Crystals rhombohedral, often pseudocubic; as druses or aggregates; usually massive; fibrous
  
Color: White, often discoloured grayish, yellowish, reddish, brownish
Diaphaneity: Transparent to nearly opaque
Luster: Vitreous, pearly on base
Hardness (Mohs): 3.5 - 4
Measured Density: 2.6 - 2.7 g/cm3
Cleavage: 2; {0001} distinct, {01-12} in traces
Tenacity: Brittle
Fracture: Conchoidal
Streak: White
  
Polymorphism and Series: Forms a series with natroalunite
Geologic Setting: Formed between 15 °C and 400 °C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification.
Mineral Association: Kaolinite, halloysite, diaspore, pyrite, gypsum, quartz
  

 

Alunite
Origin: Wendy pit near El Indio mine, Region IV, Chile
Sample size: 3 x 3 x 3 cm
Photo: Tom Loomis

 

Alunite
(zoom-in of above)

 

Alunite
Origin: Tolfa near Rome, Italy
Sample size: 4 x 3 x 3 cm
Photo: Tom Loomis

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