Mineral Information and Data:

Ammonioalunite
NH4Al3(SO4)2(OH)6 [CNMNC approved formula]

Named for its composition (Ammonia, Al).

IMA status : Approved Species 1987 (IMA1986-037)
CLASSIFICATION 
Dana
(8th edition) :
30.02.04.06  
  (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 (or 3 2/m) — hexagonal-scalenohedral
Unit cell: a = 7.013 Å, c = 17.885 Å
Crystal Habit: As rhombohedral crystals, to 20 μm, forming granular masses
  
Color: Grayish-white
Diaphaneity: Semitransparent
Luster: Vitreous
Hardness (Mohs): 2 - 3
Measured Density: 2.4 g/cm3
Cleavage: On [{0001}, perfect
Streak: White
  
Geologic Setting: Formed in hot springs under very acid conditions, rich in ammonium and sulfate, poor in potassium, below 100 °C.
Mineral Association: Ammoniojarosite, amorphous SiO2
  

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