Mineral Information and Data:

Al2(SO4)3(H2O)12 • 5H2O [CNMNC approved formula]

Named from the Latin alumen ="alum" and the Greek genos ="origin.".

IMA status : Valid species (Pre-IMA 1832) Grandfathered
(8th edition) :  
  (29) Hydrated Acid and Sulfates
  (29.08) where (A)2 (XO4)3 · x(H2O)
(10th edition) :
  (07) Sulfates (selenates, tellurates, chromates, molybdates, wolframates)
  (07.C) Sulfates (selenates, etc.) without Additional Anions, with H2O
  (07.CB) With only medium-sized cations
Crystal system: Triclinic System
Point group (H-M): 1 — pinacoidal
Unit cell: a = 7.42 Å, b = 26.97 Å, c = 6.062 Å, a = 89.95°, b = 97.566°, g = 91.888°
Crystal Habit: Crystals prismatic with a six-sided outline, small and rare; usually as an efflorescence or fibrous crusts
Color: Crystals colorless; aggregates white, yellowish, reddish
Diaphaneity: Transparent to translucent
Luster: Vitreous, aggregates vitreous to silky
Hardness (Mohs): 1.5 - 2
Measured Density: 1.77 g/cm3
Cleavage: 1; {010} perfect
Streak: White
Geologic Setting: Forms by reaction of sulfates from decomposing sulfides with aluminous minerals in shales and slates; in gossan or altered wall rock of pyritic deposits in arid regions; in coal seams; in relatively low-temperature fumaroles.
Mineral Association: Pyrite, marcasite, halotrichite, pickeringite, epsomite, alum, melanterite, gypsum
Other Properties: H2O Soluble.
Comments: Taste alum-like, acid and sharp.


Origin: Carola Mine, Freital, Dresden, Saxony, Germany
Picture size: 5.5 mm
Photo and owner: Thomas Witzke


Colorless to white tabular crystals of alunogen on jarosite
Origin: Ronneburg U deposit, Gera district, Thuringia, Germany
Picture size: 5 mm
Photo and owner: Thomas Witzke