Mineral Information and Data:

Alum-(K)
KAl(SO4)2 • 12H2O [CNMNC approved formula]

Named in 1875 ; renamed in 2008 for the dominant element K and the Latin, alumen.

IMA status : Renamed IMA Approved; 2008 ()
CLASSIFICATION 
Dana
(8th edition) :
29.05.05.01  
  (29) Hydrated Acid and Sulfates
  (29.05) where A B (XO4)2 · x(H2O)
  (29.05.05) Alum Group
  
Nickel-Strunz
(10th edition) :
07.CC.20
  (07) Sulfates (selenates, tellurates, chromates, molybdates, wolframates)
  (07.C) Sulfates (selenates, etc.) without Additional Anions, with H2O
  (07.CC) With medium-sized and large cations
  
Crystal system: Cubic System
Point group (H-M): m3 (or 2/m 3) — diploidal
Unit cell: a = 12.133 Å
Crystal Habit: Crystals octahedral if precipitated from pure water solution; cubic from alkaline solutions. Commonly massive with a columnar or granular structure; stalactitic, mealy coatings.
Twinning: On {111}, very rare
  
Color: Colorless, white; colorless in transmitted light.
Diaphaneity: Transparent
Luster: Vitreous
Hardness (Mohs): 2
Measured Density: 1.757 g/cm3
Cleavage: Poor/Indistinct {111}, traces
Tenacity: Brittle
Fracture: Conchoidal
Streak: White
  
Geologic Setting: Derived from the oxidation of sulfide minerals and potassium-bearing minerals.
  
Synonyms/varieties: Potassium-alum
Other Properties: H2O Soluble.
Comments: Potassium-alum - Renamed to Alum-(K) by Mineralogical Record, v39 (2008), p131

 

Alum-(K)
Alum-(K) colorless fragment
Origin: Tolfa, Tolfa Mts District, Rome Province, Latium, Italy
Source: American Museum of Natural History 16802
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