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Potassium Alum  (Alum-K)
Current inventory:  0 gems
 

Chemistry:  KAl(SO₄)₂·12(H₂O)
[Hydrated Potassium Aluminum Sulfate]

Discovered in 1875;   IMA status: Valid (pre-IMA; Grandfathered).
Potassium Alum is named for its content of the element potassium (K) and the Latin, alumen.

Classification

Mineral Classification:

Sulfates

Strunz 8th Ed. ID:

6/C.14-20

Nickel-Strunz 10th Ed. ID:

7.CC.20

7 : SULFATES (selenates, tellurates, chromates, molybdates, wolframates)
C : Sulfates (selenates, etc.) without additional anions, with H₂O
C : With medium-sized and large cations

Related to:

Alum Group. Sodium Alum - Zincovoltaite Series.

Crystal Data

Crystallography:

Isometric - Diploidal

Crystal Habit:

Rarely as tiny octahedral crystals, if precipitated from pure water solution; cubic from alkaline solutions. Commonly stalactitic, columnar, granular, or massive efflorescences.

Twinning:

Very rare; on [111]

Physical Properties

Cleavage:

[111] Poor/Indistinct

Fracture:

Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

2.0 - 2.5

Density:

1.757 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 124.32 (Gamma Ray American Petroleum Institute Units)

Other:

Soluble in water

Optical Properties

Color:

Colorless, White

Transparency:

Transparent to Translucent

Luster:

Vitreous

Refractive Index:

1.456  Isotropic

Birefringence:

None (Isotropic); may be weakly anomalously birefringent and sectored

Dispersion:

n/a

Pleochroism:

None

Occurances

Geological Setting:

Formed from argillaceous rocks or coal containing oxidizing pyrite or marcasite; may be a fumarolic or solfataric precipitate.

Common Associations:

Alunogen, Gypsum, Epsomite, Melanterite, Pickeringite, Sulfur

Type Locality:

Possibly Alum Grotto, Porto Levante, Vulcano Island, Eolie Islands (Aeolian Islands), Lipari, Messina Province, Sicily, Italy

Year Discovered:

1875

View mineral photos:

Potassium Alum Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Potassium Alum (also known as Alum-K) is an extremely rare sulfate mineral that is very rarely available as a faceted gem. It is a very unusual mineral in that it is both barely radioactive and water soluble.

There are very few localities to find gem quality crystals of Potassium Alum. A few of these are the El Desierto mine, San Pablo de Napa, Daniel Campos Province, Potosí Department, Bolivia; Alaunhöhle, near Dresden, Saxony, Germany; Alum mine, Alum District, Esmeralda County, Nevada, USA; Alum King Mine, Marysvale District (Marysvale Uranium area), Piute County, Utah, USA.
 

  
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