Alunite
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Alunite gets its name from the Latin word alunit for Alum.

Discovered in 1824;   IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

KAl₃(SO₄)₂(OH)₆

Potassium Aluminum Sulfate Hydroxide

Molecular Weight:

414.21 gm

Composition:

Potassium

9.44 %

K

11.37 %

K₂O

Aluminum

19.54 %

Al

36.92 %

Al₂O₃

Hydrogen

1.46 %

H

13.05 %

H₂O

Sulfur

15.48 %

S

38.66 %

SO₃

Oxygen

54.08 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Sulfates

Strunz 8th Ed. ID:

6/B.11-20

Nickel-Strunz 10th Ed. ID:

7.BC.10

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

Related to:

Alunite Supergroup. Alunite Subgroup. Natroalunite - Jarosite - Beaverite Series.

Members of Group:

Alunite Supergroup: Beudantite Subgroup, Crandallite Subgroup, Florencite Subgroup, Jarosite Subgroup.

Alunite Subgroup: Alunite, Ammonioalunite, Natroalunite.

Varieties:

None

Synonyms:

Aluminilite, Alumstone, Calafatite, Ignatiewite, Kalioalunite, K-Alunite, Lœwigite, Lowigite, Lœvigite, Newtonite

Crystal Data

Crystallography:

Trigonal

Crystal Habit:

Crystals typically pseudocubic or tabular with flat vicinal rhombohedra, to 1 cm; fibrous to columnar, porcelaneous, commonly granular to dense massive.

Twinning:

None

Physical Properties

Cleavage:

Perfect on [0001]

Fracture:

Conchoidal to Uneven

Tenacity:

Brittle

Moh's Hardness:

3.5 - 4.0

Density:

2.60 - 2.90 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 136.78 (Gamma Ray American Petroleum Institute Units)

Other:

Insoluble in water.

Optical Properties

Color:

Colorless if pure; may be White, pale shades of Gray, Yellow, Red, to Reddish Brown from impurities.

Transparency:

Opaque, Translucent, Transparent on thin edges

Luster:

Vitreous; somewhat Pearly on [0001]

Refractive Index:

1.572 - 1.592  Uniaxial ( + )

Birefringence:

0.0020

Dispersion:

None

Pleochroism:

None

Other:

Strongly pyroelectric

Occurances

Geological Setting:

Formed between 15º C and 400º C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification.

Common Associations:

Diaspore, Gypsum, Halloysite, Kaolinite, Pyrite, Quartz

Common Impurities:

Na, Fe

Type Locality:

Allumiere Quarries, Allumiere, Rome Province, Latium, Italy

Year Discovered:

1824

View mineral photos:

Alunite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Alunite, also known as Alumstone, is a source of the chemical known as alum and is a rock-forming mineral. Alunite forms from the action of sulfuric acids upon potassium rich feldspars in a process called "alunitization". Alunite can easily be mistaken for Dolomite and Calcite.

Sources of Alunite are Marysvale, Utah; Red Mountain, Custer County, Colorado; Goldfield district, Nevada, USA; and Tolfa, Italy.
 

  
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