Alunite

Chemical formula: KAl<sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>

Alunite is a potassium aluminum sulfate, occurring mainly in hydrothermally altered volcanic rocks, often as earthy masses or fine crystals.

## Characteristics Alunite is a hydrated sulfate belonging to the alunite group. It typically forms compact, granular, or earthy aggregates. Well-formed crystals are rare and most often take the form of small, tabular rhombohedra, which can resemble cubes. The mineral is most commonly white, grayish, or yellowish, although the presence of impurities can impart pink, red, or reddish-brown hues. ## Physical Properties The Mohs hardness of alunite is 3.5-4, making it a relatively soft mineral. It has a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. Its density ranges from 2.6 to 2.9 g/cm³. ## Colors and Varieties The primary color of alunite is white, but it is often colored yellow, gray, or reddish-brown by iron oxide inclusions. No formal varieties are distinguished, and nomenclature is mainly based on color or habit. ## History and Name The mineral's name, given in 1824 by François Sulpice Beudant, comes from the Latin word *alumen*, meaning alum – a chemical compound that was historically obtained from alunite. ## Uses Alunite is an important raw material for the production of alum (potassium aluminum sulfate), used, among other things, in the paper and dyeing industries. It is also used as a source of potassium in fertilizers and as a minor aluminum ore.

Properties

Mohs hardness
3.5-4
Luster
Vitreous to Pearly
Streak
White
Density
2.6-2.9
Cleavage
Distinct on {0001}
Fracture
Conchoidal to Uneven
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Alunite is identified by its occurrence in hydrothermal alteration zones of volcanic rocks, low hardness, and characteristic pearly luster on cleavage planes. It does not react with cold hydrochloric acid. ## Distinguishing from Similar Minerals Massive alunite can be confused with kaolinite, chalcedony, or microcrystalline dolomite and magnesite. It is distinguished from carbonates by its lack of reaction with hydrochloric acid. It is much softer than quartz and chalcedony. ## Crystal Forms Alunite crystals are typically small, sharply terminated rhombohedra, often flattened, forming tabular habits. Sometimes they resemble cubes. Most often, however, it occurs in compact, granular, and fibrous aggregates.

Geological environment

## Genesis Alunite is a typical mineral of hydrothermal origin. It forms as a result of the interaction of sulfur-rich, acidic solutions with potassium-rich volcanic rocks, such as rhyolites and trachytes. It also forms in oxidation zones (iron caps) of ore deposits. ## Mineral Associations It often co-occurs with quartz, pyrite, kaolinite, dickite, diaspore, hematite, and other sulfates, such as jarosite. ## Localities Important deposits and occurrences of alunite are found in Tolfa, Italy (type locality), Marysvale, Utah (USA), where it forms extensive deposits, as well as in Goldfield (Nevada, USA), Red Mountain (Colorado, USA), Bulahdelah (Australia), and Spain.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and lustrous rhombohedral crystals. Their transparency and contrasting placement on a rock matrix enhance their attractiveness. Pink or reddish coloration is also desirable. ## Popular Localities Classic crystalline specimens come from Tolfa, Italy. Beautiful, pink crystals were found in the Tambo mine in Chile. Deposits in Marysvale, Utah, provide abundant massive specimens, important from a historical and deposit perspective.

Care and storage

## Cleaning Alunite specimens should be cleaned gently, using a soft brush and distilled water. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is susceptible to scratching due to its low hardness. It is soluble in sulfuric acid, so contact with chemicals should be avoided. It does not require special protection from light or temperature. ## Storage Alunite should be stored separately, away from harder minerals, to prevent mechanical damage. It is best kept in individual boxes or on a soft lining.

External references

Sources

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