Alunite is a hydroxylated aluminium potassium sulfate mineral, formula K Al₃( S O₄)₂(O H)₆. It was first observed in the 15th century at

Tolfa Tolfa is a town and ''comune'' of the Metropolitan City of Rome, in the Lazio region of central Italy; it lies to the ENE of Civitavecchia by road. It is the main center in the Monti della Tolfa, an extinct volcanic group between Civitavecchia and ...

, near Rome, where it was mined for the manufacture of

alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...

. First called ''aluminilite'' by J.C. Delamétherie in 1797, this name was contracted by François Beudant three decades later to alunite. Mineraly

Alunite crystals morphologically are rhombohedron, rhombohedra with interfacial angles of 90° 50', causing them to resemble cubes. Crystal symmetry is trigonal. Minute glistening crystals have also been found loose in cavities in altered

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

. Alunite varies in color from white to yellow gray. The hardness on the

Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

is 4 and the specific gravity is between 2.6 and 2.8. It is insoluble in water or weak acids, but soluble in

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

Alunite Basic hydrous aluminum potassium sulfate Marysvale Piute County Utah 2324

Sodium can substitute for potassium in the mineral, and when the sodium content is high, is called natroalunite. Alunite crystal structure

Alunite is an analog of jarosite, where aluminium replaces Fe³⁺. Alunite occurs as a secondary mineral on iron sulfate ores. Alunite occurs as veins and replacement masses in trachyte,

, and similar potassium rich

volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of Volcano, volcanic origin. Like all rock types, ...

. It is formed by the action of

bearing solutions on these rocks during the oxidation and leaching of metal sulfide deposits. Alunite also is found near volcanic fumaroles. The white, finely granular masses closely resemble finely granular limestone,

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

, anhydrite, and

magnesite Magnesite is a mineral with the chemical formula (magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic ro ...

in appearance. The more compact kinds from Hungary are so hard and tough that they have been used for millstones. Historically extensive deposits were mined in Tuscany and Hungary, and at Bulahdelah, New South Wales,

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. It is currently mined at Tolfa, Italy. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; the ghost town of

Alunite, Utah Alunite ( ) is a ghost town located some south of Marysvale, near the mouth of Cottonwood Canyon in Piute County, Utah, United States. A small but important mining town during World War I, Alunite was inhabited from approximately 1915–193 ...

near Marysvale; and Red Mountain near

Patagonia, Arizona Patagonia is a town in Santa Cruz County, Arizona, United States. As of the 2010 census it had a population of 913. It developed in the mid-19th century as a trading and supply center for nearby mines and ranches. In the 21st century, it is a t ...

. The Arizona occurrence lies appropriately above a canyon named ''Alum Gulch''. Alunite is mined as an ore of both potassium and aluminium at Marysvale. Some of the ore deposits were located by airborne and satellite multispectral imaging. An article in the May/June 2019 issue of Archaeology magazine states that in China, in Henan province, an assortment of ceramic objects and jars were found, dating back 2000 years. In one of the jars, a mixture of alunite and potassium nitrate was found. The mixture was then thought to be a "mixture of immortality" mentioned in ancient Chinese texts. Obviously, this does not appear to have succeeded.

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Mineral Galleries
{{Authority control Aluminium minerals Potassium minerals Sulfate minerals Alunite group Trigonal minerals Minerals in space group 166 Luminescent minerals Industrial minerals

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External links