Alunite is a hydroxylated

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

sulfate mineral The sulfate minerals are a class of minerals that include the sulfate ion () within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal veins and as secondary mine ...

, 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 In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crysta ...

. 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 In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

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 Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...

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 Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct duri ...

, where aluminium replaces Fe³⁺. Alunite occurs as a secondary mineral on iron sulfate ores. Alunite occurs as veins and replacement masses in

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...

, 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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

and leaching of metal sulfide deposits. Alunite also is found near volcanic

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

s. The white, finely granular masses closely resemble finely granular

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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 Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

, 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 Tuscany ( ; it, Toscana ) is a Regions of Italy, region in central Italy with an area of about and a population of about 3.8 million inhabitants. The regional capital is Florence (''Firenze''). Tuscany is known for its landscapes, history, art ...

and

Hungary Hungary ( hu, Magyarország ) is a landlocked country in Central Europe. Spanning of the Carpathian Basin, it is bordered by Slovakia to the north, Ukraine to the northeast, Romania to the east and southeast, Serbia to the south, Croatia a ...

, and at

Bulahdelah, New South Wales Bulahdelah is a town and locality in the Hunter/Mid North Coast region of New South Wales, Australia in the Mid-Coast Council local government area. At the 2016 census, the locality had a population of 1,424. The main population centre, where ...

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 Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...

. In the United States it is found in the San Juan district of

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

;

Goldfield, Nevada Goldfield is an unincorporated community, unincorporated small desert city and the county seat of Esmeralda County, Nevada. It is the locus of the census-designated place, Goldfield CDP which had a resident population of 268 at the 2010 Unite ...

; the

ghost town Ghost Town(s) or Ghosttown may refer to: * Ghost town, a town that has been abandoned Film and television * Ghost Town (1936 film), ''Ghost Town'' (1936 film), an American Western film by Harry L. Fraser * Ghost Town (1956 film), ''Ghost Town'' ...

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 1915–1930 ...

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 Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April 2 ...

deposits were located by airborne and satellite

multispectral imaging Multispectral imaging captures image data within specific wavelength ranges across the electromagnetic spectrum. The wavelengths may be separated by filters or detected with the use of instruments that are sensitive to particular wavelengths, ...

. 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 Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitrat ...

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.

References

External links

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

See also

References

External links