Alunogen

alunogen

halotrichite

tschermakite

melanterite

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Formula: Al2(SO4)3(H2O)12.5H2O
Hydrated sulphate
Specific gravity: 1.65 to 1.78
Hardness: 1½ to 2
Streak: White
Colour: Colourless in crystals, aggregates white, or pale yellow or red from impurities, colorless in transmitted light
Solubility: Readily soluble in water
Melting point: About 114oC
Environments

Hydrothermal environments
Fumeroles
Coal-seam fires

Alunogen is a secondary mineral that forms by reaction of sulphates from decomposing sulphides with aluminous minerals in shale and slate; in gossan or altered wall rock of pyritic deposits in arid regions; in coal seams; and in relatively low-temperature fumaroles. Associated minerals include pyrite, marcasite, halotrichite, pickeringite, epsomite, alum-(K), melanterite and gypsum (HOM).

Localities

Near Joadja, Wingecarribee Shire, New South Wales, Australia, deposits of evaporite, consisting essentially of alunogen, occur beneath a calcareous sandstone as white, fibrous lenses, which measure up to 9 feet in diameter and 9 inches in depth. Each deposit consists of a sequence of horizontal layers of vertically stacked alunogen fibres. Small irregular-shaped quartz grains and iron oxide film coatings are also present (AM 49.1763-1766).

Near Vernon, Vernon Mining Division, British Columbia, Canada, alunogen occurs with epsomite in veins (Dana).

At the Wuda Coal mining area, Wuda District, Wuhai League, Inner Mongolia Autonomous Region, China, unique mineral assemblages that include the sulphates millosevichite, alunogen, anhydrite, tschermakite, coquimbite, voltaite and godovikovite, as well as the halide salammoniac were found as encrustations on sand and sandstone adjacent to coal-fire gas vents associated with underground coal fires (AM 90.1729–1739).

The efflorescences from a mud volcano in the Katakolo area, western Peloponnesus, Greece, contain halotrichite, alunogen, voltaite, melanterite, gypsum, and native sulphur. The sulphate minerals were formed by the reaction of carbonates, clay minerals, feldspar, and iron oxides and hydroxides of the basement rocks with H2S in the mud-volcano gases (AM 72.839-841).

In the Te Kopia geothermal area, Rotorua District, Bay of Plenty Region, New Zealand, alunogen and meta-alunogen are the dominant phases present in transient sulphate efflorescences. Meta-alunogen forms pseudomorphs after alunogen and both species occur as white, fibrous, tangled masses, as prismatic, parallel growths, and as thin, platy, crystals, 8-15 microns across. Small spherical aggregates of radiating, acicular halotrichite, kalinite, mirabilite, melanterite and tschermakite are present locally. Kaolinite formed by acid sulphate alteration is now being altered by steam to yield alunogen. In turn, alunogen can react with silica, or co-dissociate with silicic acid, to form kaolinite (MM 63.413–419).

Ruatapu Cave, Orakeikorako Thermal Area, Taupo District, Waikato Region, New Zealand has developed beneath a block of hydrothermally altered vitric tuff in the active geothermal field. The cave extends about 45 m, with a vertical drop of 23 m, to a shallow pool of clear, sulphate-rich, warm, acid water at a temperature of 43–48°C and pH equal to 3. Steam, accompanied by H2S, rises from pool surfaces, fumaroles and joints in the ignimbrite, to condense on surfaces within the cave. Oxidation of the hydrogen sulphide H2S to sulphuric acid H2SO4 produces acid sulphate fluids which react with the surficial rocks to generate assemblages of secondary minerals.
At the cave mouth the assemblage is dominated by kaolinite, opal, cristobalite, alunite and alunogen; the essential Al, K and Si are derived from the tuff.
In the main body of the cave the highly limited throughflow of water results in the more soluble of the leached constituents, notably Na and K, being retained in surface moisture and becoming available to form tamarugite and kalinite as efflorescences, in part at the expense of kaolin, along with lesser amounts of alunogen, meta-alunogen, mirabilite, halotrichite, gypsum and, possibly, tschermakite. Jarosite crusts have developed where acid sulphate pool waters have had protracted contact with ignimbrite wallrock coated with once-living microbial mats. (MM 64.125–142).

At the Javier Mine, Huac-huas, Lucanas Province, Ayacucho, Peru, by far the most common mineral is coquimbite, but colourless to white alunogen is also common as platy crystals to 1 cm (Minrec 42.2.172).

On the banks of the Rio Tinto, southwest Spain, The soluble metal sulphate salts melanterite, rozenite, rhomboclase, szomolnokite, copiapite, coquimbite, hexahydrite and halotrichite, together with gypsum, have been identified. Secondary iron sulphate minerals can form directly from evaporating, acid, sulphate-rich solutions as a result of pyrite oxidation.
Melanterite and rozenite precipitates at Río Tinto are only found in association with very acidic drainage waters of pH less than 1 draining directly from pyritic waste piles (MM 67.263–278).

At Cripple Creek Mining District, Teller county, Colorado, USA, admixtures of alunogen and epsomite are reported (Minrec 36.2.162).

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