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Formula: Al2(SO4)3(H2O)12.5H2O
Hydrated sulphate
Crystal System: Triclinic
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).
Alunogen from Ruatapu - Image
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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