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Formula: Ca(SO4).2H2O
Hydrated normal sulphate
Varieties
Selenite is a transparent variety of gypsum
Crystal System: Monoclinic
Specific gravity: 2.312 to 2.322 measured, 2.308 calculated
Hardness: 1½ to 2
Streak: White
Colour: Colourless, white, yellowish, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:
Evaporite deposits
Fumeroles
Cave deposits
Gypsum is the commonest of the sulphate minerals, found in chemical sedimentary environments, where it frequently
occurs
interstratified with limestone and
shale.
It is usually found as a layer underlying beds of rock salt, having been
deposited there as one of the first minerals to crystallise on the evaporation of salt waters. It may recrystallise in
veins forming satin spar. It is also common as a gangue
mineral in metallic veins, at fumaroles, and in the oxidation zones of
sulphide deposits. Gypsum is also found
in volcanic regions, especially where limestone has been acted on by
sulphur vapours.
Gypsum is associated with many different minerals, the more common being
halite,
anhydrite,
dolomite,
calcite,
sulphur,
pyrite and
quartz.
Localities
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper
ores overlie primary zone mineralisation consisting of
quartz-dolomite-sulphide veins hosted
in siltstone and schist.
Gypsum occurs as millimetre sized crystals associated with hematite
(AJM 22.1.24).
At Lake Crosbie, Victoria, Australia, gypsum occurs associated with
halite and glauberite
in black mud under a salt
crust which covers the lake
(AJM 10.1.17-18).
At Daye county, Hubei, China, chabazite
pseudomorphs after
gypsum have been found
(KL p266).
The Cave of Crystals, Naica Mine, Naica, Saucillo Municipality, Chihuahua, Mexico, is world-famous for giant gypsum crystals up to 14 m
long. The cave is connected to the Naica mine at 300 m level, with almost unbearable coditions of 58oC and
90 to 99 percent humidity
(Wiki).
At the Black Rock mine, Kuruman, South Africa, sturmanite
pseudomorphs after gypsum have been found
(KL p195).
At the Bristol Mineral Company's pits, Swan Inn, Yate, Gloucestershire, England, UK, gypsum has been found with minor
celestine
(RES p165).
At the Cliffs, Radcliffe on Trent, Nottinghamshire, England, UK, at an outcrop on the south bank of the river Trent,
pseudomorphs of gypsum
after halite to 3 cm occur
(RES p 245).
At Gunthorpe Weir, East Bridgford, Nottinghamshire, England, UK, pseudomorphs of
gypsum after halite occur
(RES p248).
At Shotover Hill, Oxford, Oxfordshire, England, UK, many fine crystals of gypsum variety selenite
have been found in
clay, and this occurrence has been known since the late seventeenth century
(RES p251).
At the Magma mine, Pioneer District, Pinal county, Arizona, USA, gypsum is associated with
baryte
(R&M 95.1.85-86).
At Camp Verde, Yavapai county, Arizona, USA, fine replacement pseudomorphs of
gypsum after
glauberite have been found (R&M 94.2.163, KL p190).
In Nebraska, USA, quartz pseudomorphs
after gypsum have been found
(KL p249).
At the Mid-Continent mine, Picher, Oklahoma, USA, a gypsum pseudomorph
after calcite with melanterite on
sphalerite has been found
(KL p189).
At the SK Star #2 Mining Claim, The Cove, Topaz Mountain, Thomas Range, Juab County, Utah, USA, specimens of the “rams
horn” habit of gypsum have been found in the fault zone. Colourless with a pearly luster, these occur singly and
in groups. The rams horns average under 3 cm
(MinRec 51.6.809-810).
Alteration
Glauberite dissolves in water, depositing
gypsum, so pseudomorphs of gypsum after glauberite are
not uncommon
(AJM 10.1.17-18).
anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently
formed by the hydration of anhydrite.
anorthite, H2SO4 and H2 to gypsum and
kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O →
CaSO4.2H2O + Al2Si2O5(OH)4
(DHZ 5B p65)
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