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Formula: Ca(SO4).2H2O
Hydrated normal sulphate
Varieties
Selenite is a transparent variety of gypsum
Desert rose is the name given to a (more or less) rose-like crystal group formed by precipitation in (usually) arid desert
regions containing trapped sand particles. Usually, gypsum is the host mineral, but baryte,
celestine and other minerals can form Desert Rose groups, too
(Mindat).
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).
Gypsum from Daye County - Image
At an unspecified locality in China, a blue desert rose has been found. It may be dyed blue, or it may be coloured by
azurite. I bought it from a small mineral stall in a Shenzhen shopping centre
(AESS).
Gypsum Desert Rose from China - Image
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 conditions 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).
Gypsum at Gunthorpe Weir - Image
At Shotover Hill, Oxford, Oxfordshire, England, UK, many fine crystals of
selenite have been found in
clay, and this occurrence has been known since the late seventeenth
century
(RES p251).
Gypsum from Shotover Hill - Image
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).
Gypsum from Camp Verde - Image
In Nebraska, USA, quartz
pseudomorphs
after gypsum have been found
(KL p249).
Gypsum from Nebraska - Image
At the Mid-Continent mine, Picher, Oklahoma, USA, a gypsum
pseudomorph
after calcite with melanterite
on sphalerite has been found
(KL p189).
Gypsum from the Mid-Continent Mine - Image
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).
Gypsum from the SK Star #2 Mining Claim - Image
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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