Images
Formula: Ca(SO4)
Sulphate
Crystal System: Orthorhombic
Specific gravity: 2.98 measured, 2.95 calculated
Hardness: 3 to 3½
Streak: White
Colour: Colourless, white, grey, blue
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Common impurities: Sr,Ba,H2O
Environments:
Carbonatites
Evaporite Deposits (typical)
Hydrothermal environments
Basaltic cavities
Anhydrite occurs in chemical sedimentary deposits and in the oxidation zone of high temperature hydrothermal
replacement deposits. It is found in
limestone and in some
amygdaloidal cavities in basalt.
It is a common alteration mineral in some porphyry (rock with coarse phenocrysts in a finer groundmass)
copper deposits, particularly those associated with diorite and
granodiorite intrusions.
Gypsum
is a common associate
(AofA).
In carbonatites it is associated with
baryte and
gypsum (AS).
Localities
The Simplon Railway tunnel (north section), Termen, Brig, Valais, Switzerland, is famous for magnificent
anhydrite specimens that occurred as lustrous, translucent purple crystals to several centimetres, that were
unearthed during the construction of the tunnel
(Minrec 54.4.479-500).
At Maricopa, Arizona, USA, quartz
pseudomorphs after anhydrite have been found
(KL p245).
At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, anhydrite occurs only as purple
inclusions in quartz
(R&M 96.6.494).
At the Upper New Street quarry, Paterson, Passaic county, New Jersey, USA, a
datolite
pseudomorph after anhydrite has
been found, with dozens of datolite crystals forming a flat cast where a
blade of
anhydrite once existed, but has since dissolved away. Prehnite
pseudomorphs after anhydrite are also found here
(KL p223, 241).
Alteration
anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently
formed by the hydration of anhydrite.
Back to Minerals