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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).
Anhydrite from the Simplon Railway Tunnel -
Image
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).
Anhydrite from the Upper New Street Quarry -
Image
At Balmat, Fowler, St. Lawrence County, New York, USA, anhydrite is an extremely common component of the
evaporite beds which are believed to be the source of the sulphide that drove the original mineralisation.
Anhydrite masses were a common constituent in the cubic
magnetite-bearing zones at the #4 mine. Masses of anhydrite,
sometimes containing subhedral gem-quality crystals to 15 cm or more, have been found in every Balmat zinc mine at
one time or another. Recent (in 2025) finds at the #4 mine have produced the finest anhydrite specimens
recovered in Balmat’s history, and the best of these are amongst the finest in the world. Lavender-coloured, gemmy
crystals from the new finds are typically 3 to 5 cm in length, exceptionally reaching more than 20 cm. Specimens
found on the 4051-level are typically pale blue, but beautiful sprays of pink and purple anhydrite were also
found. Accessory minerals such as sphalerite,
pyrrhotite and pyrite grew on
some of the anhydrite crystals
(MinRec 56.1.22).
Anhydrite from Balmat - Image
Alteration
anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently
formed by the hydration of anhydrite.
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