Anhydrite

anhydrite

baryte

hydrothermal

gypsum

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Formula: Ca(SO4)
Sulphate
Specific gravity: 2.98
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

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.

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