Anhydrite

sulphate

gypsum

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
Environments:

Carbonatites
Sedimentary environments (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).

Alteration

anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently formed by the hydration of anhydrite.

Common impurities: Sr,Ba,H2O

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