Gypsum

sulphate

limestone

Formula: Ca(SO4).2H2O sulphate
Specific gravity: 2.3 to 2.4
Hardness: 1½ to 2
Streak: White
Colour: Colourless, white, yellowish, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Sedimentary environments
Fumeroles
Hydrothermal environments

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.

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 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).

At Shotover Hill, Oxford, Oxfordshire, England, UK, many fine crystals of gypsum variety selenite have been found in clay, and this occurrence has been known since the late seventeenth century (RES p251).

At Camp Verde, Yavapai county, Arizona, USA, fine replacement pseudomorphs of gypsum after glauberite have been found (R&M 94.2.163).

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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