Gypsum

gypsum

anhydrite

glauberite

halite

Images

Formula: Ca(SO4).2H2O
Hydrated normal sulphate

Varieties

Selenite is a transparent variety of gypsum

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:

Evaporite Deposits
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.

Localities

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in siltstone and schist. Gypsum occurs as millimetre sized crystals associated with hematite (AJM 22.1.24).

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 Daye county, Hubei, China, chabazite pseudomorphs after gypsum have been found (KL p266).

The Cave of Crystals, Naica Mine, Naica, Saucillo Municipality, Chihuahua, Mexico, is world-famous for giant gypsum crystals up to 14 m long. The cave is connected to the Naica mine at 300 m level, with almost unbearable coditions of 58oC and 90 to 99 percent humidity (Wiki).

At the Black Rock mine, Kuruman, South Africa, sturmanite pseudomorphs after gypsum have been found (KL p195).

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 the Magma mine, Pioneer District, Pinal county, Arizona, USA, gypsum is associated with baryte (R&M 95.1.85-86).

At Camp Verde, Yavapai county, Arizona, USA, fine replacement pseudomorphs of gypsum after glauberite have been found (R&M 94.2.163, KL p190).

In Nebraska, USA, quartz pseudomorphs after gypsum have been found (KL p249).

At the Mid-Continent mine, Picher, Oklahoma, USA, a gypsum pseudomorph after calcite with melanterite on sphalerite has been found (KL p189).

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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