Mineralogy

1981 Edition

Anhydrite and gypsum

Reference work entry
DOI: https://doi.org/10.1007/0-387-30720-6_7

The calcium sulfate in evaporites sometimes occurs as gypsum, CaSO4 · 2H2O, sometimes as anhydrite, CaSO4, and sometimes as both minerals together. Near-surface material is almost always gypsum because of the ease of weathering and hydration of CaSO4, and deep-seated subsurface material is always anhydrite because of dehydration effects. Numerous examples of replacement of one of these minerals by another are known (Murray, 1964; Stewart, 1953; Borchert and Baier, 1953; Ogniben, 1955; Sund, 1959).

The water solubilities of gypsum and anhydrite have been investigated by Posnjak (1938), Bock (1961), Marshall and Slusher (1966), and others; and MacDonald (1953) has made thermochemical calculations. For many years, it was believed that gypsum crystallized out of pure water at a temperature less than about 40°C, that anhydrite was the stable phase above this temperature, and that increasing salinity lowered the transition point (see Fig. 1).
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References

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© Hutchinson Ross Publishing Company 1981