Transition of anhydrite into gypsum


There are five mineral phases in nature of CaSO4-H2O: gypsum (dihydrate), bassanite (semihydrate) and three types of waterless anhydrite: I, II and III (Prieto-Taboada i in. 2014). The most common minerals in nature are: gypsum and anhydrite II, characteristic for various environments and places of occurrence, often sharing them or appearing separately  (fig. 1) creates in different environments on the earth’s surface during the evaporation of water with an appropriate degree of salinity, while the primary anhydrite most often crystallizes as a result of gypsum dehydration (fig. 2) in shallow or deeper burial conditions under the influence of either high pressure, elevated temperature and/or the influence of underground brines (Hardie i in. 1967, Zanbak i Arthur 1986). Anhydrite can also crystallize on the earth’s surface in the sebhka environment (Butler i in. 1982).

Fig. 1. Gypsum from the copper mine in Lubin (Poland)
Fig. 2. Massive anhydrite with gypsum veins formed as a result of gypsum dehydration, taken from the closed Dingwall quarry (Canada)

Diagenetic cycle of calcium sulphates

The system of calcium sulphate and water reacts in different ways with each other being responsible for the formation of the above minerals and at the same time being susceptible to the geochemical environment, which consists of several factors, such as: temperature, pressure, presence of water (Mossop and Shearman 1973). These factors vary with the degree of burial, affecting the temperature and pressure increase, and also altering the surrounding water conditions. The changes that then take place in the rock are included in the diagenetic cycle (sensu: Murray 1964), which shows the transformation of calcium sulphate and water through the stages: sedimentation, during which gypsum crystallizes, burial stage causing dehydration and transition to anhydrite and exhumation, and weathering which sets the anhydrite to gypsification (fig. 3). The key temperature during the transformation of anhydrite into gypsum is 42 ° C, which is characteristic for a depth of approx. 900-1200 m (Mossop and Shearman 1973). However, this depth is not a factor in maintaining such a temperature, and the occurrence of anhydrite on the earth’s surface, despite the temperature drop, still occurs (Holliday 1970, Jarzyna et al. 2020, Bąbel et al. 2020.

Fig. 3. Diagenetic cycle of gypsum-anhydrite-gypsum transformation according to Murray 1964.


During the last stage of the diagenetic cycle, anhydrite is exposed to the influence of meteoric and groundwater. The access of water molecules into the structure of calcium sulphate is responsible for the crystallization of secondary gypsum. Such a transformation may cause an increase in the molar volume of a solid to 62.6% (Zanbak and Arthur 1986) and transformation of crystal strukture (fig. 4), which in turn leads to the swelling of the gypsum-anhydrite rock (Anagnostou 1993). Swelling, on the other hand, is responsible for the generation of stresses leading to numerous rock deformations with unpleasant consequences for humans, such as cracks in tunnels or walls of buildings (Steiner 1993, Oldecop and Alonso 2012), but also the formation of hydration caves, rare in the world, tepee-structure forms, domes and accompanying numerous fractures (fig. 5, 6; Reimann 1991, Jarzyna et al. 2020).

porównanie gipsu i anhydrytu
Fig. 4. Change of crystal structure of anhydrite (black line) into gypsum (green line) according to Mossop and Shearman 1973.
Fig. 5. One example of deformation due to the hydration of anhydrite in Dingwall (hydration form no 20).
Fig. 6. Extensive fractures formed in the lower part of the hydration form at Dingwall (hydration form no 27)