**2. Crystallization and alternation: Hydratation and dehydratation (gypsification, anhydritization)**

#### **2.1 Crystallization**

In most of the cases during evaporation processes, the gypsum crystallizes first, than the anhydrite (higher concentration of solution, 5-6 times higher than the normal sea water

Fig. 10. Lenticular gypsum (Wapno cap-rock, Poland) phot. J. Jaworska

Fig. 11. Anhydrite crystals (from Dębina salt dome, Poland), phot. A. Kyc

12. Enterolithic anhydrite (Wieliczka mine, Poland) phot. J. Jaworska

**(gypsification, anhydritization)** 

**2.1 Crystallization** 

**2. Crystallization and alternation: Hydratation and dehydratation** 

In most of the cases during evaporation processes, the gypsum crystallizes first, than the anhydrite (higher concentration of solution, 5-6 times higher than the normal sea water

Fig. 13. Anhydrite rock - massie agreggates (Niwnice, Poland) phot. J. Jaworska

Fig. 14. Sabre-like gypsum (Nida region, Poland) phot. J. Jaworska

Fig. 15. Laminated gypsum (Niemeyer quarry, Germany) phot. J. Jaworska

salinity and in temperature about 40°C). Not until the concentration of solution reaches values close to NaCl concentration, the only phase of calcium sulphate which crystallizes and accompanies the rock salts is anhydrite; even if the temperature does not exceed 18°C. The thick rock salt deposits seldom form salt pillows together with salt swells and diapirs; their roof surfaces are located close to the Earth's surface (at the boundary of the salt mirror) and easily undergo leaching, leaving less soluble residue of - among the others - anhydrite grains and next - the anhydrite sandstone (fig. 20.), forming so-called cap-rock that forms the natural cover of the salt deposit. The anhydrite sandstone can – depending on the conditions - undergo further transformation typical for this very mineral (fig. 21.).

Crystallization, Alternation and Recrystallization of Sulphates 473

The sulphates – mainly the products of the hypergenic processes – very easily undergo the diagenetic processes, in which the dominant role is played by: hydration (gypsification) of anhydrite and dehydration (anhydritization) of gypsum; both processes are reversible and

CaSO4 • 2H2O CaSO4 + 2H2O

Fig. 19. Gypsum-karst (Nida region, Poland). phot. J. Jaworska

Fig. 20. Anhydrite sandstone; phot. J. Jaworska

the reaction takes place as follows:

**2.2 Alternation** 

Fig. 16. and 17. Outcrop of 2.5-3 m senlenite gypsums, regional named szklica (Nida region, Poland) phot. J. Jaworska

Fig. 18. Second native sulphur in gypsum rock (Niemeyer quarry, Germany) phot. J. Jaworska

In the recent evaporation basins mainly the gypsum precipitates; anhydrite crystallizing under more extreme conditions occurs more rarely. Whereas among the sediments – particularly at the depths of few hundreds to few thousand meters – the anhydrite dominates. In many cases the anhydrite occurs as a product of the dehydration of gypsum; usually it is easily recognized pseudomorph of gypsum (e.g. selenite gypsum). The primary anhydrite, as well as the secondary one (dehydrate), as a result of tectonic processes, intense weathering of the overburden, climate changes etc., can be placed within the range of the underground or subsurface water (ground, meteoric) – where the hydration processes occur resulting in substitution of anhydrite by gypsum.

Fig. 16. and 17. Outcrop of 2.5-3 m senlenite gypsums, regional named szklica (Nida region,

Fig. 18. Second native sulphur in gypsum rock (Niemeyer quarry, Germany)

resulting in substitution of anhydrite by gypsum.

In the recent evaporation basins mainly the gypsum precipitates; anhydrite crystallizing under more extreme conditions occurs more rarely. Whereas among the sediments – particularly at the depths of few hundreds to few thousand meters – the anhydrite dominates. In many cases the anhydrite occurs as a product of the dehydration of gypsum; usually it is easily recognized pseudomorph of gypsum (e.g. selenite gypsum). The primary anhydrite, as well as the secondary one (dehydrate), as a result of tectonic processes, intense weathering of the overburden, climate changes etc., can be placed within the range of the underground or subsurface water (ground, meteoric) – where the hydration processes occur

Poland) phot. J. Jaworska

phot. J. Jaworska

Fig. 19. Gypsum-karst (Nida region, Poland). phot. J. Jaworska

Fig. 20. Anhydrite sandstone; phot. J. Jaworska
