**2.2 Alternation**

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 the reaction takes place as follows:

$$\text{CaSO}\_4 \bullet \text{2H}\_2\text{O} \bullet \text{CaSO}\_4 + \text{2H}\_2\text{O}$$

Crystallization, Alternation and Recrystallization of Sulphates 475

When the gypsum deposits are buried, their transformation into anhydrite can theoretically start at the depth of about 450-500 m (Murray, 1964; Hardie, 1967; Jowett et al. 1993); those are the depths where temperature reaches 20°C, so the dehydration should not appear, however it is compensated by high overburden pressure (10 MPa; Kubica, 1972); on the other hand, according to Sonnenfeld (1984), gypsum can be found at the depth of 1200 m; and according to Ford and Williams (2007) even at 3000 m. The depth of the gypsum dehydration among others is modified by the geotectonic environment and the lithology of the overburden. The weakly heat conducting overburden, e.g. schists and gneisses, upon the areas seismically active, volcanic, causes the increase of the hydration speed – anhydrite can substitute the gypsum already at the depth of about 400 m; whereas well conducting overburden, e.g. rock salt of the cratonic areas, causes the process of transformation of the gypsum into anhydrite to occur hypothetically at the depth of even 4 km (Jowett et al., 1993). But the anhydrite gypsification process during the exhumation occurs usually at the depth of about 100-150 m (Murrey, 1964; Klimchouk &Andrejchuk, 1996). It starts either when the anhydrite appears in the area of influence of the ground water, or when it is

The crystallization process of calcium sulphates, as well as their gypsification or anhydritization are affected by the solutions (and their pressure). The NaCl solution occurring in the pore fluids plays special role; it modifies the temperature of the gypsumanhydrite phase transformation. If the composition of pore fluids corresponds to the composition of sea water, the water activity (αH2O) is 0.93 and the transformation of gypsum into anhydrite occurs at the temperature of 52°C; however if the pore fluids are NaCl saturated, then the water activity reaches 0.75 and the transformation occurs at 18°C (Jowett et al., 1993). The temperature of gypsum-anhydrite transformation is increased by: the presence of alkaline metal ions (Conley and Bundy, 1958) up to 98°C and the solution of CaSO4 up to 95°C, but with lack of the anhydrite nuclei (Posnjak, 1940). Additionally it is necessary to take into account the regime of pore fluids pressure; if it is hydrostatic, then the temperature of the gypsum transformation decreases along with depth from 52°C under surface conditions to about 40°C at the depth of 3 km, and in the case of the lithostatic

exposed to rain water.

Fig. 23. Lenticular gypsum; phot. J. Jaworska

regime – rises to about 58°C at 2 km (Jowett et al., 1993).

#### gypsum anhydrite + water

There are many factors affecting the start and course of this reaction:


Fig. 21. Lenticular gypsum with anhydrite inclusions; phot. J. Jaworska

Fig. 22. Fine-crystalline gypsum; phot. J. Jaworska

#### **2.2.1 Conditions**

Anhydrite under surface conditions or close to the surface can be formed as a result of intense heating (over 50°C) of primary gypsum by the sun under hot and arid conditions.

gypsum anhydrite + water


2. chemical composition and concentration of solution, pore fluid pressure and the activity

Anhydrite under surface conditions or close to the surface can be formed as a result of intense heating (over 50°C) of primary gypsum by the sun under hot and arid conditions.

and lithology of the overburden (thermal conductivity of the overburden),

4. presence of cracks and pores in the sulphates as well as in the surrounding rocks.

There are many factors affecting the start and course of this reaction:

1. temperature and environmental pressure – depending on: - climate (for sulphates on the surface or close below it) - depth of the deposits – thickness of the overburden,

3. presence of micro-organisms and organisms (changes in Eh),

Fig. 21. Lenticular gypsum with anhydrite inclusions; phot. J. Jaworska

Fig. 22. Fine-crystalline gypsum; phot. J. Jaworska

**2.2.1 Conditions** 

of water,

When the gypsum deposits are buried, their transformation into anhydrite can theoretically start at the depth of about 450-500 m (Murray, 1964; Hardie, 1967; Jowett et al. 1993); those are the depths where temperature reaches 20°C, so the dehydration should not appear, however it is compensated by high overburden pressure (10 MPa; Kubica, 1972); on the other hand, according to Sonnenfeld (1984), gypsum can be found at the depth of 1200 m; and according to Ford and Williams (2007) even at 3000 m. The depth of the gypsum dehydration among others is modified by the geotectonic environment and the lithology of the overburden. The weakly heat conducting overburden, e.g. schists and gneisses, upon the areas seismically active, volcanic, causes the increase of the hydration speed – anhydrite can substitute the gypsum already at the depth of about 400 m; whereas well conducting overburden, e.g. rock salt of the cratonic areas, causes the process of transformation of the gypsum into anhydrite to occur hypothetically at the depth of even 4 km (Jowett et al., 1993). But the anhydrite gypsification process during the exhumation occurs usually at the depth of about 100-150 m (Murrey, 1964; Klimchouk &Andrejchuk, 1996). It starts either when the anhydrite appears in the area of influence of the ground water, or when it is exposed to rain water.

Fig. 23. Lenticular gypsum; phot. J. Jaworska

The crystallization process of calcium sulphates, as well as their gypsification or anhydritization are affected by the solutions (and their pressure). The NaCl solution occurring in the pore fluids plays special role; it modifies the temperature of the gypsumanhydrite phase transformation. If the composition of pore fluids corresponds to the composition of sea water, the water activity (αH2O) is 0.93 and the transformation of gypsum into anhydrite occurs at the temperature of 52°C; however if the pore fluids are NaCl saturated, then the water activity reaches 0.75 and the transformation occurs at 18°C (Jowett et al., 1993). The temperature of gypsum-anhydrite transformation is increased by: the presence of alkaline metal ions (Conley and Bundy, 1958) up to 98°C and the solution of CaSO4 up to 95°C, but with lack of the anhydrite nuclei (Posnjak, 1940). Additionally it is necessary to take into account the regime of pore fluids pressure; if it is hydrostatic, then the temperature of the gypsum transformation decreases along with depth from 52°C under surface conditions to about 40°C at the depth of 3 km, and in the case of the lithostatic regime – rises to about 58°C at 2 km (Jowett et al., 1993).

Crystallization, Alternation and Recrystallization of Sulphates 477

Fig. 27. 'Kink bands' and result of subgrain rotation in gypsum; phot. J. Jaworska

of processes.

**2.2.2 Time** 

even within several years.

**2.2.3 Volume** 

Shahid et al. (2007) comparing the crystallization and transformation conditions of sulphates in salt lakes and sabkhas in north Africa (Libia) and those from the Persian Gulf (Abu Dhabi) noted that while the climate is comparable, in the first case the anhydrite occurs very rarely, unlike in the area of the Arabian Peninsula. The main causes of this difference are the geochemical environment conditions: in the African sabkhas and salt lakes, the environment is more reducing and there is an occurrence of the organic material, the hydrogen sulphide releases and the sediment is dark; while sabkhas from the Persian gulf are more oxidised with lack of hydrogen sulphide - the sediment is light. The presence of fractures and joints in sediments/rocks surrounding the sulphates, as well as the microfractures and pores in the sulphates themselves strongly affect the start of the gypsification and anhydritization. Those free spaces allows the water to migrate and solutions to start and catalyse the course

The anhydritization and gypsification (dehydration and hydration) under natural conditions can occur very quickly: within few years (Farnsworth, 1925) or even within one year (Moiola & Glover, 1965); and experiments showed that even within several/several dozen of days (i.e. Sievert et al., 2005), what depends on physical and chemical conditions under which the process occurs. We can see for ourselves the speed of these processes, when inside a brick (ceramic material) we note the anhydrite grains, which with infiltrating water are being gypsificated and expand destroying the material – the damage of walls occurs

The volumetric change comes along with hydration and dehydration processes of the sulphates – the increase of volume of anhydrite by its gypsification is about 30-50% according to Petijohn (1957), and according to Azam (2007) - close to 63%. Whereas the gypsum anhydritization decreases its volume of about 39% (Azam, 2007); sometimes it occurs together with many alterations, especially of the primary rock structure. The different situation takes place in case of sulphate deposits which already contain water; according to Farnsworth (1924), 1000g of gypsum fills 431 cm3, while the sum of anhydrite and water

Fig. 24. Grain boundary migration between two gypsum crystals; phot. J. Jaworska

Fig. 25. Large gypsum with kink folds; phot. J. Jaworska

Fig. 26. 'Kink bands' and result of subgrain rotation in gypsum; phot. J. Jaworska

Fig. 27. 'Kink bands' and result of subgrain rotation in gypsum; phot. J. Jaworska

Shahid et al. (2007) comparing the crystallization and transformation conditions of sulphates in salt lakes and sabkhas in north Africa (Libia) and those from the Persian Gulf (Abu Dhabi) noted that while the climate is comparable, in the first case the anhydrite occurs very rarely, unlike in the area of the Arabian Peninsula. The main causes of this difference are the geochemical environment conditions: in the African sabkhas and salt lakes, the environment is more reducing and there is an occurrence of the organic material, the hydrogen sulphide releases and the sediment is dark; while sabkhas from the Persian gulf are more oxidised with lack of hydrogen sulphide - the sediment is light. The presence of fractures and joints in sediments/rocks surrounding the sulphates, as well as the microfractures and pores in the sulphates themselves strongly affect the start of the gypsification and anhydritization. Those free spaces allows the water to migrate and solutions to start and catalyse the course of processes.
