**2.2.2 Time**

476 Advances in Crystallization Processes

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

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

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

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 even within several years.

#### **2.2.3 Volume**

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

Crystallization, Alternation and Recrystallization of Sulphates 479

1. during the first quick phase, there is an initial partial dissolution of CaSO4 and

2. during the second – the slower one, there is an increase of thickness of adsorbed

3. during the third phase, there is a crack formation in the absorber layer and counter

4. during the fourth phase - the formation of gypsum nuclei at the surface of anhydrite

This process takes place in the presence of water (in the active phreatic zone), in temperatures below 40°C (process takes place faster in lower temperatures), and its speed depends on the presence of chemical activators, for example K2SO4, MgSO4•7H2O or H2SO4 (Sievert et al., 2005) and CO2, which speeds and eases the hydration. At first, it covers the most fractured parts of the rock, taking place along the cracks and grain boundaries. As a result of hydration, the anhydrite rock transforms into gypsum rock with fine-grained (alabaster), fibrous, porphyroblastic texture (Warren, 1999), coarse/lenticular-crystalline gypsum (sometimes with preserved relic of the anhydritic precursor) – they result from the dissolution of primary sulphates (fine-crystalline anhydrites); see fig. 22. and 23. The secondary gypsum can also be formed as a pseudomorph of the primary anhydrite (e.g. the floor of the cap-rock) or the coarsecrystalline gypsum (selenitic gypsum), which underwent anhydritisation and furthermore gypsification – in this case, despite the multi-stage characteristics of the diagenetic processes, the primary rock structure is preserved. There is an example of the Zechstein (Permian) sulphates, which were uplifted close to the surface as a result of diapirism, and further incorporated into a cap-rock, while being anhydritised and later gypsificated

Inclusions or remains of the primary precursor minerals (e.g. the remains of anhydrite in gypsum) can appear in the primary as well as in the secondary sulphates. Particularly valuable are the inclusions in the primary minerals which can be liquid, solid, gaseous, or even organic. They reach diameters between few and several hundred of μm. Sometimes

a. solid – most often occur: clay minerals, quartz, chalcedony, barite, halite, carbonates -

Part of the solutions can be saturated with gases (CO2, N2, CH4, H2 and H2S), e.g. originating from the organic decomposition (Petrichenko et al., 1995). For example, in the badenian gypsum of Carpathian Foredeep, the presence of: fragments of characean algas, filamentous algas, and colony of unicellular cyanobacterium, insects, coccoids, and multicellular organisms – most probably fungi, has been confirmed. The good state of preservation of

they are arranged zonally, rhythmically – as the crystal grew. Among the inclusions:

b. liquid – mainly the chlorine-sulphate solutions of various mineralization,

2- ions at the surface of anhydrite;

The process of hydration was described in detail by Sievert et al. (2005):

adsorption of hydratem Ca2+ and SO4

migration of H2O and Ca2+, SO42- ions;

(Jaworska & Ratajczak, 2008).

calcite, dolomite, magnesite

**2.3 Inclusions** 

occurs and in the end gypsum crystals are formed.

**2.2.4.2 Gypsification** 

layer

needed to form the same amount of gypsum fills 473 cm3, 9% more – then under natural conditions, when the anhydrite deposit is porous/fractured and water supersaturated, the gypsification process can result not in increase but decrease of volume of the newly formed rock.
