**2.2.4.2 Gypsification**

478 Advances in Crystallization Processes

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

According to Hardie (1967) there are three models describing transformation of gypsum into

1. dissolution of gypsum, and furthermore precipitation of anhydrite (during anhydritisation) or dissolution of anhydrite and later precipitation of gypsum (during

2. direct dehydration of gypsum, that is loosing of the crystallization water (during anhydritisation) or adding the water – hydration of anhydrite (during gypsification).

3. dehydration or hydration with mid stage, with participation of bassanite (mineral rarely occurring in nature). During the hydration, the reaction (occurring very slowly) is

4CaSO4·½H2O → CaSO4·2H2O + 3CaSO4

bassanite gypsum anhydrite

Petrichenko (1989) stated that the process of anhydritisation of gypsum began with its dissolution. This process is accompanied by the appearance of the nuclei of the new mineral phase – bassanite. During the second stage, bassanite transforms into anhydrite. The structural rearrangement of this mineral occurs, resulting in increase of thickness at the cost of length. Sheets (plates) of anhydrite crystals form with corroded edges. However in case of the presence of anhydrite "nuclei", the bassanite does not form, but anhydrite continues its crystallization at the cost of the calcium sulphate from dissolved gypsum. On the basis of examination of the inclusions in minerals, Petrichenko (1989) determined the conditions of the origin of anhydrite: this process takes place in the presence of concentrated brine solutions and under the conditions of high pressure and temperature, but not above 40-

Depending on time and speed of the sulphates transformation there are three kinds of the process: syndepositional, early- and late-diagenetic. The syndepositional anhydritisation occurring during the deposit formation, in shallow basin, sabkhas, in the subsurface environment, causes the substitution of gypsum to take place so fast that the anhydrite remain in its primary form. Anhydritisation during the later stages, according to the solutions of lower salinity, causes the primary sedimentary structures to disappear and the nodular structures to form – gypsum is substituted by incohesive mass of fine anhydritic strips and water, whereas the anhydritisation under the influence of highly concentrated brines can lead to the preservation of the primary gypsum pseudomorphs (Peryt, 1996; Warren, 1999), especially apparent in the coarse-crystalline gypsum forming "the grass-like

**2.2.4 Models of gypsification and anhydritisation** 

anhydrite (or backwards – anhydrite into gypsum):

This mechanism results in change of the rock volume;

rock.

gypsification);

as follows:

**2.2.4.1 Anhydritization** 

50°C.

selenite".

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


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 (Jaworska & Ratajczak, 2008).

## **2.3 Inclusions**

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


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

Crystallization, Alternation and Recrystallization of Sulphates 481

local salt pans), sulphate-rich (Peryt, 1995 and 1996). This process starts from the edges of the grain/crystal and proceeds with deep embayments into the core – the anhydrite/gypsum grain disintegrates into smaller parts that undergo polyhalitization more

Sulphates – gypsum in particular – are common ingredients of the lithosphere and often occur close to the Earth's surface. Additionally, the gypsum easily undergo physical weathering (is soft and has ductile rheology), as well as chemical (dissolves in water). Gypsum dissolution rates reach 29 mm/year and have been measured in Ukraine (Klimchouk & Aksem, 2005). Therefore upon the areas of gypsum deposits karst processes and forms occur (fig. 19.). Gypsum-karst features commonly develop along bedding planes, joint or fractures; sometimes up to 30 m below the Earth's surface. The evidence is the presence of: caves, sinkholes, karren, disappearing streams and springs, collapse structures (Johnson, 2008). One of the longest reported gypsum caves is D.C. Jaster Cave (SW Oklahoma, USA) where main passage is 2,413 m long but total length of all the passages reaches 10,065 m (Johnson, 2008). Speleothems in gypsum caves may provide information about paleoclimate and climate changes in the past, because in arid or semi-arid climates, the speleothems in gypsum cave are mainly composed of gypsum, whereas in contrast, in humid or tropical climate – of carbonate (calcite). The dating of speleothems could provide

b. wet periods in arid zone, when calcite speleothems were deposited (Calaforra et al.,

Gypsum-karst area could be dangerous and should be monitored due to the risk of danger. Some sinkholes and collapse structures, commonly being few hundreds m wide and tens of m deep, may cause the loss of human lives and damages, e.g. in Spain in Oviedo and Calatayud situated on cavernous gypsum area, direct economic losses by collapse events were estimated to be 18 mln euro in 1998 and 4.8 mln euro in 2003 (Gutiérrez et al., 2004 and

The process of the sulphates dissolution is visible not only in developement of karst features; it reveals itself in the smaller scale for example in development of stylolites as a result of pressure solution. The development of the stylolitization process has been usually described among the carbonate rocks - mainly limestones; in the evaporites the stylolites are exceptional. Bäuerle et al. (2000) took under consideration the problem of stylolites genesis in the main anhydrite deposits located in the salts of the Gorleben diapir (Germany). Detailed studies of these forms led to estimation of the amount of dissolved material thanks to the measurements of the maximum amplitudes of the stylolitic sutures visible inside the core. The calculations showed that over 26% rock mass were dissolved. Moreover the microscopic observations indicated the gaps in the sutures – the sutures were 'cut' by the anhydrite crystals formed as pseudomorphs after gypsum. This fact proves that the stylolitization had developed before the gypsum underwent anhydritization. In the article summary, the authors plotted the conditions of the stylolites formation in sulphates,

easily (Stańczyk, 1970).

**2.6 Dissolution and Karst** 

the paleoclimatic data relating to:

2008).

2008).

a. dry periods, when gypsum speleothems were deposited,

these microorganism tissues indicates anaerobic conditions during gypsum precipitation (Petrichenko et al., 1995). The detailed inclusion analyses led to a series of conclusions on the environment, chemical (basin type: open sea or inland ?; brine type: e.g. Na- (Ca)-SO4-Cl or Mg-Na-(Ca)-SO4-Cl or Na-CO3-SO4-Cl ?) and biochemical conditions during the sulphate sedimentation; the variations of the solution chemical composition (e.g. indication of the fresh sea water inflow direction). In addition, the analyses of one-phase liquid inclusions provide information on the water temperature in the crystallization basin.
