**2.6 Dissolution and Karst**

480 Advances in Crystallization Processes

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

a. bacterial reduction in deposits rich in organic substances – the most effective process, - sulphates are altered by S-reducing bacteria to form H2S, pyrite and other sulphides,

b. infiltration of meteoric water rich in carbonate ions – occurs during the sulphates

c. thermal reduction of sulphates – late diagenetic process, occurs in temperatures over 100°C, under atoxic conditions and with presence of the hydrocarbons (Machel,

Dissolution of sulphates in presence of hydrocarbons leads to biogenic SO4 reductions and

CH4 + CaSO4 → CaCO3 + H2S + H2O Calcitization of the sulphates can be a multi-stage process (Scholle et al., 1992), which begins with (1) dissolution (or at least corrosion) of anhydrite, (2) hydration of anhydrite and gypsum formation, (3) dissolution of gypsum (this process can be accompanied by the formation of collapse breccia), and afterwards (4) precipitation of calcite inside free spaces and pores arisen after leached sulphates. Sometimes sulphur is the secondary product of

Generally, the gypsum – more easily than the anhydrite – can be substituted by calcite. In case where this process occurs in bigger scale, the post-gypsum limestones form. They can occur in the highest parts of the cap-rock, covering the upper parts of some diapirs - upon the area of Costal Gulf the shallowest subsurface cap-rock levels are usually formed as the calcitic deposits and therefore named as calcitic cap-rocks. However, the microscopic analyses of the cap-rock deposits demonstrated that among the secondary coarse-grained gypsum with the anhydrite remains, the calcification process starts exactly with these

The sulphate rocks can also undergo the polyhalitization process. It proceeds during the early stages of the diagenesis of evaporites as a result of infiltration of hot brines into the sulphate deposits (in the peripheral zones of the evaporite basins): halite saturated, with high contents of Mg2+ and K+ (originating from the dissolution of the potassium salts in the

provide information on the water temperature in the crystallization basin.

Sulphates, as well as gypsum and anhydrite can undergo calcification by:

**2.4 Calcitization** 

1987).

native S and calcite (Holster, 1992)

calcite precipitation according to reactions:

calcitization of sulphates (see fig. 18.).

anhydrite inclusions, not with the gypsum.

**2.5 Polyhalitization** 

exposition onto water activity (Warren, 1999),

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 the paleoclimatic data relating to:


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 2008).

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,

Crystallization, Alternation and Recrystallization of Sulphates 483

anhydrite, gypsum shows limited ability of Sr ions incorporation into its crystal lattice and is not able to incorporate them completely. Dissolution and recrystallization purify gypsum and anhydrite from impurities, and activate strontium lowering its content in newly created mineral comparing to the primary mineral, i.e. some secondary gypsums from Wapno Salt Dome consist only 159 ppm Sr (Jaworska & Ratajczak, 2008), primary anhydrite from which

B likewise Sr is a sensitive indicator of changing conditions in the evporite sedimentary

Systematic increase of B content in the profile of sulphate sediments indicates progressive increase of basin salinity during the crystallization of successive generations of sulphates – evaporites containing the highest amounts of B originate from the most concentrated solutions. Any decrease/variation/fluctuation of this element concentration indicates fresh

Sea water contains 4,45 ppm of boron, mainly in the form of undissociated ortho-boric acid. Solutions of this element deriving from the terrigenic sediments, submarine exhalations and decomposing clay minerals (especially illite) constitute the source of borate ions in the sedimentary basins. Ions of BO32- can isomorphically replace SO42- and form their own

The highest B concentrations are noted during the latest stages of evaporation – when the K-Mg salts precipitate accompanied (under favour conditions) by borates' crystallization. The B content in sulphate rocks (gypsum, as well as anhydrite) can fluctuate between 2 and 5500 ppm; in the Zechstein anhydrite the content ranges from 16 to 500 ppm, and in polyhalite reaches 800 ppm (Pasieczna, 1987) – generally, there are high B contents noted in polyhalite. Sulphates can be analysed from the point of view of Mn and Fe contents; increased concentrations of both elements usually indicate the terrigenic deposit (siliciclastic

Another indication of the genesis and diagenesis of sulphates are the isotopic analyses of 87Sr/86Sr ratio, S (δ34S) and O (δ18O) in SO4, and in the case of gypsum, also O (δ18O) of the crystallization water. δ34S and δ18O in SO4 does not change despite of many transformations, the sulphate molecule maintain its primary isotopic composition, what allows to determine the primary sedimentary conditions, but dynamic and multiple transformation can affect the δ18O of crystallization water, so in gypsum we have to

The present-day 34S/32S (δ34S) ratio of sulphates in oceanic water is constant and reaches +20±0.5‰ with respect to V-CDT (Pierre, 1988) and the fractionation between dissolved

environment, as its concentration in the sediment depends on the salinity.

it has been created consist 1700 ppm Sr.

minerals (borates, e.g. boracite).

indicate two δ18O – in SO4 and H2O.

(sea or meteoric) water supply to the evaporite basin.

sediments, clay minerals) supply into the sedimentary basin.

**3.2 Boron (B)** 

**3.3 Isotopes** 

**3.3.1 Sulphur (S)** 

especially in gypsum as the primary deposit where such forms appear. The process requires:

