**3.3 Isotopes**

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 indicate two δ18O – in SO4 and H2O.

## **3.3.1 Sulphur (S)**

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

Crystallization, Alternation and Recrystallization of Sulphates 485

Present-day strontium isotope ratio equilibrated between 87Sr-depleted young oceanic basalts and hydrothermal activity along mid-oceanic ridges (ca. 0.7035) and 87Sr-enriched continental sediments (from old continental granites) transported into the basin by wind and rivers (ca. 0.7119 and more; Chaudhuri & Clauer, 1992; Dickin, 2005). It is the same reason why primary Sr isotopic ratio of evaporites could not be the same as that of contemporaneous sea water – e.g. sediments may have deposited in closed basin with inflow of continental water and continental Sr - the Sr ratio of such sulphates is higher than the one of contemporaneous ocean water, so any variation of Sr isotopic composition may relate to the paleohydrology of the basin. Additionally, variations of Sr isotopic ratio may be explain by contamination with more radiogenic Sr or by diagenesis (Hess et al., 1986;

In the classic approach recrystallization means the transformation of fine-crystalline minerals/rocks into coarse-crystalline ones and makes sometimes the continuation of the recovery process, when the mineral/rock or the whole material tries to loose the excess of the internal energy generated during the deformation/strain, when the crystal lattice defects occur. During those processes the shape and size of grains change and the crystallographic axes rotate; they are also accompanied by progressive loss and disappearance of the primary

In the case of recrystallization of cap-rock gypsum, a reverse process can be generally observed (looking upwards) - the size reduction of the mineral grains (dominant or

The boundaries between adjacent fine gypsum grains are usually blurred and irregular, what results from transformation of the larger grains into smaller ones, which successively

The recrystallization of gypsum can occur via: grain boundary migration or subgrain rotation. The grain boundary migration is characteristic for the mineral grains with large variety of lattice defects density, whereas the subgrain rotation occurs in grains with

If the adjacent grains differs in defects density, the defect-poor one bulges into the defectrich one; see fig. 24. It results in the removal of grains with many dislocations. It also enables the spontaneous crystallization and the growth of new grains - "nuclei" (either defectless or with few dislocations) inside the defect-rich grain; these fine new grains are called

The deformation bands formed during the recovery tighten progressively, creating a grid determined by subgrain walls that developed successively within the grain. The subgrains are fragments of larger grain with fine boundaries. As a result of rotation, the crystalline axis

Saunders et al., 1988; Chaudhuri & Clauer, 1992).

uniformly dispersed defects (Passchier & Trouw, 1998).

**4. Recrystallization** 

rock texture/structure.

subordinate components).

**4.1 Grain boundary migration** 

become individual.

'subgrains' as well.

**4.2 Subgrain rotation** 

sulphates in oceanic water and crystallized sulphates is negligible (Thode & Monster, 1965; Raab & Spiro, 1991). δ34S was changing in the geological past and its general trends are known as the sulphur-isotope age curve (Claypool et al., 1980). This curve allows to define the time of evaporate crystallization.
