**4. Recrystallization**

484 Advances in Crystallization Processes

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 present-day 18O/16O (δ18O) ratio of sulphates in oceanic water reaches 9.5±0.5‰ with respect to V-SMOW (Longinelli & Craig, 1967) but during crystallization of the oceanic sulphates, the δ18O is raised up to 3.5‰ (Lloyd, 1968; Pierre, 1988) and δ18O value of this

Primary gypsum and its crystallization water are formed in isotopic equilibrium with the mother brine (Sofer, 1978), but gypsum can easy loose its original crystallization water during further dehydration and hydration. During hydration sulphates interact with meteoric-, ground-, residual or sea water and gypsum absorbs this new, fresh or sometimes mixed primary water. In the areas of several-, several dozen of m long profiles consisting gypsum rocks, basing on the determination of δ18O of their crystallization water, it is possible to indicate the type and range of individual water types which affected the sulphates. E.g. in profiles of the cap-rock of the Wapno and Mogilno salt diapirs (Jaworska, 2010) there is gypsum, which shows δ18O of crystallization water indicating the influence of: cold period post-glacial water – δ18O reaches values from -11 up to -13‰ in the lowest part of the profile (Wapno and Mogilno), recent (or similar to) meteoritic water - δ18O reaches values of -9 to - 10‰ (Wapno), cap-rock water - δ18O reaches -4.3 to -6.6‰ (Mogilno), "mixing" water or

warmer period water - δ18O is -5.6‰ (Wapno) and from -6.9 to -8.7‰ (Mogilno).

radioactive waste or for the storage of hydrocarbons, as well as salt mine.

diagenetic fluids. Strontium does not fractionate (Holster, 1992).

The presence of water described as recent or originated from the colder periods inside the lowest and the middle parts of the cap-rock is very important for further management plan of such salt structure. The influence of present day water or the water from colder periods in the lowest part of the cap-rock indicates free flow of surface water into the area of so called salt mirror; the presence of this water in the middle part of the cap-rock indicates the occurrence of cracks, fractures and karst forms in cap-rock body. In consequence it means, that such cap-rock is not a hermetic cover and does not fulfil the requirements for a seal which protects the rock salt and salt mirror against inflow of freshwater. This information is of great importance for salt structures which are prepared for underground disposal of

The 87Sr/86Sr ratio of modern oceanic water is uniform and reaches 0.70901 (Burke et al., 1982) but has been changing in time. Main reasons of these irregular changes were contribution of Sr with high 87Sr/86Sr ratios from continents and input of Sr with low 87Sr/86Sr ratios from active mid –oceanic ridges (Veizer, 1989; Chaudhuri & Clauer, 1992). The general trends and variations of the marine Sr isotopes during the Phanerozoic carbonates are known (Burke et al., 1982) and this curve (the same as S-curve) allows us to study the age of evaporates precipitation. In evaporites the 87Sr/86Sr ratios reflect the isotopic composition of the brines or

the time of evaporate crystallization.

sulphates reaches 13.0±0.5‰.

**3.3.2 Oxygen (O)** 

**3.3.3 Strontium (Sr)** 

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 rock texture/structure.

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 subordinate components).

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 become individual.

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 uniformly dispersed defects (Passchier & Trouw, 1998).
