**3.1 Strontium (Sr)**

Sr can substitute Ca ions in minerals (mainly in carbonates and sulphates) or create their own minerals (celestine or strontianite), which very often occur dispersed in the marine sediments. High level of Sr characterizes the rocks formed during the final stage of carbonates' sedimentation and during the first stage of calcium sulphates crystallization; generally in sulphates the Sr content increases in direct proportion to the brine concentration (Rosell et al., 1988). The primary gypsum precipitated from evaporated seawaters is expected to have a Sr content of 0.1-0.2% (Ichikuni & Setsuko Musha, 1978) and the one from K-Mg brines - the content of 0.97% Sr (Usdowsky, 1973). Butler (1973) thinks that gypsum precipitated from the celestine saturated solution should consist about 0.09% Sr, and anhydrite – about 0.24% Sr, but primary selenitic gypsum from the Eastern Betics basin contents strontium only between 493-625 ppm Sr (Warren, 2006) and primary Zechstein (Permian) anhydrites content 0.61% Sr (Polański & Smulikowski 1969).

Multimodal distribution of Sr compound in the primary sulphates (gypsum in particular) profiles indicates various sources of this element and multi-stage process of its concentration. The Miocene selenite gypsum from the southern border of the Holy Cross Mts. shows high Sr content (averagely 1300-1500 up to max. 2575 ppm); and scarce variations of the content indicate only episodic salinity fluctuations of the basin, probably connected with the inflow of fresh sea or meteoric water; the gypsum was formed in the sub-aqueous environment characterized by high salinity, whereas the laminated stromatolitic gypsum is characterized by high variations of Sr content (from max. 3695 to 179 ppm), simultaneously indicating high salinity fluctuations (Kasprzyk, 1993).

Strontium can also originate from diagenetic processes: bacterial sulphates reduction, dissolution and recrystallization – they may favour the liberation of strontium ions from the sulphate and could locally form higher concentrations within the other sulphate rocks (Kasprzyk, 1994).

Apparent decrease of Sr concentration occurs during rock transformation in the open system with unbounded circulation of the solution in free pore spaces, whereas the residual products of these transformations are often enriched in strontium. During the hydration of 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 it has been created consist 1700 ppm Sr.
