**3.2 Boron (B)**

482 Advances in Crystallization Processes

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

a. the presence of interbeds different than the sulphate rocks; the lithological

b. the presence of overburden in which the increase of thickness and its chemical characteristics favour the conditions where the lower gypsum is under conditions

The analysis of chemical (including isotopes) contents of the sulphate rocks leads to the conclusions regarding their genesis and diagenetic transformations; e.g. strontium and

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

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

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

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

(Permian) anhydrites content 0.61% Sr (Polański & Smulikowski 1969).

179 ppm), simultaneously indicating high salinity fluctuations (Kasprzyk, 1993).

balancing between pressure solution and the gypsum-anhydrite transformation.

requires:

boron.

**3.1 Strontium (Sr)** 

(Kasprzyk, 1994).

heterogeneity,

**3. Geochemystry of sulphates** 

B likewise Sr is a sensitive indicator of changing conditions in the evporite sedimentary environment, as its concentration in the sediment depends on the salinity.

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 or meteoric) water supply to the evaporite basin.

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 minerals (borates, e.g. boracite).

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 sediments, clay minerals) supply into the sedimentary basin.
