The influence of marine and continental waters on the clay minerals transformation processes of evaporite deposits (on the example of the Kalush-Holin’ deposit, Carpathian Foredeep)

Abstract

The influence of the chemical composition of marine and continental waters on the formation and transformation of clay minerals is considered on the example of evaporites of the Kalush-Holin’ potash deposit of the Carpathian Foredeep. Clay minerals under variable physical and chemical conditions become unstable and transformed, adapting to new conditions. The main factor that causes changes in their composition and structure is the concentration of brines. The increased concentration of brines at the stage of deposition of potassium salts contributed to the aggradational transformation of clay minerals, the transformation of labile minerals into illite and chlorite that are stable in the hipersaline environment. These two minerals – illite and chlorite are characteristic of the Kalush-Holin’ potash deposit. Further arrangement of the structure leads to the transformation of part of the illite into mica. On the clay fraction diffractograms of some potash rocks, the basal reflex 001 is wide and bifurcated at the top on a line with interplanar distances of 0.99 and 1.0 nm, this indicates the presence of structurally similar minerals – mica and illite. Under conditions of hypergenesis, when evaporite deposits are washed away by fresh surface waters, a reverse process (degradational transformation) takes place, which occurs in the leaching of potassium from the interlayer space of a part of illite and the formation of labile clay structures. The clay mineral association of the gypsum-clay caprock of evaporite deposits, in addition to illite and chlorite, also contains mixed-layer illite-smectite and kaolinite – the appearance of these clay minerals in hypergenic deposits is the result of degradational transformation (illite-smectite) and formation de novo (kaolinite) under conditions of decreased saline brine concentration. Capture of potassium by the structure of the mixed-layer illite-smectite and its transition into illite (aggradational transformation) occurs more easily than the reverse process – potassium leaching and transformation of illite into a mixed-layer illite-smectite (degradational transformation).