Cationic Exchange between Montmorillonite and Pore Solution Salts in Hydrothermal Clays from Steam Hydrotherms of the Mutnovsky Geothermal Field

Abstract

Abstract —The montmorillonite samples from shallow parts of the thermal fields within the Mutnovsky hydrothermal steam deposit (the Upper Mutnovsky thermal field and the Dachny site), along with the composition of salts in the clay pore solutions have been studied in the context of ion exchange between them. This aspect represents one of the topical problems due to geothermal clays (montmorillonite) enrichment with elements transferred by steam hydrotherms and concentrating in the near-surface horizons, while montmorillonite as a natural cation exchanger will absorb them, thus acting as a geochemical barrier. The composition of the interlayer cationic complex of montmorillonite samples was studied on isolated clay fractions and from the ion-exchange experiment to determine the salt composition of clay pore solutions and the mineral composition of clay fractions. Geothermally heated soils within the studied thermal fields are dominated by kaolinite-alunite-jarosite assemblage with subordinate amounts of montmorillonite, while montmorillonite prevailing in the mud-water pots is subjected to degradation when they dry out. Crystallization of salts from pore solutions in the near-surface horizons of the studied thermal fields can be exemplified by szomolnokite FeSO4‧H2O, metavoltine K2Na6Fe2+Fe3+6O2(SO4)12·18H2O, leonite K2Mg(SO4)2‧2H2O, polyhalite K2Ca2Mg(SO4)4‧2H2O, mikasaite Fe2(SO4)3, alum and amorphous aluminum sulfate hydrate. Interaction of such solutions with montmorillonite will trigger the cation exchange reactions in the interlayer space of the layered silicate with attendant formation of predominantly Al,Fe-intercalated forms. Alumina hydrates entering the interlayer space of the montmorillonite are reflected by the band at ~ 2500 cm–1 on the infrared spectra attributed to the water coordinated to aluminum. In addition to aluminum and iron, other cations which are concentrated in the uppermost parts of geothermal clay blankets and penetrate into the interlayer space of montmorillonite are: Li+, K+, NH4+, Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, etc. As a result, in the thermal field conditions, montmorillonite is intercalated with easily extractable elements, including lithium, mercury, barium and strontium, ammonium, at the level of units (the first tens of grams per ton).