Gneiss-Water interaction and water evolution during the early stages of dissolution experiments at room temperature

Abstract

Gneiss-distilled water interaction at room temperature was investigated with batch-reactors to study water-rock reaction and geochemical evolution of the aqueous phase with time. The ion concentrations in water were controlled not only by the dissolution of primary minerals, but also by the precipitation of secondary minerals. The decreasing fraction sizes of gneiss could favor dissolution and precipitation simultaneously. Ca2+ and K+ were the major cations, and HCO3 − was the major anion in water. All the ions except Ca2+ increased in concentration with time. The Ca2+ release from the rock to the aqueous phase was initially much faster than the release of K+, Na+ and Mg2+. But after about 5–24 hours, the Ca2+ concentrations in water decreased very slowly with time and became relatively stable. During the experiment, the water varied from the Ca-(K)-HCO3-type water to the K-Ca-HCO3-type water, and then to the K-(Ca, Na)-HCO3-type water. The water-gneiss interaction was dominated by the dissolution of K-feldspar in the solution. The remaining secondary minerals were mainly kaolinite, illite and K (Mg)-mica.