Amphibolite dissolution mechanism under chlorine-rich conditions during freeze-thaw cycles

The low-temperature weathering behavior of amphibolite is vital for revealing the cation source of groundwater and for understanding a wide range of geochemical evolution. This study aims to observe the macroscopic and microscopic characteristics of amphibolite dissolution under the freeze-thaw cycles (-25–25 °C) and chlorides (HCl, NaCl, KCl). The results show that Mg-hornblende was dissolved in chloride solutions of the same pH or concentration before albite. The change rule of the solution composition of freeze-thaw cycles is consistent with the results obtained from the dissolution experiment at room temperature. In addition, in an HCl solution of pH 1 and NaOH solutions of pH 11, 13, and 14, the dissolved amount of quartz in amphibolite can reach 11 wt %. KCl can powerfully dissolve quartz and efficiently dissolve Si in the silica-oxide tetrahedron (especially the T2 site) of Mg-hornblende. However, NaCl significantly dissolves cations in the Mg-hornblende octahedron. Analysis indicates that the leaching of Ca2+ and Mg2+ in proton-promoted dissolution strongly depended on the concentration of H+. At the same time, the release of Mg is closely related to Cl−. The results can help understand chemical element migration in the earth, especially providing a theoretical basis for groundwater cation in the Qaidam Basin. The experiment results show that the concentrations of Ca2+ and Mg2+ dissolved by freeze-thaw cycles are not the same as those in the dissolution experiment at room temperature but have a similar change rule. KCl and NaCl have different promoting effects on the dissolution of silicon and cations and, thus, have different dissolution effects on amphibolite. This study is vital for understand chemical element migration in the earth, especially providing a theoretical basis for groundwater cation in the Qaidam Basin. [Display omitted] •The composition change rule in the solution of the freeze-thaw cycles is consistent with the room-temperature dissolution.•Ca2+ and Mg2+ are affected by the H+ concentration in the proton-promoted dissolution, and Mg is also related to the Cl−.•KCl and NaCl promote the dissolution of amphibolite in different ways.