First-principles calculations of equilibrium Ga isotope fractionations between several important Ga-bearing minerals and aqueous solutions

Abstract

This study predicts the equilibrium isotope fractionation factors for some important Ga-bearing species, including major minerals, aqueous solutions and gas phase systems. Equilibrium isotope fractionations of Ga were investigated by using the first-principles quantum chemistry method at the B3LYP/6-311+G(d) level. The 103ln(RPFR) values of orthoclase, albite, quartz, kaolinite, forsterite, montmorillonite, gibbsite, cassiterite, aragonite, sphalerite and calcite were calculated with the volume variable cluster model. The 103ln(RPFR)s of these minerals decrease in the following order: orthoclase > albite > quartz > kaolinite > forsterite > montmorillonite > gibbsite > cassiterite > aragonite > sphalerite > calcite. The solvation effect of Ga3+-bearing aqueous species is modeled by the water-droplet method, and the 103ln(RPFR)s of Ga3+-bearing aqueous species decrease in the following order: [Ga(OH)4]− > [Ga(OH)3] > [Ga(OH)]2+ > [Ga(OH)2]+ > [Ga(H2O)6]3+. The calculation results show that equilibrium isotope fractionations of Ga between different minerals, solutions and gas phases are appreciable. Among minerals, Ga isotope fractionation exhibits the largest value between orthoclase and calcite. Ga isotopic fractionation factor between these two minerals can reach 3.18 per mil at 100 °C. Ga isotope fractionations between Ga-bearing aqueous species and minerals are important for obtaining information about the different geochemical processes, such as surficial geochemistry. This study has provided important Ga isotope fractionation factors.