Equilibrium isotopic fractionation between aqueous Zn and minerals from first-principles calculations

Abstract

Theoretical mineral-solution equilibrium isotopic fractionation can contribute to the interpretation of Zn isotopic compositions. In this study, we investigate equilibrium isotopic fractionation properties of hexaaquo zinc complex, a major Zn aqueous species, using first-principles molecular dynamics (FPMD) based on density functional theory (DFT). Pentaaquo and tetraaquo zinc complexes, which can be relevant for specific mechanisms such as adsorption on mineral surfaces, are also considered. This approach takes into account both configurational and solvation effects that are less easily accounted for in molecular cluster models. The logarithmic value of the reduced partition function ratio of aqueous Zn (ln β 66 Zn/ 64 Zn) is 2.86 ± 0.04 at 295 K, while the same computational approach gave ln β 66 Zn/ 64 Zn in the range 2.2 − 4.1 for various Znbearing minerals. For example, calculations predict at 295 K, equilibrium fractionations between mineral and aqueous Zn of-0.46 for primary zinc sulfide sphalerite and of-0.20 and +0.34 for secondary phases gunningite and hydrozincite, respectively. Molecular clusters are also modelled, predicting isotopically heavier Zn with respect to related FPMD models (ln β increases by 0.09 to 0.23). These values are small but significant in the Zn isotopic system and supports the idea that a proper description of the dynamics of the system and the solvation effect are required for a reliable prediction of the isotopic properties of solvated ions.

Equilibrium barium isotope fractionation between minerals and aqueous solution from first-principles calculations

“…The bond strength can be measured by the force constant (Ducher et al, 2018;Li et al, 2019b;Mavromatis et al, 2020), as revealed by the linear relationship between the 3 lnβ and the force constant <F> of Ba (Fig. 3b).…”

“…As such, estimating the β factors of aqueous ions such as Ba 2+ , Mg 2+ , and Zn 2+ is more complicated and requires higher computational costs than calculating mineral β factors (Kowalski et al, 2013;Ducher et al, 2018;. In order to simplify the theoretical calculations, some previous studies used the crystal structures of hydroxide polyhydrates to model the structures of aqueous ions (Schauble, 2011;Li et al, 2019b;Mavromatis et al, 2020 .…”

“…Similar to previous studies (Méheut et al, 2009;Schauble, 2011;Huang et al, 2013Huang et al, , 2014Feng et al, 2014;Wang et al, 2017a, b;Li et al, 2019a, b), the equilibrium Ba isotope fractionation between different minerals were calculated from vibrational frequencies based on the DFT with the periodic boundary conditions. For aqueous Ba 2+ , we performed first-principles molecular dynamics (FPMD) simulation to obtain its structural properties and then extracted an adequately large number of snapshots from the FPMD trajectories for the β-factor calculations (Kowalski et al, 2013;Ducher et al, 2018;. We further compared our results with previous experimental measurements and discussed the Ba isotope fractionation behaviors with respect to low-temperature geochemical processes.…”

A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control

“…In natural waters and hydrothermal solutions, Zn occurs as the free ion, i.e. Zn 2+ , solvated by water molecules (Ducher et al, 2018), as Zn complexed to organic ligands or humic acids (Jouvin et al, 2009Mar ovi et al, 2017, and complexed with inorganic ligands such as chloride. Experimental and theoretical investigations on Zn complexation by organic ligands (Ban et al, 2002;Black et al, 2011;Jouvin et al, 2009, Fuji andAlbarède, 2012;Mar ovi et al, 2017) has shown that such complexes favor in general heavy Zn isotopes controlled by the affinity constant.…”

Zinc isotopic fractionation between aqueous fluids and silicate magmas: An experimental study

“…Chen et al (2014) suggested that heavier Zn isotope favors the fluid phase during high temperature water-rock reactions. Fujii et al (2011Fujii et al ( , 2014 and Ducher et al (2016Ducher et al ( , 2018 calculated the fractionation factors of Zn compounds in aqueous fluids. However, the causes for large variations in Zn isotopic compositions of fumaroles from volcanic studies (Toutain et al, 2008;Chiaradia et al, 2018) and sphalerite in hydrothermal deposits (Mathur and Wang, 2019) are still controversial.…”