Complexes Of Li/Na/K In Geo-fluids: A Molecular Dynamics Study

Fluids are ubiquitous in nature and significantly influence the circulation of matter in the Earth's sphere and the activation and enrichment of critical metal elements.The understanding of the geochemical behavior and thermodynamic properties of the components in the fluid is the basis for studying the hydrothermal mineralization process.With the rise of new energy vehicles and mobile devices,lithium metal has gradually become a strategic metal resource related to the country's future development.In recent years,more and more studies have pointed out that lithium mineralization is closely related to the hydrothermal mineralization process.The anionic ligands rich in hydrothermal fluids are very important for the extraction,migration and enrichment of metal elements in geological bodies.Traditional geochemical experiments are difficult to obtain the microstructure of fluid components,let alone in high-temperature fluids.In the field of physics and chemistry research,molecular dynamics(MD)simulation is increasingly becoming a popular research tool.The first-principles molecular dynamics(FPMD)method calculates the interaction between particles from the electronic structure,and accurately describes the movement process of the particles.For the description of the monovalent ions,such as Li~+,Na~+and K~+,the empirical force field also plays an indispensable role.It can obtain reliable calculation results while taking into account the calculation hardware,which well confirms the experimental observation.With the improvement of computing power and the development of algorithms,MD simulation methods have been widely used in the study of the properties of geological fluids.In this paper,systematic MD simulation calculations were carried out to research the geochemical behavior of alkali metal ions and halogen ions in melts or fluids at high temperatures.The results revealed the hydration structure and thermodynamic properties of K/Na-F ion pairs in fluids.And we clarified the stability and occurrence form of the Li-F/Cl ion pairs in silicate melts or hot aqueous solutions.Also,we reported the association constants of solution ion pairs in a wide range of temperatures and water densities.It provides a data basis for further research on the genesis of hydrothermal metal deposits.The following key points have been concluded:(1)Classical molecular dynamics(CMD)simulations were performed to study the association of K-F and Na-F ion pairs in water in a wide range of temperatures(423?1273 K)and densities(0.10?0.60 g/cm~3).The association constants(K_A)derived from the potentials of mean force(PMF)exhibit a rising tendency as temperature increases or density decreases.The association of Na-F is similar to K-F under most conditions but evidently larger at higher temperatures and lower densities.The radial distribution function(RDF)and coordination number(CN)reveal the characteristics of ion-water microstructures.There is significant water clustering near the K~+/Na~+at high temperatures or low densities.The results indicate that fluoride ion acts as an important ligand complexing with K~+/Na~+and promotes their migration and concentration in hydrothermal fluids.(2)To reveal the magmatic-hydrothermal geochemical behavior of Li~+,the FPMD and CMD simulations were conducted to investigate the stability and occurrence of Li-F/Cl ion pairs in granite melts and hydrothermal fluids.The FPMD results showed that in granitic melts,Li~+could not form stable ion pair with F~-or Cl~-,but formed four-fold coordination with O.Association constants of Li-F/Cl ion pairs in hydrothermal fluids at a wide range of T-P conditions,i.e.,temperatures from 423 K to 1273 K and densities from 0.10 g/cm~3 to 0.60 g/cm~3,were derived from a series of constrained CMD simulations.The results showed the increasing Li-F/Cl log K_A(m)values with increased temperature or decreased fluid density.In high-temperature low-density vapor-like hydrothermal fluids,strong Li-F complexing was observed.Comparison among several alkali halides ion pairs(i.e.,Li/Na/K-F and Li-Cl)indicated that Li-F ion pairs are dominant in geological vapors.These results suggest that during the magmatic-hydrothermal ore-forming processes of Lithium,F/Cl may play an indirect role in granitic melts,but can form stable ion pairs with Li in hydrothermal fluids.The data provided by this research is the basis for further understanding of the genesis of hydrothermal metal deposits.