In3+-Cl− complexation in hydrothermal fluids: Insights from ab initio deep potential molecular dynamics

Abstract

Given the rising global demand for indium (In) in electronic devices, research has increasingly focused on its natural mineralogy. The transportation and enrichment of indium are primarily driven by magmatic-hydrothermal processes. However, the speciation of In³⁺ in hydrothermal fluids remains poorly understood. This study developed a first-principle-based deep potential model for the InCl₃-H₂O system aimed to investigate the species of In-Cl complexes. The dissociation pathways of In³⁺-Cl⁻ complexes and the corresponding association constants (logK) for $\text{In}\text{C}{\text{l}}_{\text{n}}^{\text{3-}\text{n}}\text{(}\text{n = 1}\text{-}\text{4}\text{)}$ were investigated via deep potential molecular dynamic simulations (DPMD). These new thermodynamic properties provide the first dataset on $\text{In}\text{C}{\text{l}}_{\text{n}}^{\text{3-}\text{n}}\text{(}\text{n = 1}\text{-}\text{4}\text{)}$ dissociation in high P-T fluids (up to 800 °C and 50 kbar). The Helgeson-Kirkham-Flowers (HKF) parameters for In-Cl complexes were fitted with our DPMD derived logK for In-Cl complexes, which can be serves as a pioneering framework for understanding the stability and speciation of In-Cl complexes in hydrothermal fluids, particularly in the absence of current experimental data. Thermodynamic modeling reveals that In³⁺ has a strong chemical affinity for Cl⁻, with high-order In-Cl complexes (InCl₃ and InCl₄^-) playing a critical role in indium transport. Notably, InCl₄^- becomes the predominant species in a very low NaCl concentration (< 1 wt%). Its chemical affinity for Cl⁻ is stronger than Sn²⁺, and the chemical-physical conditions that favor Sn²⁺-Cl^- complex transportation are equally conducive to indium, resulting in their co-transport when they share the same source. Indium and zinc exhibit similar speciation in hydrothermal fluids, which may advantageously promote the substitution of indium into sphalerite.