Hydrothermal processes control the mineralogy and grade distribution of several important REE deposits, such as Gallinas Mountain in New Mexico or Thor Lake in Canada . The mobility of elements in hydrothermal fluids is primarily controlled by the chemical composition and speciation, which have been shown to be controlled by temperature and pressure . Commonly formed REE complexes include carbonates, sulfates, fluorides and chlorides . Therefore, accurate knowledge of the speciation of REEs in dilute to concentrated hydrothermal brines is necessary to improve existing geochemical models and to complement existing mineral recovery and extraction processes. Lanthanum (La), which is classified as a light rare earth element (LREE), is generally enriched by hydrothermal processes that form primary ore deposits. Several minerals like bastnasite, monazite, zircon and apatite are important sources of REEs in such deposits . Although LREEs form stronger complexes with F -with increasing temperature, Cl -is considered to be the main transport ligand of LREEs mainly because of its much higher availability in hydrothermal fluids . Several experimental and theoretical studies have been conducted to determine the properties of La 3+ aqua and chloride complexes at room temperature (Table in Guan et al. ). Experimental and molecular dynamics (MD) simulation studies have shown that the hydration structure of La 3+ at room temperature is a tricapped trigonal prism with nine water molecules. Weak chloride complexation was found by Raman spectroscopy in LaCl 3 solutions with a solute concentration above 0.1 mol/kg , which was confirmed by AIMD simulations . al. used a combination of pair potential-based MD and AIMD simulations and found an average configuration of La(H 2 O) 8 Cl - 2 in concentrated 14 mol/kg solutions. However, fully hydrated La 3+ complexes were seen in 1 mol/kg solutions .