The role of fluorine in the formation of hydrothermal beryllium deposits: insights from experiments and modeling.

Introduction

Beryllium (Be) is a strategic and critical metal with unique physicochemical properties, including a relatively low density compared to most other metals, high hardness and thermal stability, and excellent transparency to X–rays. These properties make Be an indispensable raw material widely applied in the aerospace, defense, nuclear, medical, electronics and semiconductor industries (Grew, 2002, Taylor et al., 2003, Foley et al., 2017; Lyudmila et al., 2018). As an incompatible lithophile element, beryllium tends to be concentrated in crustal granitic magmas, and its mineralization is considered to be genetically associated with highly evolved felsic rocks (Barton and Young, 2002; Linnen and Cuney, 2005; Zhang et al., 2023). To date, more than 120 Be–minerals have been identified in nature, among which only bertrandite [Be₄Si₂O₇(OH)₂] and beryl [Be₃Al₂Si₆O₁₈] are commercially exploitable (Grew and Hazen, 2014, Rao et al., 2022). Currently, global beryllium resources principally come from two classes of deposits: (i) the pegmatite–hosted beryl deposits and (ii) the hydrothermal bertrandite–(beryl) deposits associated with volcanic rocks, carbonates, skarns, greisens and quartz veins (Barton and Young, 2002, Trueman and Sabey, 2014). Compared with the former, the latter generally have larger reserves and higher BeO grades and are therefore becoming increasingly important exploration targets (London and Evensen, 2002, Ayuso and Foley, 2023). The Spor Mountain volcanogenic bertrandite deposit in the USA, for instance, is one of the largest hydrothermal beryllium deposits in the world, with a reported 11.6 Mt BeO resource and grades up to 0.7 wt%, and currently contributes ? 85 % of global beryllium production (Dailey et al., 2018, Ayuso and Foley, 2023). Understanding the origin of hydrothermal beryllium deposits can greatly improve the accuracy and efficiency of exploration for this type of deposit. However, the mechanisms of beryllium transport and precipitation in hydrothermal fluids are still not fully constrained. The key to addressing this issue is to determine the geochemical behavior of beryllium in hydrothermal systems based on reliable experimental data and quantitative modeling of mineralization processes.

Beryllium has only one oxidation state (+2) in aqueous solution. Because of its small ionic radius (0.27 Å; Shannon, 1976) and moderate charge, Be²⁺ behaves as a hard acid and is expected to form strong complexes with hard bases such as F^–, OH^–, CO₃^2– and SO₄^2- (Pearson, 1963). Among these ligands, F^– may be the most important ligand for Be²⁺ complexation, given the frequent association of Be minerals with fluorite or topaz in hydrothermal beryllium deposits (e.g., Damdinova et al., 2018, Zhang et al., 2019, Zhang et al., 2024). An early potentiometric study at ambient temperature showed that in fluorine–bearing geological fluids Be²⁺ occurs in the form of monomeric fluoride complexes, i.e., BeF⁺, BeF₂⁰_(aq), BeF₃^– and BeF₄^2- (Mesmer and Baes, 1969). These Be–F complexes were predicted to remain stable and occur at elevated temperatures up to 250 ? (Wood, 1992). Nevertheless, limited experimental work has been conducted by earlier Soviet researchers who measured the solubility of bromellite [BeO_(s)] in hydrofluoric acid solutions (HF–H₂O system) at 25–350 ? and argued that Be²⁺ is transported either by mixed fluoride–hydroxide complexes of BeFOH⁰_(aq) and BeF(OH)₂^– (Soboleva et al., 1975, Soboleva et al., 1984, Tugarinov, 1976) or by BeF₂⁰_(aq) and BeFOH⁰_(aq) (±BeOH^–; Kozmenko et al., 1985, Kozmenko et al., 1987). The discrepancies among the aforementioned studies may be attributed to three possible reasons: (i) accurate determination of the beryllium and fluoride concentrations may not have been achieved using the analytical techniques available 40 years ago; (ii) there is a paucity of pH dependence experiments in previous studies, making it difficult to confirm whether hydroxide is involved in bonding with Be²⁺; (iii) significant bias might be generated when predicting the stability of a metal complex under high-temperature conditions based on the data at ambient temperature using theoretical methods (Migdisov and Williams-Jones, 2007; Stefanski and Jahn, 2017). Therefore, beryllium speciation in fluorine–bearing fluids at geologically reasonable conditions, especially in medium- to low-temperature fluids where most hydrothermal beryllium deposits are formed (e.g., Wood, 1992, Li et al., 2015), remains controversial and requires re–evaluation.

In this paper, we carried out systematic experimental investigations of the solubility and speciation of beryllium in acidic, fluoride–bearing aqueous solutions (NaCl–HCl–NaF–H₂O system) at 100–250 ? and vapor–saturated water pressure, and conducted complementary first-principles molecular dynamics simulations. This work allows us to identify the dominant beryllium species in fluoride–bearing fluids and determine its thermodynamic and geometrical properties. We also provide quantitative models to assess the possible mechanism(s) that may contribute to the precipitation of beryllium ore minerals and the formation of economic hydrothermal beryllium mineralization.