# Tantalum in hydrothermal fluids

Bin Hu, Barbara Etschmann, Denis Testemale, Weihua Liu, Qiushi Guan, Harald Müller, Joël Brugger

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## To cite this version:

Bin Hu, Barbara Etschmann, Denis Testemale, Weihua Liu, Qiushi Guan, et al., Tantalum in hydrothermal fluids. Geochimica et Cosmochimica Acta, 2025, 406, pp.243-261. 10.1016/j.gca.2024.10.019. hal-05353216

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# Tantalum in hydrothermal fluids

Bin Hu<sup>a</sup>, Barbara Etschmann<sup>a</sup>, Denis Testemale<sup>b</sup>, Weihua Liu<sup>c</sup>, Qiushi Guan<sup>c</sup>, Harald Müller<sup>d</sup>, Joël Brugger<sup>a,*</sup>

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## ARTICLE INFO

Associate editor: Jean-François Boily

**Keywords:**

Tantalum

Fluoridotantalate complexes

Hydrothermal geochemistry

HERFD-XAS

Thermodynamics

## ABSTRACT

Understanding the behaviour of tantalum (Ta) in hydrothermal systems is pivotal for understanding its geochemical enrichment processes and economic extraction via hydrometallurgy. Yet, its behaviour in hydrothermal systems remains poorly characterised. This study investigates the coordination chemistry, speciation, and solubility of pentavalent Ta(V) in fluoride (F⁻) – and chloride (Cl⁻) –rich hydrothermal solutions up to 413 °C and 800 bar, utilising in-situ High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS). The results reveal the stability of high order fluoridotantalate complexes in fluoride-rich fluids solutions up to the highest investigated temperature, highlighting fluoride’s paramount role in enhancing Ta solubility through the formation of stable fluoridotantalate complexes in aqueous solutions. A transition from nonafluoridotantalate tetraanion (TaF₉⁻) to heptafluoridotantalate dianion (TaF₇²⁻) complexes was observed as a function of temperature in solutions containing ≥1 *m* fluoride. Conversely, our findings indicate a negligible role for chloride in Ta complexation even in high Cl⁻ (~6 *m*) aqueous solutions, suggesting that Ta chloride complexes do not contribute significantly to Ta transport in hydrothermal systems. Existing solubility data were reinterpreted based on an updated speciation model that integrates the in-situ XAS results. This confirms that Ta (OH)₅(aq) predominates in solutions containing <0.02 *m* fluoride; oxyfluoridotantalate anions such as [TaF₃(OH)₃]⁻ dominate in solutions containing intermediate fluoride concentrations (0.02–1 *m*), and the fluoridotantalate anions [TaF₉⁻ to TaF₇²⁻] occur in more concentrated fluoride solutions (>1 *m*) at hydrothermal conditions (~100–400 °C). Derived thermodynamic data for these species enable better understanding and geochemical modelling of Ta transport in hydrothermal fluids, highlighting the potential of F-rich fluids to transport significant amounts of Ta.

## 1. Introduction

Tantalum (Ta), one of the rare metal elements, has broad and strategic applications in the electronics and green energy industries due to its unique properties, such as high capacitance and excellent corrosion resistance (U.S. Department of the Interior and U.S. Geological Survey, 2017; Australian Government Department of Industry, Science and Resources, 2023). Highly fractionated peraluminous granites and pegmatites are among the most favourable geological settings for Ta (Linnen, 1998; Van Lichtervelde et al., 2007; Filella, 2017; Soloviev et al., 2019; Chen et al., 2022). Initial Ta concentration through magmatic processes may be modified by volatile-rich fluids exsolved from the highly evolved parent magmas (e.g., London, 1986, 1997; Černý et al., 2005; Thomas

and Davidson, 2012; Ballouard et al., 2020; Diao et al., 2024). However, the mechanisms of Ta mobilisation and the role of magmatic-hydrothermal processes remain elusive and debatable (Van Lichtervelde et al., 2007, 2009; Stepanov et al., 2014; Ballouard et al., 2020). While most researchers highlight the low solubility of Ta in aqueous fluids (Zaraisky et al., 2010) and the fact that magmatic differentiation processes alone can lead to economic Ta mineralisation in granite-pegmatite systems (Linnen and Cuney, 2005), several studies have shown that the presence of fluoride (F⁻) greatly increases the ability of fluids to transport Ta during late magmatic-hydrothermal processes (e. g., Timofeev et al., 2017; Kaeter et al., 2021). In particular, Timofeev et al. (2017) suggested that Ta minerals are less soluble than Nb minerals, and hence hydrothermal alteration may cause fractionation of

<footnote><sup>a</sup> School of Earth Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia</footnote>

<footnote><sup>b</sup> CNRS, Université Grenoble Alpes, Institut NEEL, Grenoble F-38000, France</footnote>

<footnote><sup>c</sup> CSIRO Mineral Resources, Clayton, VIC 3168, Australia</footnote>

<footnote><sup>d</sup> European Synchrotron Radiation Facility (ESRF), CS 40220, 38043 Grenoble, France</footnote>

<footnote>* Corresponding author.
E-mail address: joel.brugger@monash.edu (J. Brugger).</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Concentrations of dissolved Ta species in equilibrium with Ta₂O₅(s) in F-bearing fluids calculated at 25 °C using the thermodynamic properties selected by Filella and May (2019), for a closed system containing 1 µm Ta and at fixed pH₂₅ of 2 (a), 3 (b) and 4 (c).</figcaption></figure>

these two metals.

Under most natural conditions, Ta carries a high charge (+5) and has a relatively small ionic radius, categorising it as a hard acid in Pearson's Hard and Soft Acids and Bases (HSAB) theory (Pearson, 1963; Brugger et al., 2016). Consequently, Ta forms strong complexes with hard bases such as the fluoride (F⁻; Akinfiev et al., 2020), hydroxyl (OH⁻; Timofeev et al., 2017), and carbonate anions (CO₃²⁻; Kotova, 2011; Linnen et al., 2012), and weaker complexes with softer bases such as chloride (Cl⁻; Anderson et al., 2019). Among these common geological ligands, fluoride emerges as the most favourable complexing agent for promoting Ta solubility (e.g., Zaraisky et al., 2010) and its mobilisation in hydrothermal systems (Williams-Jones and Migdisov, 2014; Akinfiev et al., 2020). In fact, leaching in HF at mild hydrothermal conditions (80–100 °C) is currently one of the most effective metallurgical methods used for recovering Ta from ores and recycled electronic waste (e.g., Agulyansky, 2004a; Mackay and Simandl, 2014; Chen et al., 2019; Shikika et al., 2020; Schütte et al., 2021; Wang et al., 2022).

Despite the vital role that fluoride complexes may play in the geological cycling of Ta, the nature and thermodynamic properties of the predominant Ta complexes in hydrothermal systems remain poorly known and have been the subject of only a few experimental studies. Most research on Ta complexation was conducted at ambient temperature, as reviewed by Filella and May (2019), who noted that the utility of the available thermodynamic data is limited due to the lack of adequate information, even at room temperature. In general, there is agreement that Ta(OH)₅(aq) predominates in acidic solutions at low concentrations of ligands such as fluoride (Fig. 1). It is widely recognised that Ta(V) fluoride complexes become predominant at moderate F concentrations in acidic fluids (Caletka et al., 1989; Wilson et al., 2016). However, the nature of the complex with the highest F-coordination remains controversial, varying from TaF₇²⁻ (Matwioff et al., 1970, Raman spectra; Baumann, 1972, potentiometry; Wilson et al., 2016, EXAFS), to TaF₃³⁻ (Bukhsh et al., 1966, infrared spectroscopy and potentiometry), and up to TaF₄³⁻ (Varga and Freund, 1962, potentiometry). In most cases, Ta is assumed to exist as six- and seven-coordinated fluoride complexes or as oxyfluoride-tantalate such as TaOF₃²⁻, depending on F concentrations (e. g., Agulyansky, 2004b; Zhu and Cheng, 2011; Shikika et al., 2020).

Rather remarkably, the mere existence of Ta(V) chloride complexes remains controversial. Anderson et al. (2019) identified TaCl₆⁻ as the prevailing species in a 2 wt% (1.020 *m*) HF solution with dissolved TaCl₅ at room temperature. Kinsman et al. (2020) argued that Cl was a better medium for Ta aqueous extraction via solvent extraction (as TaCl₆⁻) than F, pointing out that TaCl₅(s) and TaF₅(s) are both water-soluble. However, other studies have shown that chlorine had no significant contribution to the solubility of tantalite and Ta₂O₅(s) in fluorine-free systems (Korzhinskaya et al., 2014), and no thermodynamic properties are

available for any Ta(V) chloride complex (Filella and May 2019; Akinfiev et al., 2020).

The few studies that have investigated the behaviour of Ta under hydrothermal conditions, mostly using quenched techniques, are listed in Table 1. According to the solubility measurements of Timofeev et al. (2017), Ta(OH)₅(aq) is predominant at low HF concentrations, in agreement with previous ambient studies (section 2.1), while hydroxy-fluoride or fluoride complexes such as TaF₃(OH)₃ and TaF₅(aq) become dominant at higher HF concentrations at temperatures up to 250 °C. Quantitative solubility data above 250 °C (up to 550 °C at 1000 bar) are provided by Kotova and Zaraisky (2006) and Zaraisky et al. (2010) for F-bearing solutions; Korzhinskaya et al. (2014) for Cl-bearing systems; and Kotova (2011) for basic systems (Table 1 summarises experimental conditions). Akinfiev et al. (2020) reinterpreted these solubility data, proposing that the predominant Ta(V) species are Ta(OH)₄⁺, Ta(OH)₅(aq), Ta(OH)₆⁻, TaF₃(OH)₂(aq), and TaF(OH)₅⁻.

Anderson et al. (2019) provided the only *in-situ* study of the nature and coordination geometry of Ta(V) complexes in hydrothermal fluids to date. Based on *in-situ* XAS data obtained up to 400 °C, they proposed that the local configuration of Ta(V) complexes transitions from an octahedral TaCl₆⁻ complex at ambient condition to octahedral Ta-O(H) complexes with increasing temperature in a 2 wt% (1.020 *m*) HF solution containing Ta(V) introduced as TaCl₅(s). In contrast, Ta(V) complexes existed primarily in the form of TaFₙ⁵⁻ⁿ, with *n* ~ 8 ± 1, at 350 and 400 °C (no data reported < 350 °C) in a chloride-free (Ta(V) added as Ta₂O₅(s)) solution with an HF concentration of 5 wt% (2.631 *m*). These results are at odds with the interpretation of solubility studies (Table 1), which concluded that the maximum F coordination is three; TaF₃(OH)₃ according to Timofeev et al. (2017); or TaF₃(OH)₂(aq) according to Akinfiev et al. (2020).

The present study aims to assess the coordination structure and solubility of Ta(V) in basic (NaOH) and in acidic fluoride- and chloride-bearing hydrothermal solutions at temperatures up to 413 °C and at a pressure of 800 bar, based on *in-situ* High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) (Glatzel et al., 2009; Pokrovski et al., 2022). This is the first HERFD-XAS study at the Ta L₃ edge. Compared to traditional measurements of dilute solutions via solid-state detectors (SDD), HERFD-XAS provides an energy resolution (< 1 eV in our case) lower than the core-hole lifetime broadening (4.68 eV for Ta L₃ edge; Campbell and Papp, 2001), thereby enhancing the spectral resolution to reveal more detail (Hämäläinen et al., 1991), especially in the X-ray Absorption Near Edge Structure (XANES) region (de Groot et al., 2002). HERFD-XAS can also result in a better signal-to-noise ratio of the XAS data collected from dilute aqueous solutions as the contributions of the strong elastic (Llorens et al., 2012) and Compton scattering originating from the solutions are minimised
<table><caption>**Table 1**

Summary of experimental and thermodynamic investigations into the structure of tantalum complexes under hydrothermal conditions.</caption><thead><tr><th>Publication</th><th>Technique(s) / PT range</th><th>Speciation</th><th>Experimental conditions</th></tr></thead><tbody><tr><td>Kotova and Zaraisky (2006)</td><td>Hydrothermal high-pressure apparatus / 550 °C, 1000 bar</td><td>—</td><td>Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in HF, KF solutions of 0.01 to 2.0 m H<sub>2</sub>O, NaF solutions of 0.01 to 1.0 m H<sub>2</sub>O, and LiF solutions of 0.01 to 0.3 m H<sub>2</sub>O under Co-CoO oxidising conditions.</td></tr><tr><td>Zaraisky et al. (2010)</td><td>Hydrothermal high-pressure apparatus / 300–550 °C, 1000 bar</td><td>K<sub>4</sub>TaO<sub>2</sub>+xF<sub>1-x</sub> (x = 0.4 to 0.6) Na<sub>2</sub>Ta<sub>4</sub>O<sub>11</sub>/ Na<sub>2</sub>Ta<sub>2</sub>O<sub>5</sub>F<sub>2</sub></td><td>Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in 0.1 and 1.0 m HF, KF, NaF and 0.03 and 0.1 m LiF at 300 and 350 °C (30 days), 400, 450, and 500 °C (21 days), and 550 °C (15 days) under Co-CoO oxidising conditions.</td></tr><tr><td>Kotova (2011)</td><td>Hydrothermal high-pressure apparatus / 550 °C, 500–1000 bar</td><td>—</td><td>Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in Na<sub>2</sub>CO<sub>3</sub> and NaOH solutions with the concentration (0.01; 0.1; 0.5, 1.0, 2.0 m) under Co-CoO oxidising conditions (21 days).</td></tr><tr><td>Korzhinskaya et al. (2014)</td><td>Hydrothermal high-pressure apparatus / 500 °C, 1000 bar</td><td>—</td><td>Synthetic MnTa<sub>2</sub>O<sub>6</sub>(s) and Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in chloride solutions HCl, KCl, NaCl, and LiCl for 7–10 days under the Co-CoO buffer.</td></tr><tr><td>Timofeev et al. (2017)</td><td>Titanium autoclaves / 100–250 °C, P<sub>sat.</sub></td><td>Low HF concentration Ta(OH)<sub>5</sub>(aq) [100 to 250 °C] High HF concentration TaF<sub>5</sub>(aq) [only ≤ 150 °C] TaF<sub>3</sub>(OH)<sub>3</sub> [100 to 250 °C]</td><td>Synthetic Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in variable pH and HF concentration aqueous solutions at 100, 150, 200, and 250 °C and vapor-saturated water pressure. The pH ranged from 0.8 to 3.2, mostly ~2.0; the HF concentration ranged from 10<sup>-5</sup> to 10<sup>9</sup> m.</td></tr><tr><td>Anderson et al. (2019)</td><td>EXAFS / Room Temperature-400 °C, P<sub>sat.</sub> – 400 bar</td><td>Solution A &lt; 350 °C: not reported 350 °C: TaF<sub>n</sub>(<sup>5-n</sup>) [n = 7.9(1.1)] 400 °C: TaF<sub>n</sub>(<sup>5-n</sup>) [n = 8.3(0.9)] Solution B 25 °C: TaCl<sub>n</sub>(<sup>5-n</sup>) [n = 6.2(0.6)] 360 °C: Ta (OH)<sub>n</sub>(<sup>5-n</sup>) [n = 6.4(0.7)] 400 °C: Ta (OH)<sub>n</sub>(<sup>5-n</sup>) [n = 5.6(0.6)]</td><td>Solution A: Ta<sub>2</sub>O<sub>5</sub>(s) dissolved in 2.6308 m HF solution from 300 to 400 °C at HDAC sample chamber; Ta concentration 7600 µg/mL. Solution B: TaCl<sub>5</sub> in a 1.0201 m HF solution from RT to 400 °C at HDAC sample chamber; Ta concentration 1000 ± 3 µg/mL.</td></tr><tr><td>Akinfiev et al. (2020)</td><td>Thermodynamic (review) / 0–600 °C, 1–3000 bar</td><td>Acid HF-bearing solution TaF<sub>2</sub>(OH)<sub>3</sub>(aq) Near neutral pH solution Ta(OH)<sub>5</sub>(aq) Alkaline F-bearing solution HTaO<sub>3</sub>F<sup>-</sup></td><td>—</td></tr></tbody></table>

<table><caption>**Table 2**

Preparation and composition of solutions studied by *in-situ* HERFD-XAS.</caption><thead><tr><th>Sample</th><th>HF (mol/kg)</th><th>Ta (mol/kg)</th><th>F (mol/kg)</th><th>Cl (mol/kg)</th><th>pH<sub>calc</sub> @ 319 °C, 800 bar</th><th>Recipe</th><th>T (°C)</th></tr></thead><tbody><tr><td>Stock-A</td><td>2.697</td><td></td><td></td><td></td><td></td><td>Stock solution A – HF (5.12 wt%)</td><td></td></tr><tr><td>Stock-B</td><td>1.009</td><td></td><td></td><td></td><td></td><td>Stock solution B – HF (1.98 wt%)</td><td></td></tr><tr><td>Sol-03</td><td></td><td>0.034</td><td>2.868</td><td></td><td>1.3</td><td>Stock-A + TaF<sub>5</sub></td><td>25 – 413</td></tr><tr><td>Sol-04</td><td></td><td>0.023</td><td>1.124</td><td></td><td>2.0</td><td>Stock-B + TaF<sub>5</sub></td><td>25 – 413</td></tr><tr><td>Sol-05</td><td></td><td>0.015</td><td></td><td>6.006</td><td>0.0</td><td>TaCl<sub>5</sub> + HCl (36 wt %) + H<sub>2</sub>O</td><td>25 – 413</td></tr><tr><td>Sol-09</td><td></td><td></td><td></td><td></td><td>10.1</td><td>1 m NaOH + 7.2 mg Ta<sub>2</sub>O<sub>5</sub> pellet</td><td>25 – 413</td></tr><tr><td>Sol-02</td><td></td><td>0.019</td><td>1.009</td><td>0.094</td><td>1.3</td><td>Stock-B + TaCl<sub>5</sub></td><td>25 – 413</td></tr><tr><td>Sol-08</td><td></td><td>0.027</td><td>0.135</td><td>5.971</td><td>0.0</td><td>TaF<sub>5</sub> + HCl (36 wt %) + H<sub>2</sub>O</td><td>25 – 413</td></tr></tbody></table>

(Hazemann et al., 2009). Extended X-ray Absorption Fine Structure (EXAFS) data were quantified using shell-by-shell fitting and combined with *ab initio* XANES simulations to constrain the nature of the geometry of Ta(V) complexes in acidic fluoride- and chloride-bearing solutions. We use the new understanding of Ta(V) coordination chemistry in fluoride brines to reinterpret the solubility data from previous studies (Zaraisky et al., 2010; Timofeev et al., 2017) and then apply the Helgeson-Kirkham-Flowers (HKF) or the modified Ryzhenko-Bryzgalin (MRB) equation of state to extrapolate values to magmatic hydrothermal conditions.

## 2. Materials and methods

### 2.1. Experimental solutions

Experimental solutions (Table 2) for HERFD-XAS measurements were prepared by dissolving appropriate amounts of Ta<sub>2</sub>O<sub>5</sub>(s) (ThermoScientific, 99 % purity); TaCl<sub>5</sub>(s) (ThermoScientific, 99.8 % purity) and TaF<sub>5</sub>(s) (ThermoScientific, 98 % purity) in HF-, HCl-, and NaOH-stock solutions. Analytical-grade NaCl(s) and NaF(s) were added to achieve the desired salinity. All solutions we prepared by accurately weighting the various components. Following Anderson et al. (2019), two stock solutions of hydrofluoric acid were prepared by diluting 48 wt % HF gravimetrically with MilliQ water, to 5.12 wt% and 1.98 wt% HF, i.e. 2.697 m and 1.009 m, respectively. As Ta<sub>2</sub>O<sub>5</sub>(s) is poorly soluble in the NaOH solution at room temperature, a small (7.2 mg) pellet of Ta<sub>2</sub>O<sub>5</sub>(s) was added to the NaOH solution.

### 2.2. HERFD-XAS data collection

Tantalum L<sub>3</sub>-edge *in-situ* HERFD-XAS data were acquired using the high-P-T XAS cell developed by the Néel Institute (Testemale et al., 2005; Brugger et al., 2010; Louvel et al., 2022; Guan et al., 2022), at the European Synchrotron Research Facility (ESRF) in Grenoble, France, on the FAME-UHD beamline (BM16; Proux et al., 2017). Photons at the Ta L<sub>α1</sub> fluorescence line (8146 eV) were selected using a set of bent Si(444)
<figure><img src="image_6.png" /><figcaption>Fig. 2. Comparison between the Ta L$_{3}$-edge XANES spectra of the standard sample of Ta$_{2}$O$_{5}$(s) (low temperature modification, < 1360 °C) and the results of ab initio FDMNES calculation. The grey lines are the raw calculations for the 12 symmetrically independent Ta(V) sites in the structure, for the most common configuration according to the disorder parameters refined by Stephenson and Roth (1971); results are similar for other (less likely) configurations. Different Fermi level trun- cations were attempted for the core–hole lifetime broadening set at 1.52 eV. A value of 2.00 eV provides a best fit with the experimental spectrum.</figcaption></figure>

crystal analyser spectrometers (CAS; Llorens et al., 2012) that achieved an energy resolution of ~1 eV. This resolution is substantially better than solid-state or silicon drift detectors that typically have an energy resolution of ≥ ~120 eV, and most importantly it is significantly higher than the core–hole lifetime broadening of the Ta L$_{3}$-edge (4.68 eV; Campbell and Papp, 2001). Because of this, the apparent core–hole lifetime broadening is reduced to a lower value, Γ$_{app}$, which can be calculated as (de Groot et al., 2002; Proux et al., 2017):

$$ \Gamma_{\text{app}} = 1/\sqrt{\Gamma_{\text{int}}^{-2} + \Gamma_{\text{fin}}^{-2}} \qquad (1) $$

For Ta L$_{3}$, the photo-electron transition is from the intermediate M$_{5}$ (5d$_{5/2}$) to the final L$_{3}$ (2p$_{3/2}$) states, with respective energies of Γ$_{int}$ = 1.61 (M$_{5}$) and Γ$_{fin}$ = 4.68 (L$_{3}$) eV (Campbell and Papp, 2001). Γ$_{app}$ is calculated to be 1.52 eV, indicating that the core–hole lifetime broad- ening is effectively dominated by the intermediate state. Comparison of

the HERFD versus transmission experimental spectra for the Ta$_{2}$O$_{5}$(s) standard (Fig. 2) demonstrates that the narrower apparent lifetime broadening in HERFD dramatically improves the peak characteristics of the XANES spectra at the Ta L$_{3}$ absorption edge; in this case HERFD reveals that the Ta L$_{3}$-XANES white line consists of two components (a sharp intense band and a broader shoulder), thereby providing extra information for the interpretation of the XAS results.

The photons reflected by the Si(444) crystal analyser spectrometers were counted using a KETEK detector. This energy-resolved detector permits the removal of unwanted contributions resulting from beam scatter and, in our case, provides an improved signal-to-noise ratio compared to total fluorescence measurements (Pokrovski et al., 2022). The incident and transmitted beam intensities I$_{0}$ and I$_{1}$ were collected simultaneously using Si diodes measuring photons scattered by Kapton foils placed in the primary beam. The monochromated beam energy was calibrated with a Ta metal foil, such that the maximum of the first
<table><caption>**Table 3**
EXAFS fit results (N: ligand number; ΔE: energy shift; σ²: Debye-Waller factor; R: bond distance).</caption><thead><tr><th>Sample</th><th>Temperature</th><th>ligand</th><th>N</th><th>ΔE (eV)</th><th>σ²(Å²)</th><th>R(Å)</th></tr></thead><tbody><tr><td rowspan="6">Sol-03<br>(2.868 m F)</td><td>25 °C</td><td>F</td><td>9.0(4)</td><td>5(2)</td><td>0.005<br>(1)</td><td>1.94<br>(1)</td></tr><tr><td>88 °C</td><td>F</td><td>8.1(5)</td><td>5(2)</td><td>0.005<br>(2)</td><td>1.91<br>(3)</td></tr><tr><td>164 °C</td><td>F</td><td>7.1(6)</td><td>5(2)</td><td>0.003<br>(2)</td><td>1.89<br>(5)</td></tr><tr><td>242 °C</td><td>F</td><td>7.1(7)</td><td>5(2)</td><td>0.003<br>(2)</td><td>1.89<br>(6)</td></tr><tr><td>319 °C</td><td>F</td><td>7.0(6)</td><td>5(2)</td><td>0.003<br>(2)</td><td>1.89<br>(6)</td></tr><tr><td>413 °C</td><td>F</td><td>7.0(8)</td><td>5(2)</td><td>0.003<br>(3)</td><td>1.89<br>(6)</td></tr><tr><td rowspan="4">Sol-04<br>(1.124 m F)</td><td>25 °C</td><td>F</td><td>9.0(5)</td><td>6(2)</td><td>0.005<br>(2)</td><td>1.94<br>(1)</td></tr><tr><td>88 °C</td><td>F</td><td>8.0(5)</td><td>6(2)</td><td>0.005<br>(2)</td><td>1.91<br>(4)</td></tr><tr><td>164 °C</td><td>F</td><td>7.1(7)</td><td>6(2)</td><td>0.003<br>(2)</td><td>1.89<br>(6)</td></tr><tr><td>242 °C</td><td>F</td><td>7.2(6)</td><td>6(2)</td><td>0.004<br>(2)</td><td>1.89<br>(6)</td></tr><tr><td rowspan="5">Sol-02<br>(1.009 m F,<br>0.094 m Cl)</td><td>25 °C</td><td>F</td><td>9.0(3)</td><td>5(2)</td><td>0.005<br>(1)</td><td>1.94<br>(2)</td></tr><tr><td>88 °C</td><td>F</td><td>8.0(3)</td><td>5(2)</td><td>0.006<br>(1)</td><td>1.91<br>(4)</td></tr><tr><td>164 °C</td><td>F</td><td>7.1(5)</td><td>5(2)</td><td>0.003<br>(2)</td><td>1.90<br>(6)</td></tr><tr><td>242 °C</td><td>F</td><td>7.1(4)</td><td>5(2)</td><td>0.003<br>(1)</td><td>1.89<br>(6)</td></tr><tr><td>319 °C</td><td>F</td><td>7.0(3)</td><td>5(2)</td><td>0.005<br>(3)</td><td>1.89<br>(5)</td></tr><tr><td rowspan="5">Sol-05<br>(6.006 m Cl)</td><td>25 °C</td><td>O</td><td>6.0(1)</td><td>-12<br>(3)</td><td>0.007<br>(2)</td><td>1.81<br>(3)</td></tr><tr><td>88 °C</td><td>O</td><td>6.0(1)</td><td>-12<br>(3)</td><td>0.008<br>(2)</td><td>1.81<br>(3)</td></tr><tr><td>164 °C</td><td>O</td><td>6.0(1)</td><td>-12<br>(3)</td><td>0.008<br>(3)</td><td>1.81<br>(3)</td></tr><tr><td>242 °C</td><td>O</td><td>6.0(1)</td><td>-12<br>(3)</td><td>0.007<br>(3)</td><td>1.82<br>(2)</td></tr><tr><td>319 °C</td><td>O</td><td>5.0(2)</td><td>-12<br>(3)</td><td>0.005<br>(3)</td><td>1.82<br>(2)</td></tr><tr><td rowspan="6">Sol-08<br>(0.135 m F,<br>5.971 m Cl)</td><td>25 °C</td><td>O</td><td>3.0<br>(3)</td><td>-5<br>(3)</td><td>0.003<br>(1)</td><td>1.79<br>(4)</td></tr><tr><td>88 °C</td><td>F</td><td>3.0<br>(3)</td><td>-3<br>(5)</td><td></td><td>1.90<br>(5)</td></tr><tr><td></td><td>O</td><td>3.0<br>(fix)</td><td>-5<br>(3)</td><td>0.005<br>(1)</td><td>1.79<br>(5)</td></tr><tr><td></td><td>F</td><td>3.0<br>(fix)</td><td>-3<br>(5)</td><td></td><td>1.90<br>(5)</td></tr><tr><td>164 °C</td><td>O</td><td>5.0<br>(2)</td><td>-5<br>(3)</td><td>0.006<br>(2)</td><td>1.85<br>(1)</td></tr><tr><td>242 °C</td><td>O</td><td>5.0<br>(2)</td><td>-5<br>(3)</td><td>0.007<br>(2)</td><td>1.86<br>(2)</td></tr></tbody></table>

derivative of the Ta L$_{3}$-edge transmission signal was at 9881 eV.

All experiments were conducted at a pressure of 800 bar, at temperatures ranging from ambient up to 413 °C. The autoclave comprises an inner tubular (4 mm ID) glassy carbon sample cell in a small cylindrical resistive heater, where the solution is loaded between two glassy carbon pistons. This assembly is then placed in an external water-cooled pressure vessel equipped with three 1.5 mm thick beryllium windows that enable the simultaneous collection of fluorescence and transmission signals. The autoclave was tilted from its vertical position to match the required Bragg angle of the crystal analysers. Helium was used as the

pressure medium, and pressure was read from a Bourdin gauge with a precision better than 5 bar. An automated regulation device maintains pressure within ± 0.5 bar of the set value during measurements (Bruyère et al., 2008). Temperature measurements were obtained from a K-type thermocouple placed outside the glassy carbon cell but near the sample. The solution temperature at the beam position was calibrated following the procedure described by Xing et al. (2019), based on measuring the water density from the measurement of the X-ray absorbance of water at 9850 eV and comparison with the equation of state of pure water (NIST database; Lemmon et al., 2024). The X-ray mass attenuation coefficients tabulated by Chantler (1995) were used in the calculations. Based on this calibration, temperature accuracy was better than ± 3 °C. Temperature regulation maintained the temperature within ≤ 0.2 °C during data collection.

## 2.3. HERFD-XAS data reduction and EXAFS data fitting

XANES and EXAFS data were analysed with the Athena and Artemis codes from the DEMETER package (Ravel and Newville, 2005) using FEFF version 6 (Zabinsky et al., 1995). Initial configurations were taken from Gutmann and Jack (1951) ([TaF$_{6}$] moiety in the crystal structure of TaF$_{3}$(s)) and Aleshina and Loginova (2002) ([TaO$_{6}$] moiety in the crystal structure of β-Ta$_{2}$O$_{5}$(s)). The amplitude reduction factor ($S_0^2$) was determined to be 1.0 by fitting the standard Ta$_{2}$O$_{5}$(s) and fixed to 1.0 for all subsequent refinements. The threshold energy ($E_0$) was defined as the maximum of the first derivative, and 1-σ errors calculated by Artemis are reported. One energy shift per ligand, ΔE$_{0}$, was fitted for each solution from ambient to 413 °C. EXAFS data [$\chi(k)$] were refined in R-space and were weighted by $k^n$, where n = 1, 2 and 3.

## 2.4. Ab initio XANES simulations

Tantalum L$_{3}$-edge XANES spectra of selected stoichiometries and geometries were calculated using the Finite Difference Method Near Edge Structure (FDMNES) package (Joly, 2001; Bunău et al., 2021). FDMNES uses density functional theory (DFT) to compute the final states and the corresponding absorption cross-sections of the Schrödinger equation. Implementing the Finite Difference Method (FDM) permits a free potential shape, enabling a more precise simulation of electron behaviour near the atomic nucleus compared to the Muffin-Tin (MT) approximation (e.g., Brugger, 2007), especially for low symmetry and/or non-dense aqueous complex structures.

For ease of comparison with the experimental spectra, the raw absorption cross-section calculated by FDMNES is convoluted by the core-hole lifetime broadening (Γ), which is 4.68 eV for the Ta L$_{3}$ absorption edge (Campbell and Papp, 2001), and is reduced to 1.52 eV in our HERFD measurements (Eq. 1). Theoretical calculations using FDMNES only broadly reproduce the experimental HERFD spectrum of Ta$_{2}$O$_{5}$(s) (Fig. 2), possibly indicating a different order state for the measured material than that provided by Stephenson and Roth (1971). Note that the individual bands in the raw calculated spectra are often very sharp, with widths of just ~0.05 eV. Based on the comparison of convolutions performed with different Fermi energies, the Fermi levels used for the convolution of the FDMNES raw simulations were universally set as 2.0 eV (Fig. 2).

## 2.5. Fitting thermodynamic properties and thermodynamic modelling

We refitted the existing high quality Ta$_{2}$O$_{5}$(s) solubility data of Timofeev et al. (2017) (100–250 °C) and Zaraisky et al. (2010) (300–550 °C) to obtain a thermodynamic model that is consistent with these data as well as the new XAS results (Table 3).

Timofeev et al. (2017)'s solutions were prepared using HClO$_{4}$, NaF, and NaClO$_{4}$, with compositions reported in terms of total ClO$_{4}$, total F content, and measured pH$_{25°C}$. Hence, we calculated the HClO$_{4}$ to NaClO$_{4}$ ratio required to obtain the measured pH$_{25°C}$ listed by Timofeev

<footnote>* The numbers in parentheses are 1σ errors.</footnote>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Tantalum L₃-edge XANES spectra for Sol-03 (TaF₅ in 2.868 m F) and Sol-04 (TaF₅ in 1.124 m F); (a) and (c) are normalized spectra, (b) and (d) first derivative.</figcaption></figure>

et al. (2017) using our thermodynamic model (Supp. Mat. 2).

A customised Matlab code (Mei et al., 2016; Guan et al., 2023) was used to compare the experimental solubility values with those calculated using different thermodynamic models. The solubilities of Ta₂O₅(s) were calculated for each experiment based on the full distribution of species calculations performed using the EQBRM (Equilibrium-Based Reaction Model; Anderson and Crerar, 1993) implementation from the BeerOz package (Brugger, 2007). The equilibrium constants for the mineral solubility (Kₛₚ) and selected ligand exchange reactions were optimised in order to reflect the measured solubility and the speciation derived from our XAS results:

$$Ta_2O_5(s) + 5H_2O = 2Ta(OH)_5(aq) \qquad (2)$$

$$Ta(OH)_5(aq) + nHF(aq) = TaF_n^{5-n} + (n-5)H^+ + 5H_2O \quad (n = 5, 7, 8, 9) \quad (3)$$

$$Ta(OH)_5(aq) + 3HF(aq) = TaF_3(OH)_3^- + H^+ + 2H_2O \qquad (4a)$$

$$Ta(OH)_5(aq) + 3HF(aq) = TaF_3(OH)_2(aq) + 3H_2O \qquad (4b)$$

$$Ta(OH)_5(aq) + HF(aq) = TaF(OH)_5^- + H^+ \qquad (5)$$

A large number of different speciation models were tested, in particular using different combinations of TaF₄⁻, and the different mixed fluoride-hydroxide complexes proposed by Timofeev et al. (2017) (equation 4a) and Akinfiev et al. (2020) (equations 4b, 5). The quality of the fits was quantified using the residual function, χ²:

$$\chi^2 = \sum_i \left[ \left( [Ta_{calc}] - [Ta_{exp}] \right)^2 / [Ta_{calc}] \right] \qquad (6)$$

Average F coordination numbers were calculated based on the thermodynamic model and compared with the experimental values obtained from the EXAFS refinements; models that deviated significantly from the measured coordination numbers were discarded.

For all thermodynamic calculations, the properties of Ta₂O₅(s) were taken from Jacob et al. (2009), and Ta(OH)₅(aq) from Akinfiev et al. (2020). Activity coefficients were calculated using the extended Debye-Hückel equation (Helgeson, 1969),

$$\log \gamma_i = - \frac{z_i^2 \cdot A \cdot \sqrt{I}}{1 + a_i \cdot B \cdot \sqrt{I}} + b \cdot I \qquad (7)$$

where zᵢ is the charge of ion i; I is the ionic strength; A and B are the Debye-Hückel coefficients (Helgeson and Kirkham, 1974). The b parameter depends on the main electrolyte in solution; the b values for NaClO₄ solutions derived by Migdisov and Williams-Jones (2007) were used for Timofeev et al. (2017)'s measurements, and those for NaCl solutions derived by Oelkers and Helgeson (1990) for Zaraisky et al. (2010)'s measurements. The value of the closest approach, aᵢ, was set as 9 Å for H⁺ and OH⁻ (Kielland, 1937); 3.5 Å for F⁻ (Kielland, 1937), Cl⁻, NaCl(aq) and NaF(aq); 4 Å for Na⁺ (Garrels and Christ, 1965), HF₂ and NaOH(aq); 4.5 Å for ClO₄⁻ and TaF₃(OH)₃⁻ (Timofeev et al., 2017) and for HCl(aq), Ta(OH)₅(aq), TaF(OH)₅⁻ (Akinfiev et al., 2020), TaF₅(aq), TaF₇⁻, TaF₈⁻ and TaF₉⁻.

Extrapolations of the thermodynamic properties for new and refined complexes were conducted either using the Helgeson-Kirkham-Flowers (HKF) model (Tanger and Helgeson, 1988; Sverjensky et al., 1997), or the Modified Ryzhenko-Bryzgalin (MRB) model (Ryzhenko et al., 1985;
<figure><img src="image_8.png" /><figcaption>Fig. 4. Tantalum L₃-edge XANES spectra for Sol-05 (TaCl₅ in 6.006 m Cl) and Sol-09 (Ta₂O₅ pellet in 1.0 m NaOH); (a) and (c) are normalized spectra, (b) and (d) first derivative.</figcaption></figure>

Borisov and Shvarov, 1992; Shvarov, 2015). The HKF model is the most widely used for modelling hydrothermal geological fluids. We used OptimB (Shvarov, 2015) as well as a customised Matlab code to fit HKF parameters. This custom code permits all the HKF parameters (including derived parameters such as partial molar volume and heat capacity) to be constrained within meaningful ranges, and also implements the equation of states and correlations introduced in the DEW model (Huang and Sverjensky, 2019).

The empirical MRB model was used for species with limited information. MRB fits were conducted using OptimC (Shvarov, 2015). MRB uses correlations with the dissociation constant of water ($K_w(P, T)$) to model the P-T dependence of the dissociation constant ($pK = -\log K$) for aqueous complexes as follows:

$$pK(P, T) = 298.15/T \cdot pK_{(298K, 1bar)} + f(P, T) \cdot (A + B/T) \quad (8)$$

where $pK_{(298 K, 1 bar)}$ is the stability constant of the dissociation reaction at 25 °C, 1 bar; $T$ is the temperature (K), $P$ is the pressure (bar), and $A$ and $B$ are empirical parameters. The $f(P, T)$ is a species-dependent function:

$$f(P, T) = 1/1.0107(\log K_w(P, T) - T/298.15K^{\log K_w(298.15K, 1bar)}) \quad (9)$$

The geochemical thermodynamic modelling at hydrothermal conditions was executed utilising the HCh package (Shvarov and Bastrakov, 1999). The species included in the model are listed in the Supplement Material (Supp. Mat. 1-Table S1). The distribution of species at room temperature based on Filella and May (2019)'s data (Fig. 1) was calculated using Geochemist's Workbench (Bethke, 2022).

# 3. Results

## 3.1. XANES analysis

The XANES spectra for the Ta-HF-H₂O system, Sol-03 (2.868 m F, Fig. 3a, b) and Sol-04 (1.124 m F, Fig. 3c, d), have two to three peaks that occur within two main regions labelled as Band 1 and Band 2 (highlighted on Fig. 3). The occurrence of two main bands is consistent with the XAS data of Wilson et al. (2016) for Ta(V) in a 27 M (~23.5 m) HF solution, and can be attributed to the splitting of the d-states in the 2p-to-5d electron excitation (Bare et al., 1993). Two states are expected for an octahedral coordination (Wilson et al., 2016), and the presence of a third peak indicates a lower symmetry. The first peak in the HERFD data (Peak-1; Fig. 3) shifts slightly with temperature in Sol-03, which contains the higher F concentration but remains at constant energy for Sol-04, with the lower fluorine concentration. Peaks-2 and -3 together form Band 2, and their relative intensities systematically decrease and increase, respectively, with increasing temperature (Fig. 3). This is highlighted in the first derivative of the XANES (Fig. 3b, d).

The XANES spectra of Sol-05 (TaCl₅ in 6.006 m HCl; Fig. 4a, b) and Sol-09 (Ta₂O₅(s) pellet in a 1.0 m NaOH; Fig. 4c, d) are similar but differ from those of F-bearing solutions. Ta₂O₅(s) is only sparingly soluble in Cl- and F-free alkaline solutions, resulting in noisier spectra. As for the Ta-HF-H₂O system, the XANES spectra display two bands (Fig. 4), but Peak-1 (Band 1) has a higher intensity than the peaks in Band 2 (Fig. 4a, c). In the first-derivative spectrum, it is apparent that Band 2 is comprised of two peaks (Peak-2d and -3d, Fig. 4b, d). Changes in the relative intensity of Peak-2d and -3d with increasing temperature
<figure><img src="image_9.png" /><figcaption>Fig. 5. Tantalum L₃-edge EXAFS spectra in k-sapce for Sol-03 (TaF₅ in 2.868 m F) and Sol-05 (TaCl₅ in 6.006 m Cl). The phase change of the high fluorine system Sol-03 with the increase of temperature, especially the transition from room temperature to 88 °C to higher temperatures, symbolises the change of complex structure and coordination number in the system. The phase of the fluorine-free system Sol-05 did not change significantly.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 6. Tantalum L₃-edge XANES spectra for Sol-02 (TaCl₅ in 1.009 m F) and Sol-08 (TaF₅ in 5.971 m Cl); (a) and (c) are normalised spectra, (b) and (d) first derivative.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Fourier transforms (no phase correction) of the measured Ta L<sub>3</sub>-edge EXAFS spectra (solid lines) with fits (dashed lines) for (a) Sol-03 (TaF<sub>5</sub> in 2.868 *m* F), (b) Sol-04 (TaF<sub>5</sub> in 1.124 *m* F), and (c) Sol-02 (TaCl<sub>5</sub> in 1.009 *m* F).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Fourier transforms (no phase correction) of the measured Ta L<sub>3</sub>-edge EXAFS spectra (solid lines) with fits (dashed lines) for (a) Sol-05 (TaCl<sub>5</sub> in 6.006 *m* HCl), and (b) Sol-08 (TaF<sub>5</sub> in 5.971 *m* HCl).</figcaption></figure>

correlate with a slight shift of Peak-2 towards lower energy levels.

The differences observed in the XANES spectra between fluorine-
bearing and fluorine-free solutions suggest variations in the ligands
and structure of the Ta-complexes. This is corroborated by comparing

the *k*-space EXAFS data of Sol-03 and Sol-05 in (Fig. 5). It should be
noted that the similarity in the spectra from ambient to 413 °C for Sol-05
indicates that there is little change of in the nature and geometry of the
Ta(V) complex in the HCl-solution, while small systematic changes
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Ab initio calculations of the Ta L₃-edge XANES spectra of the selected Ta fluoride and oxygen complexes using the FDMNES code. The black spectral lines are the experimental data collected from this study; the coloured spectral lines are represented as convolution lines with energy truncations of the Fermi level (2 eV); the light-coloured spectral lines are represented as convolution lines without energy truncations of Fermi level. **TTP:** Tricapped trigonal prism; **SA:** Square antiprism; **CO:** Capped octahedral; **TF:** Triangular frustum; **TA:** Triangular antiprism.</figcaption></figure>

affect the Ta(V) complex in the HF-rich solution between 25–164 °C, in line with the XANES data (Fig. 3a, b).

Mixed Ta-F-Cl solutions were also measured to test the potential competition between the fluorine and chloride ligands. In these solutions, variations in XANES spectra are intricately tied to the relative fluoride and chloride concentrations. The XANES spectra of Sol-02 (1.0 m F + 0.1 m Cl, Fig. 6a, b) show shifts in peak positions and intensities remarkably similar to those observed for Ta(V) in HF-solutions (Fig. 3). In contrast, the Ta(V) XANES spectra of a solution with lower F concentrations and higher Cl:F molar ratio (Sol-08; 0.13 m F + 6.0 m Cl, Fig. 6c, d) are analogous to those of the F-free HCl solution (Sol-05, Fig. 4a, b).

## 3.2. EXAFS analysis

The EXAFS spectra predominantly reflect changes related to the atomic number, quantity, and bond distance of the neighbouring atoms. The EXAFS data and corresponding fits are depicted in Figs. 7 and 8, with the final fitting results detailed in Table 3. Due to correlations among fitting parameters and the similar X-ray cross-sections of O and F atoms (Kelly et al. 2008; Xing et al. 2019), external constraints are required to obtain fits that are internally consistent. Several approaches were tested:

1. Single $\Delta E_0$ with Debye-Waller factor restraint: each solution set was fitted using one $\Delta E_0$, while the EXAFS Debye-Waller factor was restrained to increase twofold from ambient temperature to 413 °C. This method yielded anomalously high ligand numbers, in some instances exceeding 12.

2. Ligand number restraints: for solutions with high fluoride concentrations (Sol-03 and Sol-04), we compared the results of fits where the number of ligands was fixed at values between 6 and 10. The relative quality of each fit was evaluated by comparing reduced $\chi^2$ values and by visual estimation of the fits in R-space (Supp. Mat. 1-

Fig. S1). This analysis indicated that configurations with more than 10 ligands were excessive, while those with fewer than 8 ligands produced inferior fits. Models incorporating mixed ligands did not align well with the experimental data, as evidenced by a negative EXAFS Debye-Waller factor, suggesting insufficient electrons in the fitting system.

3. Unconstrained Debye-Waller factors and constrained ligand numbers: Debye-Waller factors were unconstrained, and the ligand number was restrained between 7 and 9 (ambient to 88 °C) and 5 to 7 (164 to 413 °C) for F-rich systems, and between 4 and 6 for the oxychloride dominated systems (sol-05, sol-08). The constraint of the F ligand number is reasonable based on the previous room temperature studies showing ligand numbers between 5 and 9 (Agulyansky, 2004b; Wilson et al., 2016). This approach provides results that vary in a systematic manner with temperature and solution compositions and are consistent with previous studies, and hence was used to provide the fit results listed in Table 3.

In the Ta-F systems, both Sol-03 (Fig. 7a) and Sol-04 (Fig. 7b) were fitted with only F-ligands. Attempts to include O atoms within the first shell yielded poor fits and large errors. The Ta-F bond lengths decreased with increasing temperature, as the number of F atoms decreased from nine to seven.

For the F-free, high-Cl solution (Sol-05, Fig. 8a), our fits (Table 3) support the preference for Ta-O bonds over Ta-Cl bonds. We found no significant Ta-Cl complexes even when the HCl concentration was 6.0 m. Ta(V)-Cl bonds are expected to be significantly longer (2.3–2.5 Å in ionic liquids, Roeper et al., 2010; 2.28–2.57 Å in TaCl₅(s), Rabe and Müller, 2000) than Ta(V)-O bonds (fitted distance of 1.81(1), compared to Ta (V)-O distances of 1.85–2.19 Å in Ta₂O₅(s) for example, Aleshina and Loginova, 2002). This interpretation aligns with the predictions of by Zaraisky et al. (2010) and Akinfiev et al. (2020). Sol-5 contains the [TaO₆] moiety at room temperature; with increasing temperature, the (O, OH, OH₂) ligand number decreases from 6 to 5 (Table 3).
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Solubility of crystalline Ta₂O₅(s) from 100 to 250 °C and water vapour-saturated pressure. (a, c, e, g) Comparison of predicted solubility of Ta₂O₅(s) in acidic HClO₄-HF-NaF solutions with the experimental data of Timofeev et al. (2017). Multiple points at similar x-axis positions reflect experiments at different pH; the initial pH₂₅-c are indicated by the colour of the dot. (b, d, f, h) Calculated solubility of Ta₂O₅(s) in HF solutions, using optimised values listed in Supp. Mat. 3. Inset plots are calculated theoretical average number of F coordinated to Ta and corresponding pH values. Round points and error bars correspond to our XAS data (Table 3).</figcaption></figure>

The EXAFS and XANES results of the mixed ligand solution with high-F and low-Cl concentrations (Sol-02, Fig. 7c) are similar to those of the HF-only system (Sol-03, Fig. 7a) with only Ta-F bonds detected through fitting (substantiated by the bond lengths), and the numbers of F ligands decreasing with increasing temperature. In contrast, Sol-08 (Fig. 8b) with high chlorine and low fluorine contents is more complex; Ta-F bonds are observed at 25 and 88 °C despite the low F concentration (0.13 m F vs 6.0 m Cl). While the XANES spectra are similar to the fluorine-free system, fitting the EXAFS data shows the presence of a [TaF₃O₃] moiety at 25 °C and 88 °C. At higher temperatures, as the

ligand number reduces to 5, oxygen becomes the dominant coordinating element, the solution being predominated by the [TaO₅] moiety.

### 3.3. Exploration of the geometry of Ta(V) complexes with ab initio XANES simulations

Observed differences in the XANES spectra for different samples, e. g., shifts in the positions and intensities of peaks, may be attributed to subtle modifications in the geometry of (predominantly) the first coordination shell of the aqueous complex. To explore this, we simulated the
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Solubility of Ta₂O₅(s) from 300 to 550 °C at 1000 bar: Comparison of predicted solubility of Ta₂O₅(s) in HF-NaF solutions as a function of pH with the experimental data of Zaraisky et al. (2010) (a, b, d, e, g, h, j, k). Calculated solubility of Ta₂O₅(s) in HF solutions (c, f, i, l), using optimised values listed in Supp. Mat. 3.</figcaption></figure>

XANES spectra for various geometries and ligands using *ab initio* XANES simulations. Atoms beyond the first coordination shell can be disregarded as they are expected to be strongly disordered, and this approach has been shown to provide reliable information for other small inorganic complexes (e.g., Borg et al., 2012; Etschmann et al., 2010; Testemale et al., 2005). Several structures were systematically investigated to attempt to correlate XANES features with specific geometries for the complexes.

The results of our FDMNES calculations are summarised in Fig. 9. Initially, for the nonafluoridotantalate tetraanion [TaF₃⁻], we employed a tricapped trigonal prism (TTP) structure. By symmetrically varying the polar angles (θ) between the triangular prism's upper and lower atoms, both bimodal positions and relative intensities of theoretical spectral lines shifted (Supp. Mat. 1-Fig. S2a). A polar angle of ~57 ° provides the best match between calculated and experimental XANES (Fig. 9a).

For the 8-coordinated configuration [TaF₃⁻], we referred to the results of Anderson et al. (2019) and designed the structure as a square antiprism (SA). We find that the calculated spectra are highly sensitive to changes in azimuthal angles (Supp. Mat. 1-Fig. S2b) and are closest to the experimental results when the atoms above and below the axis are

aligned (φ = 0°). This configuration accurately reproduced the separations and relative intensities of the three distinct peaks that contribute to the white line (Fig. 3). Note that only two bands are produced for the simulation of the nonafluoridotantalate tetraanion, suggesting that Sol-03 at 25 °C may be a mixture of TaF₃⁻ and TaF₃⁺.

The spectral manifestations of 7-coordinated fluoridotantalate complexes are more diverse. We compare three potential structures: pentagonal bipyramid (PB), augmented triangular prism (ATP) and ATP's variant as a capped octahedral (CO). The calculated convoluted spectra for PB and ATP structure form a single peak over a range in bond lengths and angles, rather than the two distinct peaks observed in the experiments. Only the CO structure approximates the experimental spectra, with a distorted configuration that approaches a triangular pyramid combined with a quad pyramid with Ta at the apex (Fig. 9a; Supp. Mat. 1-Table S4 & Fig. S3c). Note that the relative strength and spacing of the twin peaks also depend on the polar angles.

Based on the EXAFS data (Table 3), Sol-05 contains the [TaO₆] moiety, which corresponds to the Ta(OH)₄⁺ complex in this highly acidic solution (6 m HCl; Filella and May 2019). We find that an octahedral configuration does not fit the experimental spectra, and the six ligands
<table><caption>**Table 4**

HKF and MRB parameters for Ta species adopted in the study for the modelling of Ta in hydrothermal systems.</caption><thead><tr><th>Species</th><th>ΔfG°<sub>298</sub></th><th>ΔfH°<sub>298</sub></th><th>S°<sub>298</sub></th><th>C°<sub>p, 298</sub></th><th>V°<sub>298</sub></th><th>α<sub>1</sub> 10</th><th>α<sub>2</sub> 10<sup>-2</sup></th><th>α<sub>3</sub></th><th>α<sub>4</sub> 10<sup>-4</sup></th><th>c<sub>1</sub></th><th>c<sub>2</sub> 10<sup>-4</sup></th><th>ω 10<sup>-5</sup></th><th>Reference</th></tr></thead><tbody><tr><td> TaO<sub>2</sub><sup>+</sup></td><td>-186385</td><td>-210781</td><td>-38.48</td><td>127.380</td><td>70.180</td><td>11.7607</td><td>20.9342</td><td>-2.4779</td><td>-3.6444</td><td>90.0908</td><td>22.9127</td><td>1.1263</td><td>Akinfiev et al. (2020)</td></tr><tr><td> HTaO<sub>5</sub>(aq)</td><td>-242833</td><td>-275401</td><td>-10.15</td><td>90.396</td><td>91.282</td><td>14.0782</td><td>28.7832</td><td>-7.9346</td><td>-3.9689</td><td>58.6881</td><td>15.7442</td><td>0.0347</td><td>Akinfiev et al. (2020)</td></tr><tr><td> TaO<sub>5</sub></td><td>-232325</td><td>-266837</td><td>-13.62</td><td>-46.481</td><td>68.840</td><td>11.8213</td><td>21.0822</td><td>-2.5361</td><td>-3.6505</td><td>-6.0342</td><td>-12.5026</td><td>1.8272</td><td>Akinfiev et al. (2020)</td></tr><tr><td> HTaO<sub>3</sub>F<sup>-</sup></td><td>-296835</td><td>-300079</td><td>128.05</td><td>-148.17</td><td>86.400</td><td>14.0434</td><td>25.5077</td><td>-4.6685</td><td>-3.8748</td><td>-69.9033</td><td>-33.2161</td><td>1.3079</td><td>Akinfiev et al. (2020)</td></tr><tr><td> Ta(OH)<sub>5</sub>F<sub>3</sub></td><td>-464794</td><td>-</td><td>49.87</td><td>102.537</td><td>60.003</td><td>8.5170</td><td>72.6759</td><td>-126.1730</td><td>-6.8500</td><td>94.3167</td><td>6.63230</td><td>0.5706</td><td>This study</td></tr><tr><td> TaF<sub>2</sub><sup>+</sup></td><td>-585453</td><td>-</td><td>46.13</td><td>-57.1404</td><td>60.309</td><td>4.4045</td><td>-109.9730</td><td>371.9720</td><td>0.8805</td><td>268.3180</td><td>-150.1730</td><td>3.3215</td><td>This study</td></tr><tr><td> TaF<sub>3</sub><sup>+</sup></td><td>pK(298 K)</td><td>A</td><td>-3.294</td><td>B</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td> TaF<sub>4</sub><sup>+</sup></td><td>7.852</td><td></td><td>-7.818</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

are more inclined towards an antiprismatic structure (Fig. 9b). The relative positions of the peaks gradually converge as the polar angles θ increase; at an angle of ~48° the calculated spectrum approximates the experimental spectra for Sol-05 at 25 °C (Fig. 9b; a large set of configurations is shown in Supp. Mat. 1-Fig. S4b). In the case of the mixed ligand complex, according to the speciation model derived in this study (section 5.2 below), TaF<sub>3</sub>(OH)<sub>3</sub>⁻ is the predominant species at room temperature, replaced by Ta(OH)<sub>4</sub>⁻ at 319 °C. The positions and relative intensities of the two bands in the XANES spectrum of Sol-08 are well reproduced by triangular frustum (TF) structures (Fig. 9b).

## 4. Discussion

### 4.1. Nature of Ta(V) complexes in F- and Cl-rich waters

The primary aim of this study was to resolve the discrepancy in our understanding of the aqueous speciation of Ta(V) in F- and Cl-bearing geofluids:

* Room-temperature studies highlight the importance of high-order fluoride complexes, which become predominant at F-concentrations as low as 0.01 *m* in acidic fluids according to the thermodynamic properties selected by Filella and May (2019) (Fig. 1). The precise nature of the [TaF<sub>n</sub><sup>5-n</sup>] complexes remains poorly constrained (n ≤ 9).

* At hydrothermal conditions (≥ 100 °C), Timofeev et al. (2017) found no evidence for fluoridotantalate complexes with more than five coordinated F ligands in solutions with up to ~0.8 *m* F, and suggested that Ta(OH)<sub>5</sub>(aq), TaF<sub>3</sub>(OH)<sub>3</sub> and minor TaF<sub>5</sub>(aq) are the main complexes in natural H<sub>2</sub>O-Ta-HF-NaF fluids. Compared to Timofeev et al. (2017), Akinfiev et al. (2020) replaced Ta(OH)<sub>3</sub>F<sub>3</sub> with the neutral Ta(OH)<sub>2</sub>F<sub>3</sub>(aq) complex; noticed that TaF<sub>5</sub>(aq) was not required to fit Timofeev et al. (2017)'s results; and added extrapolations for the hydroxy-complexes Ta(OH)<sub>4</sub>⁺ and Ta(OH)<sub>5</sub>(aq). These models are in stark contrast with Anderson et al. (2019)'s EXAFS study, which determined that Ta was coordinated to eight fluorine ligands at 350 °C and 400 °C.

In our study, Sol-02, Sol-03 and Sol-04, which contain ≥ 1 *m* HF show similar XANES features at room temperature and were best fitted with 9 fluoride ligands. The fitted Ta-F bond lengths for these solutions are around 1.93(1) Å, similar to those obtained by Wilson et al. (2016) (1.907(8) to 1.950(5) Å) for HF-rich solutions (1–27 M), but Wilson et al. (2016) obtained coordination numbers of ~7. Varga and Freund (1962) introduced the 9-fluoride ligand complex, though this interpretation was subsequently debated by Bukhsh et al. (1966), who favoured a maximum 8-fold coordination.

Both our XANES and EXAFS data of F-bearing solutions demonstrate differences between room temperature and elevated temperatures, indicating that the Ta(V)-F speciation changes with increasing temperature between 25 °C and 164 °C. We interpret this change as a decrease in coordination number from 9 to 7, based on EXAFS fits as well as the interpretation of the XANES spectra. Our results are consistent, within error, with those of Anderson et al. (2019), who determined that Ta was coordinated to 7.9 ± 1.1 F at 350 °C and 8.3 ± 0.9 F at 400 °C in a 2.631 *m* HF solution, with bond lengths of 1.91(2) Å.

Our results demonstrate the strong preference of Ta(V) to form complexes with F rather than Cl. Sol-05, with ~6 *m* HCl, is similar to the Cl- and F-free solution dominated by Ta(V)-oxy-hydroxy-complexes (Fig. 4), and no evidence for chloride complexing was found by EXAFS fitting (Supp. Mat. 1-Fig. S2). Anderson et al. (2019) suggest that [TaCl<sub>6</sub>] predominates at 25 °C in a solution containing 2 wt% HF with dissolved TaCl<sub>5</sub>(s). This result is anomalous in light of the lack of consensus for the existence of Ta(V) chloride complexes at room temperature (Filella and May 2019), and the fact that the refined Cl:Ta ratio is higher in the complex (6.2 ± 0.6:1) than in the bulk solution (Cl:Ta =

<footnote>†Short-hand ‘anhydrous’ notation for hydroxy species (Ta(OH)<sub>5</sub>+, Ta(OH)<sub>5</sub>(aq), Ta(OH)<sub>6</sub>-, and Ta(OH)<sub>5</sub>F-).</footnote>
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Solubility of Ta₂O₅(s) as a function of NaF/HF molar ratio and total F concentration and predominant Ta and F aqueous species in the Ta₂O₅(s) + Quartz system at 500 °C, 1000 bar.</figcaption></figure>

5). The prevalence of Ta-F complexation can be explained by the Hard and Soft Acids Bases Theory (Pearson, 1963). Highly charged Ta⁵⁺ is a typical hard acid that complexes with a hard base such as F⁻ rather than the borderline Cl⁻.

## 4.2. Extrapolation of thermodynamic properties for Ta-F complexes to elevated temperatures

Quantitative knowledge about Ta transport in F-rich geological fluids is derived from solubility experiments (Akinfiev et al., 2020; Timofeev et al., 2017; Zaraisky et al., 2010). These experiments provide a direct measurement of mineral solubility, but their thermodynamic interpretation depends upon the choice of speciation model, and in many cases different models can represent the solubility data equally well (e.g., Mei et al., 2015). This issue arises from the fact that solubility datasets are often under-determined. Indeed this difficulty was illustrated in the case of the Ta-HF-NaF-H₂O system by the studies of Timofeev et al. (2017) and Akinfiev et al. (2020), who interpret the same Ta₂O₅(s) solubility dataset with different main species (Ta(OH)₅(aq), Ta (OH)₃F₃ and TaF₅(aq) versus Ta(OH)₅(aq) and Ta(OH)₂F₃(aq)). Selecting an inaccurate model will not affect the prediction of mineral solubility near the experimental conditions, since both studies reproduce the experimental data with a similar degree of accuracy, but may lead to inaccurate extrapolations beyond the range in solution compositions and temperatures of the experiments (e.g., Kalintsev et al., 2023).

All studies of the thermodynamics of Ta(V) in hydrothermal fluorine-bearing solutions underestimated the stability of high order Ta(V)-fluoridocomplexes (Zaraisky et al., 2010; Timofeev et al., 2017; Akinfiev et al., 2020). Hence, we attempted to fit the existing solubility data with a new model that is consistent both with the XAS results and the solubility data. We used Akinfiev et al. (2020)'s model as a basis; this model was extended by including TaF₅⁻ⁿ species with n = 5, 7, 8, 9. Similarly to Akinfiev et al. (2020), we find that TaF₅(aq) is not required to fit the current solubility data. We illustrate this by including TaF₅(aq) in the solubility fits shown in Fig. 10; these fits show that TaF₅(aq) is not a predominant complex under any experimental conditions, as model where it contributes more than ~10 % of total dissolved Ta overestimate total Ta₂O₅(s) solubility. From this analysis, the maximum equilibrium constants (logK) for the association reaction Ta(OH)₅(aq) + 5HF(aq) = TaF₅(aq) + 5H₂O are ~4 (100 °C), ~3.5 (150 °C), ~3 (200 °C) and ~2.5 (250 °C). No HKF parameters were derived given the lack of direct experimental evidence for the Ta(OH)₅(aq) complex. In addition, Akinfiev et al. (2020) used the neutral Ta(OH)₂F₃(aq) complex instead of the TaF₃(OH)₃⁻ selected by Timofeev et al. (2017). In our fits, TaF₃(OH)₃⁻ provided better solutions for Timofeev et al. (2017)'s pH

dependent data points, and hence this species was favoured over Ta (OH)₂F₃(aq).

The results of the final solubility fitting are illustrated in Figs. 10 and 11. The recommended species include Ta(OH)₄⁻, Ta(OH)₅(aq), Ta(OH)₆, Ta(OH)₅F⁻, Ta(OH)₃F₃, TaF₇⁻, TaF₈⁻, and TaF₉⁻, with updated or new properties provided by this study for the last four complexes. HKF equation of state parameters recommended for modelling Ta(V) solubility in hydrothermal fluids are presented in Table 4, and the formation constants for TaF₃(OH)₃⁻ in this study are compared to those of Timofeev et al. (2017) in Supp. Mat. 1-Table S5.

Our fit results show that, similar to Timofeev et al. (2017)'s results (100–250 °C), Ta(OH)₅(aq) predominates in low HF (< 0.01 m) solutions, and TaF₃(OH)₃ becomes predominant with increasing fluorine concentrations up to ~0.1 m. However, high order fluoride complexes (TaF₈⁻ and TaF₉⁻ at 100 °C, TaF₇⁻ at 150–250 °C) become predominant at HF concentrations >~1 m, in line with the new XAS results: the calculated ligand numbers obtained at different fluorine concentrations maintain a good agreement with the EXAFS results (insets in Fig. 10b, d, f, h).

When fitting the solubility data of Zaraisky et al. (2010) at higher temperatures (Fig. 11), we encountered challenges due to the scarcity of experimental data. Our findings indicate that at elevated temperatures, Ta fluoride complexes, in particular TaF₇⁻, exist in F-rich (> 1 m HF) aqueous solutions (consistent with Anderson et al., 2019), whereas mixed fluoride-hydroxy complex, such as TaF(OH)₅⁻ and TaF₃(OH)₃⁻, exist at intermediate fluoride concentrations.

Overall, the XAS results show that Ta-F complexes predominate in high fluorine environments. The XANES spectra of F-rich solutions (Fig. 3) show little variation above ~200 °C; this suggests that there is little change in the geometry and composition of the complex at these conditions. This consistency is also reflected in the thermodynamic calculations, i.e., the decrease of TaF₄⁻ at 88 °C and the subsequent dominance of TaF₇⁻ over a wide range of temperatures. The new model resolves discrepancies between room-temperature studies that highlight the stability of high order fluoridocomplexes, and hydrothermal studies that favoured lower charge, lower order complexes even at relatively low temperatures (< 300 °C).

## 4.3. Tantalum behaviour in hydrothermal systems

As summarised by Zaraisky et al. (2010), the fractionation of Li–F granitic magmas leads to the accumulation of flux elements such as F and Li, and metal cations such as Ta and Nb in residual melts, ultimately resulting in the crystallisation of tantalite and columbite. In shallow intrusions, volatile-rich fluorinated fluids may exsolve from these melts,
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Solubility of Ta₂O₅(s) as a function of F concentration, NaF/HF molar ratio, and temperature (°C). (a–d) are fluid dominated systems (1.0 and 0.1 *m* total F), (e, f) a rock-buffered (quartz-topaz-paragonite-albite, Qtz-Tpz-Pg-Ab) system representative of highly evolved granitic environments. Pressure is fixed at 1000 bar. (a, c, e) display Ta concentrations and predominant Ta(V) aqueous species; (b, d, f) show pH and predominant F species in solution. Total amounts of dissolved F (log ppm) are also shown as isolines in (f).</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Results of simulation of the titration of calcite, CaCO₃(s), into 1 kg of solution containing 1 m F and saturated with Ta₂O₅(s) as a function of NaF/HF molar ratio. The predominant Ta and F aqueous species are highlighted with black solid line, and changes in mineral associations are shown with red solid lines.</figcaption></figure>

interact with the magmatic mineralisation, and possibly contribute to upgrading ore grades or forming secondary hydrothermal mineralisation. Kaeter et al. (2018) proposed that fluids rich in volatiles can create reaction channels as a result of hydraulic fracturing and F-catalysis. We simulated the behaviour of Ta in silicate systems at various pH and temperatures (Figs. 12, 13). We also provide a simulation of Ta solubility in the presence of calcite (Fig. 14), since strong fractionation of Ta and Nb is observed in the metasomatic alteration halos associated with carbonatites (Bagdasarov, 2009).

In fluid-dominated systems, the mobility of Ta is primarily influenced by solution composition and temperature. Specifically, Ta concentrations increase with total fluoride concentrations (Fig. 12). With respect to temperature, the solubility of Ta₂O₅(s) is prograde over most of the temperature range, except at low temperatures (≤ 100 °C) due to the rapid decrease in stability of the highly charged TaF₃⁻ and TaF₃²⁻ complexes. In general, Ta(V) fluoridocomplexes become increasingly stable at high temperatures (Fig. 13a-d). In a rock-buffered system (quartz-topaz-paragonite-albite), a similar prograde Ta₂O₅(s) solubility is observed (Fig. 13e, f). In such a system, representative of a highly evolved granitic system, fluid salinity plays a significant role by controlling fluid pH, but overall the predicted aqueous Ta concentrations remain high (Fig. 13e, f).

Bagdasarov (2009) note that Ta/Nb ratios increase as a result of the metasomatism associated with rare-metal-rich carbonatites. A simple calcite titration simulation (Fig. 14) (500 °C, 1000 bar, 1.0 m Ftot) shows that the interaction between acidic fluid and calcium-rich rock raises the pH. This process leads to the precipitation of fluorite, accompanied by a reduction in HF activity and subsequent deposition of Ta₂O₅(s). Consequently, the concentration of Ta in the fluid decreases rapidly, with a sharp interface between Ta-rich (> 1 ppm) and Ta-poor (< 1 ppm) fluids, indicating that reactions with Ca minerals may be an efficient precipitation mechanism for Ta (Fig. 14).

### 4.4. Contrasting behaviour of Ta in melts and fluids

Keppler (1993) initially identified that the solubility of manganotantalite (MnTa₂O₆ (s)) in water-saturated haplogranitic (HPG) melts significantly increases with higher fluorine contents, which was attributed to the formation of metal complexes by F⁻ or F-induced non-bridging oxygen growth. In contrast, Fiege et al. (2011) and Aseri et al.

(2015) discovered that fluorine appears to have minimal impact on the solubility of MnTa₂O₆ (s) in natural granite and HPG melts saturated with water. Mayanovic et al. (2013) investigated the coordination of Ta in a fluorine-free silicate melt and quenched glass through XAS, revealing that Ta is coordinated to six oxygen atoms, with a transition from regular octahedral at ambient temperature to distorted octahedral at 960 °C, 0.5 GPa. Using first-principles molecular dynamics simulations, Wang et al. (2020) found that Ta(V) coordinates with six oxygen atoms even in fluorine-rich silicate melts. They concluded that there is no direct chemical interaction between F⁻ and Ta⁵⁺ in silicate melts. Fluorine would rather influence the ore-forming processes indirectly, for example by modifying both the viscosity and chemical properties of melts, subsequently impacting the solubility and mobilisation capacity of Ta and other rare metals (Van Lichtervelde et al., 2007; Simmons and Webber, 2008).

The importance of high order fluoride complexes in F-rich fluids at high temperatures contrasts with the octahedral Ta(V)-O coordination in silicate melts (Wang et al., 2020). Hence, where volatile-rich fluids exsolve from the melt, Ta is expected to partition strongly into that fluid, possibly contributing to secondary Ta migration.

## 5. Conclusion

Economic Ta deposits are mostly associated with lithium-columbite-tantalite-type granitic pegmatites (Simandl, 2001; Zhu et al. 2024). Fractional crystallisation produces continuous enrichment of volatiles such as F as well as high field strength elements; for example, Ta concentrations increase from 24 ppm in the earliest magmatic stage to 137 ppm in the last (fifth) stage in the Tanco pegmatites (Huang et al., 2002). Ta-Nb mineralisation occurs both as magmatic minerals and in association with hydrous metasomatisms such as albitisation and greisen (Linnen et al., 2012; Diao et al., 2024). Wright (1963) observed that Li-rich muscovite and lepidolite masses in the Tanco pegmatite underwent alteration due to a Li- and F-rich aqueous fluid. An increase in the alkalinity of residual fluid can lead to the crystallisation of pyrochlore during post-magmatic sodic metasomatism (Ogunleye et al., 2006). Volatile-rich fluids released during the late stages of the fractional crystallisation of highly evolved granitic melts can create reaction pathways via hydraulic fracturing and alteration, as fluoride increases reaction rates among silicate minerals (Duan et al. 2021, 2022), and can
lead to further Ta enrichment via resorption of earlier crystallised minerals (e.g., Van Lichtervelde et al., 2009), eventually leading to local supersaturation and Ta mineral precipitation (Kaeter et al., 2018).

Overall, despite the strong potential for the high mobility of Ta in F-rich hydrothermal fluids, the evidence for hydrothermal Ta mineralisation/remobilisation remains limited and often controversial. This may be due to unusual conditions for the formation of significant volumes of aqueous fluids from pegmatites, as opposed to solute-rich supercritical hydrous melts. This is compounded by the fact that fluorine can strongly partition into the melt (together with Ta) during melt-fluid equilibration, as demonstrated by the experiments of Borodulin et al. (2009).

Fluids associated with Ta mobility may contain high levels of chloride (e.g., 41–46 wt% NaCl<sub>equiv</sub>. in beryl–columbite–phosphate pegmatites, 23–38 wt% NaCl<sub>equiv</sub>. in albite pegmatites, Alfonso and Melgarejo, 2008). Our findings contradict the notion that chloride complexes are important for Ta(V) mobility. In a chlorine system (Sol-05), Ta XAS data exhibit characteristics solely associated with –O(H) coordination, while the Ta-F coordination will gradually dominate (from Sol-08 to Sol-02) if the relative F content in the system is increased. Considering the fluorine concentrations in most crustal fluids (0.001–0.26 molal; Xing et al., 2019) and the extremely high HF concentrations (up to ~1 molal) estimated using mica geoluminometers in fluids of Ta–Nb deposits associated with Li–F granites in Eastern Transbaikalia (Aksyuk, 2002), Ta(V) fluoride complexes are expected to play a significant role in Ta-(re) mobilisation under hydrothermal conditions in some F-saturated environments. Other volatiles and chlorine may work synergistically to facilitate metasomatic processes under these conditions (Duan et al., 2022, 2023).

### CRediT authorship contribution statement

**Bin Hu:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis. **Barbara Etschmann:** Writing – review & editing, Investigation, Formal analysis, Data curation. **Denis Testemale:** Methodology, Investigation. **Weihua Liu:** Writing – review & editing, Conceptualization. **Qiushi Guan:** Investigation. **Harald Müller:** Resources. **Joël Brugger:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Investigation, Formal analysis, Conceptualization.

### Data availability

Data are available through Mendeley Data at https://doi.org/10.17632/76gftb2grg.2.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

Research funding was provided by the Australian Research Council grant DP190100216 to JB. We acknowledge travel funding provided by the International Synchrotron Access Program (ISAP), which is managed by the Australian Synchrotron, part of ANSTO, and funded by the Australian Government. We are grateful to two anonymous reviewers for helping improve this paper, and Associate Editor Jean-François Boily for handling this submission and helping to put together this special issue. This work was both inspired and enabled by Terry Seward's pioneering work in hydrothermal geochemistry. Visiting Terry's laboratory in Zurich as young post-doctoral researcher was inspirational to JB, and certainly contributed to extending his horizons from mineralogy to experimental geochemistry. Who doesn't want to play with cm-sized synthetic optical grade diamonds? Terry popularised the physical

chemistry approach to hydrothermal geochemistry applied in this paper, and pioneered the use of XAS spectroscopy to study metal complexation at hydrothermal conditions – heroic times indeed. Terry never lost his fascination for weird and exotic natural geochemical systems – a planet where minerals such as vrbaite form is indeed a wonderful one. Terry showed that studying such extreme environments can reveal crucial new knowledge about our planet.

## Appendix A. Supplementary material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2024.10.019.

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