Partition coefficients for Cl between felsic melts and a supercritical aqueous fluid (similar to 4-16 wt.% NaCleq) were experimentally determined to better constrain Cl behavior during magmatic fluid exsolution in upper-crustal magma chambers. Experiments were conducted at 850 degrees C, 200 MPa, and oxygen fugacity near NNO + 0.5, using a range of melt and fluid compositions. At constant total chlorinity of 1 mol/kg H2O, Dfluid melt Cl values range from 11.3 to 21.1, negatively correlated with both the melt's aluminum saturation index (ASI) and the HCl/total Cl ratio in the fluid. For a fixed melt composition (ASI = 1.02), Dfluid melt Cl Rayleigh fractionation modeling of fluid exsolution from upper-crustal magmas using these data indicates that during progressive crystallization, chlorinity of exsolved fluids rapidly decline before stabilizing at similar to 1 mol/kg H2O (similar to 4 wt.% NaCleq), regardless of initial fluid chlorinity or H2O content in melt. This implies that the majority of exsolution fluids released from felsic magmas in the upper crust are of low salinity (similar to 1 mol/kg H2O). Copper transfer modeling further suggests that efficient metal extraction occurs in Cl-and H2O-rich magmas, particularly where early H2O saturation is achieved, thus favoring the formation of high-grade porphyry copper deposits. (c) 2025 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). values increase linearly from 18.7 to 60.1 as total chlorinity rises from 1 to 4 mol/kg H2O.

Carbonatite-associated rare earth element (REE) deposits are currently the primary source of REE resources. Their formation requires REE enrichment during prolonged magma evolution, achieved by suppressing REE-rich mineral crystallization and promoting REE-enriched brine melt formation. Our experiments on the fractional crystallization of carbonatitic magmas indicate that pressure (emplacement depth) is the primary factor controlling REE enrichment. High-pressure ( >0.3 GPa) promotes early olivine crystallization, depleting silica and suppressing REE-rich apatite formation. Deep emplacement also delays aqueous fluid exsolution, thereby stabilizing brine melts that enhance phosphate dissolution and prevent REE dispersion into apatite. In contrast, low-pressure conditions ( <0.3 GPa) lead to exsolution of REE-poor hydrothermal fluids, dispersing REE into magmatic apatite and preventing the deposition of economically significant REE-carbonates in subsequent hydrothermal stages. Our pressure-dependent model highlights deep emplacement as crucial for passive REE enrichment in residual brine melts, driving large-scale mineralization through precipitation of burbankite and/or bastnäsite.

Hydration of the lithospheric mantle is a critical part of deep water cycling and is essential in shaping Earth's habitable environments. At low water/rock ratios, water is stored in nominally anhydrous minerals or hydrous minerals; however, at high water/rock ratios, the leaching effect of water on mantle minerals remains poorly understood. Here, we present hydrothermal experiments of the 'phlogopite + H2O' system at 500 degrees C-950 degrees C and 0.9-1.8 GPa, showing that incongruent dissolution of phlogopite in water produces corundum (Al2O3) as an insoluble residual phase at pressures above similar to 1.5 GPa. Water leaches soluble components (K, Mg and Si) from phlogopite, leaving corundum as the residue under lithospheric mantle P-T conditions. This provides a mechanistic explanation for sapphire formation in the lithospheric mantle, consistent with natural sapphire gemstones occurring as mantle xenoliths and xenocrysts within alkaline basalts. Sapphires form through hydrous leaching of phlogopite at depths greater than similar to 50 km and are subsequently transported to the surface by alkaline basaltic magmas originating from at least similar to 70 km depth. The widespread sapphire deposits associated with alkaline basalts in East China indicate extensive hydrous leaching of the lithospheric mantle, with water ultimately sourced from the subducted slab, mantle transition zone or mantle plumes.

Hydration and complexation are crucial processes for dissolving metal elements and transporting metal complexes in hydrothermal fluids. However, the impact of hydration and complexation on the transport and enrichment of metal elements, such as those involving rhodium and chloride, has not been thoroughly evaluated. Here, this study employed a hydrolysis experiment of K3RhCl6 at 200-600 degrees C and 100 MPa to determine the controlling factors and thermodynamics of Rh-Cl complexes in chloride-rich fluids. The results show that the dominant Rh-Cl complex is RhCl3-6at 200-400 degrees C, gradually converting into Rh(III)-OH-Cl complexes over 400 degrees C. The hydrolysis equilibrium constant (LnK) of RhCl3-6 at 200-400 degrees C affected by temperature (T (K)) is calculated as: lnK 49 06 7 01 48802 3896 0 T Accordingly, the DrHmH and DrSHm of the hydrolysis reaction were obtained to be 405.8 +/- 32.39 kJ mol-1 and 407.9 +/- 58.30 J mol-1 K-1, respectively. Thermodynamic parameters reveal the dependence of the stability of Rh-Cl complexes on temperature in chloride-rich fluids. For instance, the formation constants (lnb) of RhCl3-6 vary from 0.0184 +/- 0.0022 to-0.0079 +/- 0.0012 as the temperature rises from 150 to 400 degrees C. Geochemical modeling illustrates that low-temperature and acidic fluids can enhance the stability of Rh-Cl complexes, which can be dominated by Cl concentration (over 0.5 wt.%). Hydrothermal fluids with low pH and high Cl content, typically occurring in the mid-ocean ridge, promote Rh transport and subsequent enrichment in encrustations and minerals by substituting Mn and Fe for isomorphism, or in the form of alloys, forming a substantial Rh reservoir in the ocean. (c) 2026 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Fluoride-rich fluids dramatically enhance niobium (Nb) solubility; however, the existence of solubility limits and alternative Nb species during dissolution remains poorly constrained. This study systematically investigated Nb2O5 solubility and Nb speciation in 0.1 to 4.5 mol/kg HF solutions at temperatures of 150-350 degrees C under vapor-saturated pressures. Experimental results reveal that Nb2O5 solubility increases significantly with rising HF concentration from 0.1 to 2.0 mol/kg, reaching up to similar to 10,000 ppm dissolved Nb. Beyond 2.0 mol/kg, however, the dissolved Nb concentration does not rise persistently, but fluctuates around a mean value of 15,200 +/- 6300 ppm. This trend most likely results from decreased HF dissociation and enhanced solvation effects at elevated temperatures and pressures. Solubility data analysis identifies Nb(OH)(3)F-3(-) as a newly recognized dominant species, stable across a pH range of 0.4-3.2 and fluoride concentrations of 0.1-4.5 mol/kg. The formation constants (log K) for this species vary from -5.47 +/- 0.56 at 150 degrees C to -7.61 +/- 0.20 at 350 degrees C. Geochemical modeling confirms that the stability of Nb species is highly dependent on pH and, in particular, fluoride concentration, accounting for the enhanced hydrothermal mobility of Nb in acidic, high-F hydrothermal environments. These findings support the possible role of high-F hydrothermal fluids in dissolving and transporting Nb, and also demonstrate that Nb hydrothermal transport in crustal F-bearing hydrothermal fluids is mainly governed by complexes with a low number of F- ligands (<= 3).

Carbonatite is a rare but highly prospective rock that plays a significant role as a source of critical metals, notably rare earth elements (REE) and niobium (Nb). The widespread occurrence of fluorine-bearing minerals (e.g., fluorite, fluorapatite, and bastnäsite) in carbonatites indicates that their parent magmas are rich in fluorine. Nevertheless, the mechanisms by which fluorine controls REE and Nb mineralization remain unclear. Building on previous work, we systematically synthesize the regulatory role of fluorine in REE and Nb mineralization throughout the entire magmatic evolution of carbonatites. Our key findings are: (1) during partial melting, fluorine decreases the solidus of carbonated peridotite, thereby enhancing the generation and stability of carbonatitic melt. (2) In the mantle melting stage, it lowers the partition coefficients of REE and Nb between mantle minerals and melt, effectively promoting their extraction. (3) Throughout magmatic crystallization, fluorine significantly enriches REE and Nb in the residual melt. (4) During liquid immiscibility, fluorine exerts limited influence, as the partition behaviors of REE and Nb are primarily governed by structural differences between the conjugate melts. (5) In the hydrothermal stage, fluorine facilitates the mobilization, transport, and ultimate precipitation of REE- and Nb-bearing minerals, directly driving mineralization. The presented evidence implies that carbonatitic melts and fluoride-bearing minerals exert primary control over the spatial distribution and mineralization potential of REE and Nb. Future studies should focus on (1) clarifying how fluorine promotes REE and Nb enrichment in late-stage brine melts and (2) investigating its effect on the solubility and precipitation mechanisms of REE- and Nb-bearing minerals during the late stages of magma evolution.

Markedly light carbon isotope compositions (e.g., delta C-13 < -20 parts per thousand) are often regarded as a sign of biological activity, yet certain abiotic processes can also cause similar carbon isotope fractionation. However, previous studies showing abiotic C-13 depletion mostly focused on simple hydrocarbons, while generation of abiogenic C-13-depleted condensed carbonaceous matters remains poorly documented. This study investigates petrology and geochemistry of carbon species in variably serpentinized peridotite rocks from the north Qilian ophiolite, northern Tibet. It is revealed that poorly crystalline graphitic carbon, occurring as nanometer-sized veinlets included in secondary minerals, was generated by hydrogen-driven reduction of inorganic carbon during serpentinization. Moreover, the graphitic carbon probably has low delta C-13 (< -27 parts per thousand) comparable to that of biogenic carbon. The finding indicates abiogenic graphitic carbon with biogenic-like delta C-13 may be produced during water-rock interactions, highlighting concerns about reliability of carbon isotopes as a standalone biosignature. Furthermore, such graphitic carbon may contribute significantly to the carbon budget of serpentinite, which may play a critical role in deep carbon cycling during serpentinite subduction.

Serpentinization generally occurs in the presence of Cl-bearing fluids, but formation of chloride minerals and their role in serpentinization are poorly understood. Kuliginite, a hydroxychloride mineral with the ideal formula of Fe3Mg(OH)(6)Cl-2, was recently identified in serpentinized rocks from New Caledonia, and it was suggested that kuliginite may significantly influence iron distribution and hydrogen generation during serpentinization by Cl-rich fluids. In this study, we re-investigate kuliginite from New Caledonia, and reveal that the kuliginite is mostly Mg-rich and Fe-depleted compared with holotype kuliginite, in contrast to previous conclusion that the kuliginite closely resembles holotype kuliginite. Overall, Mg-rich kuliginite formed preferentially over holotype-like kuliginite in serpentinized rocks from New Caledonia, which may be ascribed to higher stability of the former than the latter. Despite some substitution of Fe by Mg, Mg-rich kuliginite still contains large amounts of iron (similar to 50 wt% FeO). Therefore, Mg-rich kuliginite can also greatly impact iron partitioning and hydrogen generation once it forms during serpentinization by Cl-rich saline fluids.

Hydrous aluminosilicates are important deep water-carriers in sediments subducting into the deep mantle. To date, it remains enigmatic how hydrous aluminosilicates withstand extremely high temperatures in the mantle transition zone. Here we systematically investigate the crystal structures and chemical compositions of typical hydrous aluminosilicates using single-crystal X-ray diffraction, electron probe microanalyzer, and nanoscale secondary ion mass spectrometry. These single crystals are synthesized at 15.5–22.0 GPa and 1400–1700 °C, featuring pervasive structural disorders. In particular, Al and Si atoms extensively occupy new tetrahedral and octahedral sites that are nominally vacant in their ordered counterparts. High temperature activates disorders leading to variable local crystal structures and more hydrogen incorporation into the crystal structure. This result suggests that the order-to-disorder transition holds the key to the high thermal stability of hydrous aluminosilicates, significantly affecting the water cycle in the deep mantle. Activated disorder in the structure of hydrous aluminosilicates improves their thermal stability and allows more hydrogen to be incorporated into the crystal structure, impacting the deep water cycle into the lower mantle.

The roles and mechanisms governing fluid mobility and hydrothermal mineralization of niobium (Nb) in geological processes have remained poorly understood, which hinders comprehensive insights into its geochemical properties and oreforming processes. This study investigates complexation, speciation, and thermodynamic stability of Nb within 0.07–0.28 mol/L fluoride-containing hydrothermal solutions at 100 MPa and 100–550°C via high-temperature hydrolysis experiments of potassium fluoniobate. The experimental and thermodynamic calculation results reveal a novel complex species, Nb(OH)4F2−, and determine for the first time its temperature-dependent hydrolysis constant through lnK=−6749/T+4.37. Combined with previous studies, this study identifies the dominant Nb species in natural acidic hydrothermal systems as Nb(OH)4F2− and Nb(OH)3F20 under low-fluoride conditions, shifting to Nb(OH)3F3− and Nb(OH)2F30 in high-fluoride environments. Based on quantitative characterization and comparative analysis of multiple metal complex stabilities, it is demonstrated that Nb, Sn(IV), and Ti exhibit remarkably similar hydrolysis behaviors and thermodynamic stabilities of their dominant complexes in F-bearing hydrothermal fluids, resulting in analogous fluid mobility. Rising temperature and decreasing F− activity, as well as changing pH, are thus identified as the primary mechanisms to trigger destabilization of these metal complexes, leading to their co-precipitation or paragenetic associations during geological processes.

Abundant indium (In) is frequently observed as an impurity element within sphalerite and closely associated with granite-related Sn-polymetallic deposits. Magmatic-hydrothermal processes have been proposed to govern In mineralization; however, the underlying mechanisms remain poorly understood. This study experimentally investigates geochemical behaviors of In, Sn, Cu, Cd, Fe, Pb, and Zn during interactions between granitic melts and 0.5-2.0 mol/L HF-bearing or 0.5-11.0 mol/L HCl-bearing solutions at 850 degrees C and 200 MPa. The results reveal a significant covariation relationship between In and these elements. Specifically, In, Sn, Zn, Cu, and Cd were enriched in all granitic melts, with their concentrations progressively increasing with higher HF concentrations but decreasing with higher HCl concentrations in both melts and solutions, while Fe and Pb were roughly depleted. This distinction is attributed to the low hydrothermal mobility of In, Sn, Zn, Cu, and Cd, which promotes their synchronous enrichment within acidic magmatic-hydrothermal systems. The results also highlight a strong pH dependence on hydrothermal mobility of In. Acidic F-or Cl-bearing fluids can enrich In within the granitic melts by a factor of 5 to 20 or 1 to 10 times, respectively. In contrast, neutral-alkaline Cl-bearing fluids may cause substantial depletion of In by factors of up to 1000 times. Consequently, the exsolution of acidic F-or Cl-bearing fluids could be an essential prerequisite for generating an In-rich magma reservoir. Subsequent activities involving neutral-alkaline fluids, such as exotic brine extraction or saline fluid exsolution, can efficiently transport In from the In-rich magmas into hydrothermal fluids, thereby facilitating In mineralization.

With the development of aviation, superconducting, and other steel industries, the demand for niobium (Nb) has significantly increased worldwide, positioning it as a critical strategic metal. The Bayan Obo rare-earth elements (REE)-Nb-iron (Fe) deposit contains over 70

The occurrence of sulfate and fluoride minerals in carbonatite-hosted rare earth element (REE) deposits suggests that sulfur and fluorine play important roles in REE mineralization. However, their influence on the partitioning behavior of REEs during the immiscibility process remains poorly understood. This study performed partitioning experiments to explore the impact of sulfur and fluorine on the liquid immiscibility between carbonatitic melt and alkaline silicate melt at 1000-1200 degrees C and 0.5-2.2 GPa. Surprisingly, the experimental results indicate that the addition of sulfur and fluorine does not significantly change the partition coefficients of trace elements between carbonatitic melt and silicate melt. The key factor determining REE partitioning is the structural difference between the two immiscible melts, which can be characterized by the non-bridging oxygen per tetrahedrally coordinated cation of the silicate melt (NBO/T). Partition coefficients tend to decrease as NBO/T increases. Importantly, REE, SO3, and F exhibit similar behaviors, making sulfate and fluoride minerals useful indicators for exploring carbonatite-hosted REE deposits. Additionally, we used rhyolite-MELTS software to simulate crystallization differentiation and liquid immiscibility in alkaline silicate melts. Modeling results show that the initial CO2 content of silicate melt determines the degree of crystallization at which liquid immiscibility occurs. Lower initial CO2 content enhances the enrichment of REEs in the immiscible carbonatitic melt.

Germanium (Ge) has been recognized as a critical strategic metal due to its high-technology implications. It is predominantly found in sphalerite within the Pb-Zn deposits, whose genesis is closely related to chloride-bearing hydrothermal activities. However, the dissolution and complexation of Ge in chloride-bearing fluids have not yet been well understood. To address this issue, this study investigates the dissolution behavior of Ge and corresponding species in HCl and NaCl aqueous solutions at 150 °C using the solubility method. The results show that the solubility of Ge in HCl solutions reaches 300 ppm and decreases with increasing HCl concentration. In contrast, the solubility of Ge in NaCl solutions is up to 1500 ppm and exhibits no significant dependence on NaCl concentration. The new findings demonstrate that the solubility of Ge in chloride-bearing solutions is primarily controlled by the solution’s pH. As the pH increases, the concentration of Ge dissolved in solutions rises substantially, suggesting that alkaline fluid environments facilitate Ge dissolution and transport. Further analysis of the experimental data indicates that the dominant species of Ge is Ge(OH)40 in acidic chloride-bearing solutions and Ge(OH)5− in neutral-basic chloride-bearing solutions, in which the species Ge(OH)5− promotes higher Ge solubility compared to Ge(OH)40. On account of high solubility in both HCl and NaCl solutions, it is proposed that Ge mineralization is not controlled by the co-precipitation process with Pb and Zn but by subsequent solid–liquid reaction between sphalerite and fluids. These experimental data and computational results not only provide new insights into the dissolution, transport, and precipitation processes of Ge in Earth’s material cycling but also offer novel perspectives for the understanding of Ge mineralization and industrial extraction and recovery of Ge.

Partition coefficients for Cu, Au and Mo between an aqueous fluid (similar to 4-16 wt% NaCl eq.) and felsic melts, were determined experimentally to better constrain the enrichment of these metals during magmatic fluid exsolution in porphyry copper systems. The experiments were conducted with a variety of melt and fluid compositions at 850 degrees C, 200 MPa and an oxygen fugacity approximating that of the Ni-NiO buffer. The results show that at a total chlorinity of 1 mol/kg H2O, the values of D-Cu(fluid/melt), D-Au(fluid/melt) and D-Mo(fluid/melt) are similar to 7.5-27.7, similar to 1.2-41.2 and similar to 0.5-2.7, respectively, and that these values vary as function of the melt aluminum saturation index (ASI). For a fixed starting glass composition with an ASI of 1.02 and a fluid with a total chlorinity that increases from 1 to 4 mol/kg H2O, the D-Cu(fluid/melt) and D-Au(fluid/melt) values increase from 27.7 to 68.8 and from 26.1 to 45.6, respectively, whereas the D-Mo(fluid/melt) value increases from 0.6 to 2.6. Rayleigh fractionation modeling of fluid exsolution from an oxidized (fO(2)>similar to FMQ + 2) and sulfide-poor arc magma using these data shows that the ratios involving Cu, Au and Mo in porphyry copper deposits are controlled by the ASI of the melt. Exsolution of a low chlorinity magmatic fluid from a metaluminous to weakly peraluminous magma (ASI= similar to 1-1.1) will produce a fluid with a high Au/Cu ratio fluid and a Cu-Au porphyry deposit, whereas a strongly peraluminous (ASI > 1.1) magma will produce a low Au/Cu ratio fluid and a Cu porphyry deposit. In contrast, both metaluminous and strongly peraluminous silicate magmas (ASI similar to 1.0 or ASI > 1.2) will exsolve fluids with high Mo/Cu (or Mo/Au) ratios. Varying the initial chlorinity of the exsolved fluid from similar to 1 to 4 mol/kg H2O and the initial H2O content of the magma have little effect on the selective partitioning of Cu, Au and Mo into the exsolved fluid, and also an insignificant effect on the bulk Au/Cu and Mo/Cu ratios of the hydrothermal ore systems.

The dissolution behavior of feldspar is fundamental to understand geological processes such as surface mass cycling, chemical weathering, mineral deposition and global climate change. In this study, we select oxalic acid – one of the most popular organic acids in nature as a buffer solution to simulate the chemical weathering process of feldspar under natural environments. A total of 34 fluid–feldspar reaction experiments are performed to investigate the effects of temperature, pH, and specific surface area (SSA) on the dissolution mechanism of feldspar. Based on PHREEQC modelling of saturation index of secondary minerals, we obtain the overall dissolution rate of feldspar, which is predominantly influenced by pH and temperature, with SSA exerting a secondary effect. The dissolution rates of Na, K, Ca and Si show a positive correlation with temperature and SSA, whereas that of Al exhibits a negative correlation with SSA and a weak correlation with temperature. Based on chemical reaction kinetics, the reaction order of feldspar dissolution in oxalic acid is estimated to be approximately 0.47, suggesting that the dissolution rate of feldspar is primarily controlled by a desorption process. Our new experimental results reveal that the anomalous Al dissolution behavior, which are likely due to the formation of aluminum complexes on the feldspar surface, have potentially significant for understanding the Al enrichment mechanism during the chemical weathering of the continental crust.

The underestimated rhenium (Re) concentration of continental crust is crucial for resolving the "missing Re puzzle" in the silicate Earth. Previous studies attributed the unknown Re reservoir in the continental crust to sulfide cumulates in the lower crust. However, the impact of aqueous fluids on Re abundance in the continental crust has been largely overlooked due to a lack of partition coefficients between fluids and silicate melts DRefluid / melt . $\left(D_{\mathrm{Re}}<^>{\text {fluid } / \text { melt }}\right) .$To address this gap, we conducted partitioning experiments at 0.5 GPa and 850 degrees C under oxidized conditions (similar to hematite-magnetite buffer) to determine the DRefluid / melt . $D_{\operatorname{Re}}<^>{\text {fluid } / \text { melt }} .$Our goal was to investigate how fluid exsolution influences Re distribution in the crust. Our experiments revealed that the D values ranged from 4 to 108 for Re. Interestingly, these D values were not related to the concentration of F-,Cl-,and CO32-, $\mathrm{F}<^>{-}, \mathrm{Cl}<^>{-}, \text {and } \mathrm{CO}_3<^>{2-},$ but increased as the H2O fugacity in aqueous fluids increased. Numerical modeling suggests that magmatic fluids can extract a significant fraction of Re (similar to 80%) during arc-magma differentiation, leading to Re depletion in the upper continental crust. Therefore, we believe that aqueous fluids play a dominant role in depleting Re content in the continental crust, whereas sulfide accumulation plays a very limited role.

Metal dissolution, complexation, and speciation are the key processes that facilitate metal mobilization and transport in fluids. Niobium (Nb), a kind of critical metal, has traditionally been regarded as a fluid-immobile element; however, it sometimes shows apparent hydrothermal mobility and even mineralization. Studying the solubility and complexation of Nb in fluids is thus crucial for understanding its dissolution, transport, enrichment, and mineralization. In this paper, we reviewed the geological observations on Nb mobility related to magmatic-hydrothermal and metamorphic fluid activities, especially compiled and reprocessed the published data on Nb solubility and related thermodynamic calculation to discuss the complexation and speciation of Nb in fluids. Previous solubility experiments demonstrate that Nb has much higher solubility in F-bearing solutions than in other solutions (Cl−, ClO4−, CO32−, HCO3−, OH−, SO42−, etc.), the maximum of which is up to ∼3 wt% in a 2 mol/kg HF solution. It is revealed that Nb solubility is related to the solution's composition, pH, ionic strength, oxygen fugacity, temperature, and pressure. High solubility could be found in neutral and weakly-basic solutions at near ambient temperature and pressure or in F-bearing fluids at high-temperature and high-pressure conditions. Modeling calculations show that Nb could be soluble and stable in fluids as the mononuclear or polynuclear complexes, such as fluoride complexes, hydroxide complexes, chloride complexes, and hexametalate ions, etc. Thereinto, Nb–OH–F complexes should play a dominant role in Nb hydrothermal mobility and be enriched in medium–high temperature, acidic, and F-bearing fluids. Experiments and modeling calculation have also inferred the existence of the species (e.g., Nb(OH)4+yFy−, Nb(OH)3+yF2y−, and Nb(OH)2+yF3y−), in which the F coordination number is no more than 3. Considering that it could have higher F contents than the experimental solutions (F < 4 wt%), we believe that natural mineralized F-rich fluids would facilitate the formation of the species with higher F coordination numbers due to the positive relationship between the F coordination number in the species and F content in fluids. Compared to the Nb–OH–F complexes, Nb–OH complexes are less stable and soluble but predominant in dilute solutions containing low concentrations of ligand anions or in alkaline solutions. In addition, further experiments need to be replenished and the problems need to be addressed are also discussed, which will enhance our understanding of how the Nb cycle and mineralization related to hydrothermal activities happen.