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<p>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)<sub>4</sub>F<sub>2</sub><sup>−</sup>, and determine for the first time its temperature-dependent hydrolysis constant through ln<i>K</i>=−6749/<i>T</i>+4.37. Combined with previous studies, this study identifies the dominant Nb species in natural acidic hydrothermal systems as Nb(OH)<sub>4</sub>F<sub>2</sub><sup>−</sup> and Nb(OH)<sub>3</sub>F<sub>2</sub><sup>0</sup> under low-fluoride conditions, shifting to Nb(OH)<sub>3</sub>F<sub>3</sub><sup>−</sup> and Nb(OH)<sub>2</sub>F<sub>3</sub><sup>0</sup> 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<sup>−</sup> 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.</p>

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Experimental investigation on niobium species and its thermodynamic stability in fluoride-bearing hydrothermal fluids

  • 期刊论文

  • 发表日期：

    2025年10月20日

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  • Ruilong Pan,
  • Xing Ding,
  • Haotian Liu,
  • Zebang Yi,
  • Cheng Xu,
  • Xiaolong Huang

摘要

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.