Experimental study of natural pyrochlore and niobium oxide solubility in Alkaline hydrothermal solutions

The concentration and temperature dependences of pyrochlore and Nb oxide solubility in Na₂CO₃ and Na₂СO₃ + NaF aqueous solutions with concentrations from 0.01 to 2.0 m at 300–550°C and 50 and 100 MPa (the Co–CoO buffer) are studied. It is established that the Nb equilibrium content in the solution increases at 550°C and 100 MPa with an increase in mNa₂CO₃ and reaches the value of 10^–4 m. The Nb₂O₅ solubility almost does not change as the concentration of Na₂CO₃ increases and is found within 10^–6 to 10^–5.5 m.     

Experimental study of fluorite solubility in acidic solutions as a method for boron fluoride complexes studying

Fluorite solubility in HCl and HF solutions with varied concentrations of boric acid was studied at 81, 155, and 208°C and saturated vapor pressure. Our experimental results demonstrate that fluorite solubility increases with increasing B(OH)₃ concentration, and this was interpreted as the formation of the BF3OH–complex (Ryss, 1956). The experimental data were used to determine, using the OptimA software, the free energies of formation of HF°(aq) and, which were then used to calculate the constants of the reactions HF = H⁺ + F^– (1) and B(OH)₃(aq) + 2H⁺ + 3 F^– (2). The pK ₁ values are 3.71 ± 0.013, 4.28 ± 0.015, and 4.89 ± 0.017 and pK ₂ 13.60 ± 0.02, 13.99 ± 0.02, and 14.95 ± 0.03 at saturated vapor pressure and 81, 155, and 208°C, respectively.     

The carbon dioxide solubility in alkali basalts: an experimental study

Experiments were conducted to determine CO₂ solubilities in alkali basalts from Vesuvius, Etna and Stromboli volcanoes. The basaltic melts were equilibrated with nearly pure CO₂ at 1,200°C under oxidizing conditions and at pressures ranging from 269 to 2,060 bars. CO₂ solubility was determined by FTIR measurements. The results show that alkalis have a strong effect on the CO₂ solubility and confirm and refine the relationship between the compositional parameter Π devised by Dixon (Am Mineral 82:368–378, 1997) and the CO₂ solubility. A general thermodynamic model for CO₂ solubility in basaltic melts is defined for pressures up to 2 kbars. Based on the assumption that O²⁻ and CO₃²⁻ mix ideally, we have:

Antimony in hydrothermal processes: solubility, conditions of transfer, and metal-bearing capacity of solutions

Based on data on the composition of ore-bearing hydrothermal solutions and parameters of ore-forming processes at various antimony and antimony-bearing deposits, which were obtained in studies of fluid inclusions in ore minerals, we investigated the behavior of Sb(III) in the system Sb–Cl–H₂S–H₂O describing the formation of these deposits.

We also performed thermodynamic modeling of native-antimony and stibnite dissolution in sulfide (m_HS⁻ = 0.0001−0.1) and chloride (m_Cl⁻ = 0.1−5) solutions and the joint dissolution of Sb_(s)⁰ and Sb₂S_3(s) in sulfide-chloride solution (m_HS⁻ = 0.01; m_Cl⁻ = 1) depending on Eh, pH, and temperature. All thermodynamic calculations were carried out using the Chiller computer program. Under the above conditions, stibnite precipitates in acid, weakly acid to neutral, and medium redox solutions, whereas native antimony precipitates before stibnite under more reducing conditions in neutral to alkaline solutions.

The metal-bearing capacity of hydrothermal solutions (200–250 °C) of different compositions and origins has been predicted. We have established that the highest capacity is specific for acid (pH = 2–3) high-chloride solutions poor in sulfide sulfur and alkaline (pH = 7–8) low-chloride low-sulfide solutions.     

The analysis of the volatiles from fluid inclusions in hydrothermal fluorite

Volatiles from primary fluid inclusions in hydrothermal fluorite were studied. The gases released were analysed with a mass spectrometer using an internal standard; water-vapour pressure was measured manometrically.To extract the volatiles, both heating and grinding in vacuum were used. In the thermal treatment, volatiles from other sources besides the inclusions were also found: H₂ and hydrocarbons, as well as additional amounts of H₂O and CO₂. The vacuum grinding, on the other hand, leads to volatile deficiency, especially with respect to H₂O and CO₂, due to retention of these components on the ground material.The study of the dependence of amount of volatiles released upon heating, on the grain size of the mineral fractions, was used as additional information for evaluating the amounts of volatiles coming from a source other than inclusions.The thermal studies were supplemented by decrepitophonic measurements.It is concluded that the volatiles from inclusions are represented practically only by CO₂ and H₂O in a 1:100 molar ratio. Conclusions about the conditions of mineral formation are drawn.     

The solubility of gold in hydrogen sulfide gas: An experimental study

The solubility of gold in H₂S gas has been investigated at temperatures of 300, 350 and 400 °C and pressures up to 230 bars. Experimentally determined values of the solubility of Au are 0.4-1.4 ppb at 300 °C, 1-8 ppb at 350 °C and 8.6-95 ppb at 400 °C. Owing to a positive dependence of the logarithm of the fugacity of gold on the logarithm of the fugacity of H₂S, it is proposed that the solubility of Au can be attributed to formation of a solvated gaseous sulfide or bisulfide complex through reactions of the type:

(A)     

Characteristics of hydrolysis of the complex Na2SnF6 in hydrothermal solutions—An experimental study

Characteristics of hydrolysis of the complex Na₂SnF₆, which is used as the starting material, in hydrothermal solutions have been studied at 200–602°C and 1 kbar. Experimental results show that intense hydrolysis of Na₂SnF₆ occurs at high temperatures and that with the rise of temperature the hydrolysis will become more intense. Under the present experimental conditions the most possible existing form of Sn in the hydrothermal solutions is SnF₃(OH) or Na₂SnF₃(OH). In addition, the hydrolysis constants for Na₂SnF₆ have also been calculated at 200–602°C, and the relationship between Na₂SnF₆ hydrolysis and temperature is discussed.     

An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymium (III) fluoride was investigated at temperatures of 150, 200 and 250 °C, saturated water vapor pressure, and a total fluoride concentration (HF°_aq + F⁻) ranging from 2.0 × 10⁻³ to 0.23 mol/l. The results of the experiments show that Nd³⁺ and NdF²⁺ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF²⁺ and a solubility product for NdF₃. The solubility product of NdF₃(logK_sp=loga_Nd³⁺+3loga_F^-) is −24.4 ± 0.2, −22.8 ± 0.1, and −21.5 ± 0.2 at 250, 200 and 150 °C, respectively, and the formation constant of NdF²⁺(logβ=loga_NdF²⁺-loga_Nd³⁺-loga_F^-) is 6.8 ± 0.1, 6.2 ± 0.1, and 5.5 ± 0.2 at 250, 200 and 150 °C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF²⁺ and the solubility of NdF₃.The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta59, 4329-4350.], which indicate that NdF²⁺ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta66, 4311-4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta70, 983-992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated.     

The solubility of iron hydroxide in sodium chloride solutions

The solubility of iron(III) hydroxide as a function of pH was investigated in NaCl solutions at different temperatures (5–50°C) and ionic strengths (0–5 M). Our results at 25°C and 0.7 M in the acidic range are similar to the solubility in seawater. The results between 7.5 to 9 are constant (close to 10⁻¹¹ M) and are lower than those found in seawater (>10⁻¹⁰) in this pH range. The solubility subsequently increases as the pH increases from 9 to 12. The solubility between 6 and 7.5 has a change of slope that cannot be accounted for by changes in the speciation of Fe(III). This effect has been attributed to a solid-state transformation of Fe(OH)₃ to FeOOH. The effect of ionic strength from 0.1 to 5 M at a pH near 8 was quite small. The solubility at 5°C is considerably higher than at 25°C at neutral pH range. The effects of temperature and ionic strength on the solubility at low and high pH have been attributed to the effects on the solubility product and the formation of FeOH²⁺ and Fe(OH)₄⁻. The results have been used to determine the solubility products of Fe(OH)₃, K¹_Fe(OH)3 and hydrolysis constants, β¹₁, β¹₂, β¹₃, and β¹₄ as a function of temperature (T, K) and ionic strength (I):log K¹_Fe(OH)3 = −13.486 − 0.1856 I^0.5 + 0.3073 I + 5254/T (σ = 0.08)log β¹₁ = 2.517 − 0.8885 I^0.5 + 0.2139 I − 1320/T (σ = 0.03)log β¹₂ = 0.4511 − 0.3305 I^0.5 − 1996/T (σ = 0.1)log β¹₃ = −0.2965 − 0.7881 I^0.5 − 4086/T (σ = 0.6)log β¹₄ = 4.4466 − 0.8505 I^0.5 − 7980/T. (σ = 0.2)Both strong ethylenediaminetetraacetic acid and weak (HA) organic ligands greatly affect iron solubility. The additions of ethylenediaminetetraacetic acid and humic material were shown to increase the solubility near pH 8. The higher solubility of Fe(III) in seawater compared to 0.7 M NaCl may be caused by natural organic ligands.     

An experimental study of Cobalt (II) complexation in Cl and H2S-bearing hydrothermal solutions

The speciation of cobalt (II) in Cl⁻ and H₂S-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300 °C and a pressure of 100 bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120-300 °C and variable pressures of H₂S. From the results of these experiments, it is evident that CoHS⁺ and predominate in the solutions except at 150 °C, for which the dominant chloride complex is CoCl₃⁻. The logarithms of the stability constant for CoHS⁺ show moderate variation with temperature, decreasing from 6.24 at 120 °C to 5.84 at 200 °C, and increasing to 6.52 at 300 °C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for , 6.44 for CoCl₃⁻, 4.94 to 5.36 for , and 2.42 for CoCl⁺. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl = 0.25 m, NaCl = 0.75 m, ΣS = 0.3 m) suggest that at temperatures ?200 °C, cobalt occurs dominantly as CoHS⁺, whereas at higher temperatures the dominant species is .     

Fluorapatite and fluorite solubility controls on geothermal waters in Yellowstone National Park

Potential solubility controls on phosphorus in Yellowstone National Park geothermal waters were investigated using the analytical phosphate estimates of Stauffer and Thompson (1978), the computer program, WATEQF, and adopting the equilibrium constant: $\text{log}\text{K}{}_{\mathrm{25°}}\text{= ?61.4}$ for fluorapatite (FAP = Ca₅(PO₄)₃F) dissolution. The near-boiling high-Cl geyser and spring effluents are at or near saturation with respect to (with) FAP. The sixteen representative springs in this category had FAP saturation indices ($\text{S.I. =}\text{log}\text{IAP/K}{}_{\mathrm{r}}$) ranging from ? 3.2 to +4.9 and averaging +0.9. The strongly positive indices were all associated with the highly alkaline conditions resulting from adiabatic cooling in the near surface environment. Hot spring waters indicating extensive dilution (reduced Cl) by meteoric water have lower pH's, and despite PO₄ and Ca concentrations an order of magnitude higher than the geysers, are still frequently undersaturated with FAP. The travertine-depositing “Mixed-water” springs are invariably supersaturated with FAP at ground surface and at or near saturation with hydroxylapatite. Supersaturation may result from kinetic inhibition of apatite crystallization by the elevated Mg²⁺, HCO₃^?, and lower temperatures in these springs. The phosphates may be residuals of the meteoric dilution water.Separately, if Strübel's temperature-dependent estimates of fluorite (CaF₂) solubility are adopted, the high-Cl geysers and springs on “Geyser Hill” and at Norris are consistently undersaturated with CaF₂ at the 90–100° orifice temperatures. The apparent disequilibrium may reflect fluorite equilibration at the much higher temperatures (> 200°C) in the deeper enthalpy reservoirs.     

An experimental study of the solubility of baddeleyite (ZrO2) in fluoride-bearing solutions at elevated temperature

The solubility of baddeleyite (ZrO₂) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400 °C and pressures up to 700 bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)₃° and ZrF₂(OH)₂°. Formation constants determined for these species (Zr⁴⁺ + nF⁻ + mOH⁻ = ZrF_n(OH)_m°) range from 43.7 at 100 °C to 46.41 at 400 °C for ZrF(OH)₃°, and from 37.25 at 100 °C to 43.88 at 400 °C for ZrF₂(OH)₂°.Although the solubility of ZrO₂ is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF³⁺-ZrF₆²⁻). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for the concentration of this metal at conditions commonly encountered in fluoride-rich natural hydrothermal systems.     

热液矿床中萤石的稀土元素地球化学及其地质意义

萤石是许多热液矿床中重要的脉石矿物,其稀土元素含量及相关参数（如∑REE、LREE/HREE、Eu/Eu^*、Ce/Ce^*、Y/La、Tb/Ca-Tb/La图解等）,能为揭示成矿流体性质、来源与演化,建立成矿模式,评价区域成矿潜力等提供重要信息。然而,随着微区分析技术的日趋成熟,原位实测数据显示,萤石中微量元素（包括稀土元素）的分配在显微尺度上可能具有不均一性,致使依据萤石溶液法获得的稀土元素含量所反映地质信息的可靠性受到质疑。因此,在应用萤石稀土元素地球化学探讨相关地质问题前,有必要加强萤石微观结构的观察和配套的流体包裹体以及相关同位素组成分析。本文在综述萤石稀土元素地球化学研究基础上,初步探讨了影响萤石稀土元素不均一分配的主要机制,以期为萤石稀土元素在热液矿床成因研究中的应用提供借鉴。     

Experimental study of the alteration process of labradorite in acid hydrothermal solutions

Labradorite was altered artificially by HC1 solution ranging from M = 1 to M = 0.003 at 245 and 230°C. The products of alteration were examined by X-ray diffraction, electron microscopy, electron diffraction, infrared spectroscopy and the electron microprobe and the solution was analyzed chemically.Amorphous silica only was formed in solutions with M_HCl = 1 and M_HCl = 0.3. In a solution with M_HCl = 0.2, amorphous silica was initially formed, later dissolved and replaced by kaolinite. A mixture of microcrystalline boehmite and amorphous aluminosilicate was formed, altering to kaolinite in solutions with M_HCl = 0.1 and 0.3. Small amounts of kaolinite were initially formed but the alteration soon stopped in solution with M_HCl = 0.003. Relationships between the alteration processes and pH of the solutions can be roughly explained by using solubility diagrams assuming the congruent dissolution of labradorite and precipitation of the products in partial equilibrium. However, these assumptions are not valid with strongly acid solutions.The rate of dissolution of labradorite is controlled not only by its surface area, but also by the diffusion of matter through the layer of alteration products.     

Physicochemical modeling as applied to study of sulfoarsenide complexes in hydrothermal solutions

System As–Na–S–Cl–H–O was studied. The research was carried out in three stages: (1) selection of the most likely complexes resulting from arsenic sulfide dissolution, (2) calculation of their thermodynamic constants, and (3) comparison of calculated data with thermodynamic database obtained in tests with the solution of inverse thermodynamic problems using the Selektor program complex. The system As–Na–S–Cl–H–O included more than 230 dependent components, which were divided into two groups, base and functional. The former group includes components of the solution (NaCl, NaOH, Na₂S, NaHS, HCl, H₂S, H₂SO₄, sulfates, H₂SO₃, sulfites, thiosulfates, Na⁺, Cl⁻,HS⁻, S²⁻), gas phase (43 components), and solid phase (orpiment, red arsenic, arsenolite, claudetite, arsenic, sulfur, sodium salts). Thermodynamic constants of the base components are contained in the Selektor database (they were borrowed from reference-books). The latter group includes 77 complexes labile in the solution but determining the solubility of arsenic and stability of its solid phases. Physicochemical modeling was performed in H₂S (≥0.01 m, pH = 1–10), Na₂S, and NaHS solutions at 25–250 °C and saturated-vapor pressure. It has been established that the dissolution of arsenic sulfide mineral phases in subneutral and alkaline solutions at low oxidation potential is favored by the formation of sulfoarsenides, which are more stable than arsenides and arsenates. Thermodynamic constants of functional complexes determining the orpiment solubility were calculated within the experimental error. It is shown that in hydrothermal iron-free systems with a low oxidation potential, the concentration of As in the solution decreases on cooling and with acidity increase.     

The H<Subscript>2</Subscript>O solubility of alkali basaltic melts: an experimental study

Experiments were conducted to determine the water solubility of alkali basalts from Etna, Stromboli and Vesuvius volcanoes, Italy. The basaltic melts were equilibrated at 1,200°C with pure water, under oxidized conditions, and at pressures ranging from 163 to 3,842 bars. Our results show that at pressures above 1 kbar, alkali basalts dissolve more water than typical mid-ocean ridge basalts (MORB). Combination of our data with those from previous studies allows the following simple empirical model for the water solubility of basalts of varying alkalinity and fO₂ to be derived: \textH ₂ \textO( \textwt% ) = \text H ₂ \textO_\textMORB ( \textwt% ) + ( 5.84 ×10 ^{- 5} *\textP - 2.29 ×10 ^{- 2} ) ×( \textNa₂ \textO + \textK₂ \textO )( \textwt% ) + 4.67 ×10 ^{- 2} ×\Updelta \textNNO - 2.29 ×10 ^{- 1} {\text{H}}_{ 2} {\text{O}}\left( {{\text{wt}}\% } \right) = {\text{ H}}_{ 2} {\text{O}}_{\text{MORB}} \left( {{\text{wt}}\% } \right) + \left( {5.84 \times 10^{ - 5} *{\text{P}} - 2.29 \times 10^{ - 2} } \right) \times \left( {{\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}}} \right)\left( {{\text{wt}}\% } \right) + 4.67 \times 10^{ - 2} \times \Updelta {\text{NNO}} - 2.29 \times 10^{ - 1} where H₂O_MORB is the water solubility at the calculated P, using the model of Dixon et al. (1995). This equation reproduces the existing database on water solubilities in basaltic melts to within 5%. Interpretation of the speciation data in the context of the glass transition theory shows that water speciation in basalt melts is severely modified during quench. At magmatic temperatures, more than 90% of dissolved water forms hydroxyl groups at all water contents, whilst in natural or synthetic glasses, the amount of molecular water is much larger. A regular solution model with an explicit temperature dependence reproduces well-observed water species. Derivation of the partial molar volume of molecular water using standard thermodynamic considerations yields values close to previous findings if room temperature water species are used. When high temperature species proportions are used, a negative partial molar volume is obtained for molecular water. Calculation of the partial molar volume of total water using H₂O solubility data on basaltic melts at pressures above 1 kbar yields a value of 19 cm³/mol in reasonable agreement with estimates obtained from density measurements.