Art. A. Migdisov

Hydrothermal transport and deposition of the rare earth elements by fluorine-bearing aqueous liquids

New technologies, particularly those designed to address environmental concerns, have created a great demand for the rare earth elements (REE), and focused considerable attention on the processes by which they are concentrated to economically exploitable levels in the Earth’s crust. There is widespread agreement that hydrothermal fluids played an important role in the formation of the world’s largest economic REE deposit, i.e. Bayan Obo, China. Until recently, many researchers have assumed that hydrothermal transport of the REE in fluorine-bearing ore-forming systems occurs mainly due to the formation of REE-fluoride complexes. Consequently, hydrothermal models for REE concentration have commonly involved depositional mechanisms based on saturation of the fluid with REE minerals due to destabilization of REE-fluoride complexes. Here, we demonstrate that these complexes are insignificant in REE transport, and that the above models are therefore flawed. The strong association of H+ and F− as HF° and low solubility of REE-F solids greatly limit transport of the REE as fluoride complexes. However, this limitation does not apply to REE-chloride complexes. Because of this, the high concentration of Cl− in the ore fluids, and the relatively high stability of REE-chloride complexes, the latter can transport appreciable concentrations of REE at low pH. The limitation also does not apply to sulphate complexes and in some fluids, the concentration of sulphate may be sufficient to transport significant concentrations of REE as sulphate complexes, particularly at weakly acidic pH. This article proposes new models for hydrothermal REE deposition based on the transport of the REE as chloride and sulphate complexes.

Solubility of chlorargyrite (AgCl) in water vapor at elevated temperatures and pressures

The solubility of chlorargyrite (AgCl) in undersaturated water vapor was investigated at temperatures of 300 to 360oC and pressures up to 180 bars. It was shown that the presence of water vapor increases the concentration of AgCl in the gas (vapor) phase by between 1.5 and 2 orders of magnitude. This phenomenon is attributed to the formation of hydrated gaseous particles. Silver chloride dissolved in water vapor without changing its stoichiometry (congruent dissolution, Ag:Cl = 1:1). On the basis of the experimental data obtained in this study, the process of chlorargyrite dissolution, and the formation of hydrated gaseous particles in water vapor can be described by the reaction: AgClcryst.+3·H2Ogas=AgCl·(H2O)3gas Considering that Ag is coordinated by three molecules of water and one molecule of chlorine in the AgCl · (H2O)3gas particle, it was assumed that the silver atom is in fourfold coordination. The properties of the AgCl · (H2O)3 particle were refined using ab initio molecular orbital calculations, and the stable geometry of the particle was deduced to have C3 symmetry. The temperature dependence of the equilibrium constant for the reaction controlling the formation of AgCl · (H2O)3gas is described by the equation: logK(P=1bar)=(22.578±5.505)−(0.0255±0.0045)·TK−(11987.6±658.5)/TK Preliminary calculations suggest that water vapor can transport significant quantities of silver, and that such transport may play an important role in mobilizing silver in natural hydrothermal systems.

A spectrophotometric study of neodymium(III) complexation in chloride solutions

The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250°C and a pressure of 50 bars. The simple ion, Nd3+, is dominant at 25°C, whereas NdCl2+ and NdCl2+ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions: Nd3+ + Cl− = NdCl2+ β1, Nd3+ + 2 · Cl− = NdCl+2 β2. The values of β1 were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of β2 obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200°C. Theoretical estimates of β1 and β2 by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl2+ and NdCl2+ at temperatures above 150°C than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for β1 and β2 similar to those obtained experimentally. Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of ∼2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth’s crust.

Estimates of the second dissociation constant of H2S from the surface sulfidation of crystalline sulfur

Abstract The adsorption of hydrogen sulfide (Γ H 2 S) and protons (Γ H + ) on the surface of crystalline sulfur was investigated experimentally in H 2 S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm −3 and log C H + values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of Γ H 2 S were very close to the highest values determined for Γ H + . This finding, combined with the lack of detectable proton adsorption in H 2 S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of S − H 2 S complexes in the presence of H 2 S. We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: S + H 2 S = SSH 2 0 β° The deprotonation of the SH ° complex occurs via the reaction: SSH 2 0 = SSH − + H + β − Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β − at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.

The behaviour of metals and sulphur during the formation of hydrothermal mercury–antimony–arsenic mineralization, Uzon caldera, Kamchatka, Russia

Abstract Uzon caldera, located in the eastern volcanic belt of the Kamchatka peninsula, is a complicated structure of Middle Pleistocene age. The composition of the co-existing solid and fluid phases, temperature and pH were determined with the aim of establishing the distribution of sulphur species, As, Sb and the main ore-forming metals. In the solid samples, the following sulphur-bearing minerals were identified: pyrite, realgar, orpiment, alacranite (As8S9), uzonite (As4S5), amorphous As-sulphide, stibnite, cinnabar and native sulphur. The following sulphur-bearing species H2S, H2S2+S52−(aq)(aqueous polysulphanes), S0(aq), SO32−(aq), S2O32−, SO42− and total concentration of sulphur were determined in solutions. Eh, pH and H2S concentration were measured potentiometrically in situ. Zero-valent sulphur (S0(aq)+H2S2+S52−(aq)) predominates in Uzon solutions. The pair H2S–Scolloidal is Eh-determining in Uzon solutions up to 75–85°C. A quantitative thermodynamic model of the mineral deposition process at Uzon was constructed using the collected data. It was obtained that the composition of the hydrothermal solution and the precipitation of Sb–As–Hg species can be described using two only main factors: the initial composition of fluid and the temperature variation.

Experimental study of polysulfane stability in gaseous hydrogen sulfide

The solubility of sulfur in gaseous hydrogen sulfide has been studied in the H2S-S system. Experiments were carried out at temperatures between 50 and 290°C and pressures up to 200 bars. The experimentally determined concentrations of sulfur in the gas phase are 6–7 orders of magnitude higher than the corresponding concentrations calculated for a system free of hydrogen sulfide. The results of experiments show significant interaction between S and H2S. These interactions can be of two kind: solvation by hydrogen sulfide (solubility), as with formation of new stable gaseous chemical compounds, like polysulfanes (chemical reaction). The data obtained can be reasonably well described by the formation of a H2S · S compound. Thermodynamic parameters for polysulfanes and equilibrium compositions of the S-H2S system have been calculated ab initio for the experimental conditions. At temperatures above 170°C, results (of calculations) are in good agreement with experimental data, although the difference between the calculated and experimental mole fraction of the sulfur in the gas phase reaches 2 orders of magnitude at 125–170°C. It is theorized that sulfur solubility in gaseous H2S is related to two main chemical reactions, dominated in the different temperature ranges: sulfur solvation by H2S (125–170°C) and polysulfane formation (200–290°C).

An experimental study of stibnite solubility in gaseous hydrogen sulphide from 200 to 320°C

The solubility of stibnite in gaseous hydrogen sulphide was investigated experimentally in the systems Sb2S3-S-H2S and Sb2S3-H2S at temperatures between 200 and 320°C (pressures of up to 200 bars). The concentrations obtained are several orders of magnitude higher than those calculated for H2S-free systems and display a prograde dependence on temperature in the range of 200 to 290°C. Concentrations of Sb 2S3 are relatively constant at temperatures ranging from 290 to 320°C with a proposed solubility maximum at 300°C. In order to be able to describe stibnite solubility in the gas phase a simple solvation model was used, namely: Sb2S3. 1 H2S gas 5 Sb2S3 z (H2S) gas . Equilibrium constant for this solvation reaction varied from 10 26.76 to 10 25.77 over the temperature range from 200 to 320°C. Copyright