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This contribution presents a thermodynamic model revealing the mechanisms responsible for separation of Heavy and Light Rare Earth (HREE and LREE) phosphates in natural hydrothermal systems at temperatures of 250–350 °C. Our calculations were performed for an isothermal column of rock containing 0.5 wt% of apatite (Ca phosphate), which served as an immobilizing agent for REE dissolved in the solution. REE were transported by 10 wt% NaCl acidic solution. The model accounted for formation of REE phosphate solid solutions through a regular mixing model. It demonstrates that hydrothermal flushing can efficiently separate REE forming xenotime (HREE phosphate solid solutions) at the beginning of the column, and re-transportation of monazite (LREE rich) to the end of the column. This separation is primarily due to the fact that at elevated temperatures stability LREE chloride complexes is significantly higher than that for HREE. The model also evaluates behavior of U and Th, which accompany REE in vast majority of natural locations. It was found that U strongly fractionates to xenotime, whereas Th fractionates to monazite. This phenomenon can be explained by the differences in crystal-chemical characteristics between monazite and xenotime, and the mobility of Th in aqueous solutions at elevated temperatures.

Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is sparse in available literature. The behavior of U3Si2 in H2O containing atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U3Si2 following exposure to pressurized H2O at temperatures from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chemistry is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concentration between 1 and 30 parts per million H2. Testing included UN, U3Si5, and UO2. Both UN and U3Si5 were found to rapidly pulverize in less than 50 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U3Si2 at 300 °C found reasonable stability through 30 days in 1–5 ppm H2. However, pulverization was observed following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. Testing performed at 350 °C resulted in rapid pulverization of U3Si2 in under 50 h.

Carbon dioxide (CO2) is a major component of volcanic gases and ore-forming hydrothermal fluids. However, CO2 has contrasting effects on the speciation of different metal complexes and ore mineral solubility, but a molecular understanding of its effects is lacking. To address this deficiency, we conducted ab initio molecular dynamics (MD) simulations of the behavior of AuCl(aq) in the CO2–H2O system at 340 °C and 118–152 bar and 800 °C and 265–291 bar for CO2 mole fractions (XCO2) of 0.1–0.9. The MD simulations indicate that the linear [H2O–Au–Cl]0 structure of gold chloride is not affected by CO2 at XCO2 up to 0.8 at 340 °C and XCO2 up to 0.5 at 800 °C, whereas the “dry” [AuCl]0 species predominates at XCO2 > 0.8 at 340 °C and XCO2 > 0.5 at 800 °C. The number of water molecules hydrating the AuCl(aq) complex decreases systematically with an increasing CO2 mole fraction and increasing temperature. Results of Au solubility experiments at 340 °C in CO2–H2O solutions show that the addition of CO2 does not en...

The solubility of Nb2O5 and the speciation of niobium in HF-bearing aqueous solutions have been determined at temperatures of 150, 200, and 250 °C and saturated water pressure. At a pH of ∼2 and at low HF concentration, niobium is transported primarily as the species Nb(OH)4+ and at high HF concentration, as the species NbF2(OH)3°. Equilibrium constants for the formation of Nb(OH)4+ range from −11.23 ± 0.26 to −10.86 ± 0.24 and for the formation of NbF2(OH)3° from −3.84 ± 0.20 to −5.08 ± 0.42, at 150 and 250 °C. The results of this study show that the solubility of Nb2O5 (solid) in aqueous fluids increases with increasing HF concentration, but is not strongly affected by temperature. The influence of pH is variable; at low pH and HF concentration, a decrease in pH increases the solubility of Nb2O5 (solid). At higher pH, the reverse may be true. Modeling of the transport and deposition of niobium suggests that simple mixing with a brine is not an effective method for removing niobium from solution. By contrast, interaction of an acidic fluid with carbonate rock results in a rapid reduction in the capacity of the fluid to mobilize niobium.

For many years, our understanding of the behavior of the REE in hydrothermal systems was based on semi-empirical estimates involving extrapolation of thermodynamic data obtained at 25 °C (Haas et al., 1995; Wood, 1990a). Since then, a substantial body of experimental data has accumulated on the stability of aqueous complexes of the REE. These data have shown that some of the predictions of Haas et al. (1995) are accurate, but others may be in error by several orders of magnitude. However, application of the data in modeling hydrothermal transport and deposition of the REE has been severely hampered by the lack of data on the thermodynamic properties of even the most common REE minerals. The discrepancies between the predictions of Haas et al. (1995) and experimental determinations of the thermodynamic properties of aqueous REE species, together with the paucity of data on the stability of REE minerals, raise serious questions about the reliability of some models that have been proposed for the hydrothermal mobility of these critical metals. In this contribution, we review a body of high-temperature experimental data collected over the past 15 years on the stability of REE aqueous species and minerals. Using this new thermodynamic dataset, we re-evaluate the mechanisms responsible for hydrothermal transport and deposition of the REE. We also discuss the mechanisms that can result in REE fractionation during their hydrothermal transport and deposition. Our calculations suggest that in hydrothermal solutions, the main REE transporting ligands are chloride and sulfate, whereas fluoride, carbonate, and phosphate likely play an important role as depositional ligands. In addition to crystallographic fractionation, which is based on the differing affinity of mineral structures for the REE, our models suggest that the REE can be fractionated hydrothermally due to the differences in the stability of the LREE and HREE as aqueous chloride complexes.

Magmatic systems host many types of ore deposits, including world-class deposits of copper and gold. Magmas are commonly an important source of metals and ore-forming fluids in these systems. In many magmatic-hydrothermal systems, low-density aqueous fluids, or vapours, are significant metal carriers. Such vapours are water-dominated shallowly, but fluxing of CO2-rich vapour exsolved from deeper magma is now recognised as ubiquitous during open-system magma degassing. Furthermore, we show that such CO2-fluxing leads to a sharp drop in element solubility, up to a factor of 10,000 for Cu, and thereby provides a highly efficient, but as yet unrecognised mechanism for metal deposition.

The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province. It was emplaced within metasedimentary rocks belonging to the Kechika Group, which have been altered to potassic fenite near the intrusion and sodic fenite at greater distances from it. The intrusion comprises a ferroan dolomite carbonatite core, which passes gradationally outward into calcite carbonatite. The potentially economic REE mineralization is hosted by the dolomite carbonatite. Three types of dolomite have been recognized. Dolomite 1 constitutes the bulk of the dolomite carbonatite, dolomite 2 replaced dolomite 1 near veins and vugs, and dolomite 3 occurs in veins and vugs together with the REE mineralization. Carbon and oxygen isotope ratios indicate that the calcite carbonatite crystallized from a magma of mantle origin, that dolomite 1 is of primary igneous origin, that dolomite 2 has a largely igneous signature with a small hydrothermal component, and that dolomite 3 is of hydrothermal origin. The REE minerals comprise REE fluorocarbonates, ancylite-(Ce), and monazite-(Ce). In addition to dolomite 3, they occur with barite, molybdenite, pyrite, and thorite. Minor concentrations of niobium are present as magmatic pyrochlore in the calcite carbonatite. A model is proposed in which crystallization of calcite carbonatite preceded that of dolomite carbonatite. During crystallization of the latter, an aqueous-carbonic fluid was exsolved, which mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite, where they precipitated mainly in response to the increase in pH that accompanied fluid-rock interaction and, in the case of the REE fluorocarbonates, decreasing temperature. These fluids altered the host metasedimentary rock to potassic fenite adjacent to the carbonatite and, distal to it, they mixed with formational waters to produce sodic fenite.

Conventional wisdom holds that aqueous solutions are the only non-magmatic fluids capable of concentrating metals in the Earth’s crust. The role of hydrocarbons in metal concentration is relegated to providing geochemical barriers at which the metals are reduced and immobilised. Liquid hydrocarbons, however, are also known to be able to carry appreciable concentrations of metals, and travel considerable distances. Here we report the results of an experimental determination of bulk solubilities of Au, Zn, and U in a variety of crude oils at temperatures up to 300 °C and of the benchtop-scale transport experiments that simulate hydrocarbon-mediated re-deposition of Zn at 25–200 °C. It has been demonstrated that the metal concentrations obtained in solubility experiments are within the range of concentrations that are typically considered sufficient for aqueous fluids to form ore bodies. It has also been shown that Zn can be efficiently transported and re-deposited by hydrocarbons. These results provide direct evidence of the ability of natural crude oils to mobilise metals available in hydrocarbon-associated host rocks, and transport them in concentrations sufficient to contribute to ore-forming processes.

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.

The solubility of copper chloride and metallic copper in low-density homogenous HCl-bearing aqueous fluids was investigated experimentally at temperatures between 350 and 550 °C. Analysis of the resulting data and those on the solubility of copper chloride reported in Archibald et al. (2002) for temperatures between 280 and 320 °C suggests that at temperatures At temperatures above 450 °C, the stoichiometry of the dominant form of the dissolved copper chloride changes from copper monochloride (Cu:Cl = 1:1) to copper dichloride (Cu:Cl = 1:2) and the hydration numbers of the corresponding clusters are constant for the range of temperatures and pressures investigated. We did not manage to determine the valence state of copper in these species, and therefore interpreted our stability data separately for two alternative sets of hydration clusters, namely; one containing monovalent copper (CuCl:HCl or CuCl2H), and the other containing divalent copper dichloride (CuCl2). The model developed in this study was used to evaluate the solubility of chalcopyrite in HCl-bearing, low density aqueous fluids for temperatures ranging between 300 and 800 °C. This evaluation showed that, even for a relatively modest proportion of HCl (0.5 vol%), the concentration of copper in low-density fluids in equilibrium with this mineral can reach thousands of ppm at temperatures between 550 and 700 °C and fO2 conditions similar to those of porphyry copper ore-forming systems.

The solubility of β-Ga2O3(s) in HCl-bearing water vapour was measured at temperatures between 150 and 400 °C and pressures up to 257 bar. These measurements indicate that Ga(III) oxide dissolves in significant concentrations in water vapour, and that its solubility depends on fHCl and fH2O. At low fHCl, the dominant gallium gas species is Ga(OH)3(g), whereas at high fHCl, Ga(OH)Cl2(g) predominates. Gallium(III) oxide solubility increases exponentially with increasing pressure at 350 and 400 °C due to the formation of the hydrated clusters, GaOHCl2(H2O)n(g), where n = 1–7. The logarithms of the dissolution constants for the reaction, 0.5Ga2O3(s) + 1.5H2O(g) = Ga(OH)3(g), are −7.70 ± 0.30 and −7.93 ± 0.30 at 350 and 400 °C, respectively, and for the reaction, 0.5Ga2O3(s) + 2HCl(g) = GaOHCl2(g) + 0.5H2O(g), are −0.32 ± 0.29, 0.03 ± 0.20, and −2.59 ± 0.15 at 150, 200, and 400 °C, respectively. The data obtained in this study were used to determine the partitioning of Ga between vapour and liquid at temperatures between 150 and 350 °C, and a pH for the liquid ranging from 0 to 10. The estimated partition coefficient (Kd) decreases with increasing temperature, and reaches a maximum value of ∼100 at 150 °C and ∼25 at 200 °C at a pH of ∼4. The values are very similar to those estimated from data for geothermal wells in Iceland and hot springs in the Kamchatka Peninsula.

New experimental data demonstrate that the solubility of metals in water vapor has been greatly underestimated. We demonstrate here that the solubility of AgCl increases exponentially with H2O fugacity rather than linearly, as previously assumed, leading to much greater hydration and correspondingly higher solubility. Our data suggest that the solubility of silver chloride in low-density aqueous fluids is related to the formation of hydrated clusters, i.e., AgCl:(H2O)n. The hydration number of the predominant cluster increases systematically with increasing pressure, and each of the gaseous solutions investigated in our experiments contains a mixture of clusters with different hydration numbers that predominate at different pressures. Extrapolation of the data to magmatic conditions, based on an observed linear relationship between the Gibbs free energy of formation of the hydrated species and reciprocal temperature, predicts a solubility silver chloride comparable to the concentration of silver observed in vapor inclusions, thereby supporting a model of metal transport by vapor.

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− = ZrFn(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.

The speciation of Nd(III), Sm(III), and Er(III) in sulfate-bearing solutions has been determined spectrophotometrically at temperatures from 25 to 250 °C and a pressure of 100 bars. The data obtained earlier on the speciation of Nd in sulfate-bearing solutions ( Migdisov et al., 2006 ) have been re-evaluated and corrected using a more appropriate activity model and are compared with the corresponding data for Sm(III) and Er(III) and new data for Nd(III). Based on this comparison, the dominant species in the solution are interpreted to be REESO 4 + and REE ( SO 4 ) 2 - , with the latter complex increasing in importance at higher temperature. Equilibrium constants were calculated for the following reactions: REE 3 + + SO 4 2 - = REESO 4 + β 1 ; REE 3 + + 2 · SO 4 2 - = REE ( SO 4 ) 2 - β 2 ; REESO 4 + + SO 4 2 - = REE ( SO 4 ) 2 - K 2 . The values of the first formation constant (β1) are in reasonably good agreement with those predicted by Haas et al., 1995 , Wood, 1990a ; those for the second formation constant (β2, K2) also agree reasonably well with predictions of Wood, 1990a , Wood, 1990b at low temperatures, but predict higher stability of REE ( SO 4 ) 2 - at T > 150 °C.

The speciation of erbium in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100 to 250 °C and a pressure of 100 bars. The hydrated ion, Er 3+ , is predominant at ambient temperature, but chloride complexes are dominant at elevated temperature. Formation constants were calculated for the following reactions

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 ( log K sp = log a Nd 3 + + 3 log a 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 NdF 2 + ( log β = log a NdF 2 + - log a Nd 3 + - log a 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. Acta 59, 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. Acta 66, 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. Acta 70, 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 speciation of erbium in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100 to 250 °C and a pressure of 100 bars. The hydrated ion, Er3+, is predominant at ambient temperature, but chloride complexes are dominant at elevated temperature. Formation constants were calculated for the following reactions: (β1) Er3+ + Cl− = ErCl2+ (β2) Er3+ + 2 Cl− = ErCl2+ The values obtained for the first formation constant (β1) were 0.88 ± 0.11, 1.59 ± 0.12, 2.34 ± 0.11, and 3.09 ± 0.14 at 100, 150, 200 and 250 °C, respectively. Values of the second formation constant could only be determined at 200 and 250 °C, and were 2.95 ± 0.34 and 4.12 ± 0.12, respectively. The values for the first formation constant (β1), are identical within experimental error to the values predicted by Haas et al. [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. Acta, 59, 4329–50.], whereas the values for the second formation constant (β2) agree relatively well with those predicted by Haas et al. (1995) and determined experimentally by Gammons et al. [Gammons, C.H., Wood, S.A., and Li, Y. (2002). Complexation of the Rare Earth Elements with aqueous chloride at 200 °C and 300 °C and saturated water vapor pressure. Special Publication — The Geochemical Society, (Water–Rock Interactions, Ore Deposits, and Environmental Geochemistry), 191–207.].

The formation constants of neodymium complexes in sulfate solutions have been determined spectrophotometrically at temperatures of 30–250 � C and a pressure of 100 bars. The dominant species in the solution are NdSO4 + and Nd(SO4)2 � , with the latter complex being more important at higher temperature. Equilibrium constants were calculated for the following reactions: Nd 3þ þ SO4 2� ¼ NdSO4 þ ; b1; Nd 3þ þ 2 � SO4 2� ¼ NdðSO4Þ 2 � ; b 2 ; NdSO4 þ þ SO4 2� ¼ NdðSO4Þ 2 � ; K s. The values of b1 and b2, were determined for 30 and 100 � C, whereas for higher temperatures it was only possible to determine the stepwise formation constant Ks. The values of the formation constants obtained in this study for 30 and 100 � C are in excellent agreement with those predicted theoretically by Wood [Wood, S.A., 1990b. The aqueous geochemistry of the rare-earth elements and yttrium. 2. Theoretical predictions of speciation in hydrothermal solutions to 350 � C at saturation water vapor pressure. Chem. Geol. 88 (1–2), 99–125] and Haas et al. [Haas, J.R., Shock, E.L., Sassani, D.C., 1995. Rare earth elements in hydrothermal sysytems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta 59 (21), 4329–4350], and those for the stepwise formation constant (Ks) agree reasonably well with the

The solubility of cassiterite was investigated experimentally in liquid-undersaturated HCl-bearing water vapour at temperatures of 300 to 350 8C and pressures up to 180 bar. Concentrations of tin varied between 0.1 and 32 ppb, depending on experimental conditions, and reached a maximum at 320 8C. These concentrations are at least four orders of magnitude higher than those reported for the water-free system. Positive correlation of the concentrations of tin with fH2O and fHCl indicate that cassiterite solubility is enhanced by solute–solvent interaction, and that this involves the formation of hydrated gaseous species of tin chloride having a tin–chlorine ratio of 1:2, and statistical hydration numbers of 2.9, 2.7, and 2.5 at 300, 320, and 350 8C, respectively. We therefore propose that the solubility of cassiterite in HCl-bearing water vapour is explained by the reaction: SnO cassiterite þ 2dHCl gas þ ndH2O vapour ¼ SnOCl2d H2OÞ vapourþ1

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.

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 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 © 2001 Elsevier Science Ltd

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.

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.

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).