Uranyl acetate speciation in aqueous solutions—an XAS study between 25°C and 250°C

Speciation of uranium (VI) in acetate solutions between 25 and 250°C, at pH values between 1.8 and 3.8 and acetate/uranium (Ac/U) ratios of 0.5 to 100 has been investigated using uranium L_III-edge X-ray absorption spectroscopy. With increasing pH the UO₂(Ac)₂⁰ species becomes more important than UO₂(Ac)⁺ species, which is predominant below pH 2. It remains the dominant species as pH is further increased to 3.8 at an Ac/U ratio of 20. Decrease in U-O_eq bond distance and coordination number with increasing solution age indicates that steric/kinetic factors are important and that equilibrium is attained slowly in this system with initial acetate coordination to the uranyl ion being monodentate or pseudo-bridging before slow conversion to bidentate chelation. Acetate coordination to the uranyl ion appears to decrease as temperature is increased from room temperature to ∼100°C before increasing in solutions of Ac/U > 2. For solutions where Ac/U ≤ 2 at pH 2.1, there is no evidence for uranyl acetate speciation at low temperatures, but at elevated temperature bidentate uranyl-acetate ion-pairing is evident. The existence of the uranyl acetate species in the temperature range 200 to 240°C demonstrates the importance of including acetate and other organic ligands in models of uranium transport at elevated temperatures.     

Liquid-vapor fractionation of boron and boron isotopes: Experimental calibration at 400°C/23 MPa to 450°C/42 MPa

We experimentally determined the boron partitioning and boron isotope fractionation between coexisting liquid and vapor in the system H₂O−NaCl−B₂O₃. Experiments were performed along the 400 and 450°C isotherms. Pressure conditions ranged from 23 to 28 MPa at 400°C and from 38 to 42 MPa at 450°C. Boron partitions preferentially into the liquid. Its overall liquid-vapor fractionation is, however, weak: Calculated boron distribution coefficients D_B^liquid-vapor are < 2.5 at all run conditions. With decreasing pressure (i.e. increasing opening of the solvus) D_B^liquid-vapor increases along the individual isotherms. Extrapolation to salt saturated conditions yields maximum boron liquid-vapor fractionations of D_B^liquid-vapor = 1.8 at 450°C and D_B^liquid-vapor = 2.7 at 400°C. ¹¹B preferentially fractionates into the vapor. Calculated Δ¹¹B_vapor-liquid = {[(¹¹B/¹⁰B)_vapor - (¹¹B/¹⁰B)_liquid]/(¹¹B/¹⁰B)_{NBS 951}}*1000 are small and range from 0.2 (± 0.7) to 0.9 (± 0.5) ‰ at 450°C and from 0.1 (± 0.6) to 0.7 (± 0.6) ‰ at 400°C. The data indicate increasing isotopic fractionation with decreasing pressure (i.e. increasing opening of the solvus). Extrapolation to salt saturated conditions yields maximum boron isotope liquid-vapor fractionations of Δ¹¹B_vapor-liquid = 1.5 (± 0.7) ‰ at 450°C and Δ¹¹B_vapor-liquid = 1.3 (± 0.6) ‰ at 400°C. The weak boron isotope fractionation suggests similar trigonal speciation in liquid and vapor. Although the boron and boron isotope fractionation between liquid and vapor is only weak, mass balance calculations indicate that for high degrees of fractionation liquid-vapor phase separation in an open system can significantly alter the boron and boron isotope signature of low-salinity hydrous fluids in hydrothermal systems. Comparing the model calculations with natural oceanic hydrothermal fluids, however, indicate that other processes than fluid phase separation dominate the boron geochemistry in oceanic hydrothermal fluids.     

Ammonium in aqueous fluids to 600 °C, 1.3 GPa: A spectroscopic study on the effects on fluid properties, silica solubility, and K-feldspar to muscovite reactions

The behavior of ammonium, NH₄⁺, in aqueous systems was studied based on Raman spectroscopic experiments to 600 °C and about 1.3 GPa. Spectra obtained at ambient conditions revealed a strong reduction of the dynamic three-dimensional network of water with addition of ammonium chloride, particularly at small solute concentrations. The differential scattering cross section of the ν₁-NH₄⁺ Raman band in these solutions was found to be similar to that of salammoniac.The Raman band of silica monomers at ∼780 cm⁻¹ was present in all spectra of the fluid at high temperatures in hydrothermal diamond-anvil cell experiments with H₂O ± NH₄Cl and quartz or the assemblage quartz + kyanite + K-feldspar ± muscovite/tobelite. However, these spectra indicated that dissolved silica is less polymerized in ammonium chloride solutions than in comparable experiments with water. Quantification based on the normalized integrated intensity of the H₄SiO₄⁰ band showed that the silica solubility in experiments with H₂O + NH₄Cl was significantly lower than that in equimolal NaCl solutions. This suggests that ammonium causes a stronger decrease in the activity of water in chloridic solutions than sodium.The Raman spectra of the fluid also showed that a significant fraction of ammonium was converted to ammonia, NH₃, in all experiments at temperatures above 300 °C. This indicates a shift towards acidic conditions for experiments without a buffering mineral assemblage. The estimated pH of the fluid was ∼2 at 600 °C, 0.26 GPa, 6.6 m initial NH₄Cl, based on the ratio of the integrated ν₁-NH₃ and ν₁-NH₄⁺ intensities and the HCl⁰ dissociation constant. The NH₃/NH₄⁺ ratio increased with temperature and decreased with pressure. This implies that more ammonium should be retained in K-bearing minerals coexisting with chloridic fluids upon high-P low-T metamorphism. At 500 °C, 0.73 GPa, ammonium partitions preferentially into the fluid, as constrained from infrared spectroscopy on the muscovite and from mass balance.The conversion of K-feldspar to muscovite proceeded much faster in experiments with NH₄Cl solutions than in comparable experiments with water. This is interpreted as being caused by enhancement of the rate-limiting alumina solubility, suggesting complexation of Al with NH₄. Nucleation and growth of mica at the expense of K-feldspar and NH₄⁺/K⁺ exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH₄⁺ into K-feldspar was distinctly faster than K-feldspar consumption.     

The effect of CO2 on the speciation of RbBr in solution at temperatures to 579 °C and pressures to 0.26 GPa

Carbon dioxide- and salt-bearing solutions are common in granulite, ore-forming and magmatic environments. The presence of CO₂ affects mineral solubilities, fluid miscibility, and viscosity and wetting properties, and is expected to affect salt speciation. EXAFS measurements of RbBr-H₂O-CO₂ fluids contained in corundum-osed synthetic fluid inclusions (SFLINCs) have been used to investigate the effect of CO₂ on salt speciation at temperatures to 579 °C and pressures to around 0.26 GPa.Forward modelling indicates that solute dehydration is difficult to distinguish from up to around 40% of Rb-Br ion-pairing, so results refer to the total number of nearest neighbours, which are likely to be mostly O present in waters of hydration, but may also include Br, if ion pairing is present. Additionally, results relate to the number of well-ordered neighbours in the first shell, because nearest neighbours with a high degree of disorder may be present but contribute minimally to the EXAFS signal. Analysis of the EXAFS results at the Rb edge for the CO₂-free solution is consistent with previous work and shows that the number of nearest neighbours for Rb in CO₂-free solutions decreases from 6 ± 0.6 to 1.4 ± 0.1 as temperature increases from 20 to 534 °C. The decrease is accompanied by a decrease in Rb-x bondlengths of 0.05 Å, where x is the first shell scatterer. Results for the CO₂-bearing solution are different to those for the CO₂-free solution. The number of nearest neighbours is 16 and 22% less than for the CO₂-bearing solution at 312 and 445 °C respectively. Changes in the numbers of nearest neighbours correlate well with calculated changes in the bulk solution dielectric constant; CO₂-bearing and CO₂-free solutions lie on the same trend, which suggests that it may be possible to calculate the number of nearest neighbours from dielectric constant. Rb-x bondlengths for the CO₂-bearing solution are statistically indistinguishable to those for the CO₂-free inclusions. Results for Br are worse quality than for Rb so EXAFS analysis could not be completed, however XANES spectra for CO₂-free and CO₂-bearing solutions are consistent with solute dehydration similar to that recorded by the Rb spectra. The conclusions of this study provide support for the notion that CO₂ has a fundamental effect on the mechanics of solubility, and that these effects should be incorporated into conceptual and quantitative thermodynamic models.     

Heat capacities of aqueous solutions of sodium hydroxide and water ionization up to 300 °C at 10 MPa

A commercial (Setaram C80) calorimeter has been modified to measure the heat capacities of highly caustic solutions at temperatures up to 300 °C and pressures up to 20 MPa. The improvements have allowed more accurate determination of the isobaric volumetric heat capacities of chemically aggressive liquids at high temperatures. Test measurements with aqueous solutions of sodium chloride showed a reproducibility of about ±0.1%, with an accuracy of ∼0.3% or better, over the whole temperature range. Heat capacities of aqueous solutions of sodium hydroxide at concentrations from 0.5 to 8 mol/kg were measured at temperatures from 50 to 300 °C and a pressure of 10 MPa. Apparent molar isobaric heat capacities of NaOH(aq) were calculated using densities determined previously for the same solutions by vibrating-tube densimetry. Standard state (infinite dilution) partial molar isobaric heat capacities of NaOH(aq) were obtained by extrapolation using an extended Redlich-Meyer equation. Values of the standard heat capacity change for the ionization of water up to 300 °C were derived by combining the present results with the literature data for HCl(aq) and NaCl(aq).     

Experimental determination and analysis of the solubility of corundum in 0.1-molal CaCl2 solutions between 400 and 600°C at 0.6 to 2.0 kbar

Corundum (α-Al₂O₃) solubility was measured in 0.1-molal CaCl₂ solutions from 400 to 600°C between 0.6 and 2.0 kbar. The Al molality at 2 kbar increases from 3.1 × 10⁻⁴ at 400°C to 12.7 × 10⁻⁴ at 600°C. At 1 kbar, the solubility increases from 1.5 × 10⁻⁴m at 400°C to 3.4 × 10⁻⁴m at 600°C. These molalities are somewhat less than corundum solubility in pure H₂O (Walther, 1997) at 400°C but somewhat greater at 600°C. The distribution of species was computed considering the Al species Al(OH)₃⁰ and Al(OH)₄⁻, consistent with the solubility of corundum in pure H₂O of Walther (1997) and association constants reported in the literature. The calculated solubility was greater than that measured except at 600°C and 2.0 kbar, indicating that neutral-charged species interactions are probably important.A Setchénow model for neutral species resulted in poor fitting of the measured values at 1.0 kbar. This suggests that Al(OH)₃⁰ has a greater stability relative to Al(OH)₄⁻ than given by the models of Pokrovskii and Helgeson (1995) or Diakonov et al. (1996). The significantly lower Al molalities in CaCl₂ relative to those in NaCl solutions at the same concentration confirm the suggestions of Walther (2001) and others that NaAl(OH)₄⁰ rather than an Al-Cl complex must be significant in supercritical NaCl solutions to give the observed increase in corundum solubility with increasing NaCl concentrations.     

Siderophore adsorption to and dissolution of kaolinite at pH 3 to 7 and 22°C

Siderophores are Fe(III)-specific ligands produced by many aerobic microorganisms under conditions of iron stress. This study examined adsorption of the commercial trihydroxamate siderophore, desferrioxamine B (DFO-B), to an iron-containing kaolinite (0.1 bulk wt.% Fe) and examined DFO-B effects on initial kaolinite dissolution and iron release rates. Adsorption experiments were conducted at pH 3 to 8 in 0.01-M NaClO₄ in the dark and at 22°C; batch initial dissolution experiments were conducted to 96 h.The adsorption envelope (i.e., adsorption as a function of pH) of DFO-B on kaolinite was consistent with cation-like behavior, with adsorption increasing above kaolinite’s pH_pznpc of 4.9. DFO-B enhanced aluminum release from kaolinite at pH 3 to 7, relative to HNO₃, which is consistent with the ligand’s high binding affinity for Al. Correlation between DFO-B adsorption and aluminum release suggested a surface-controlled, ligand-promoted dissolution mechanism. DFO-B had no effect relative to HNO₃ on silicon release at pH 3 and 5, but moderately enhanced silicon release at pH 7. DFO-B enhanced iron release from kaolinite, with dissolved iron concentrations in the 10-ppb range at 96-h reaction time. These results show that kaolinite may serve as a source of iron to aerobic microorganisms in iron-limited conditions and that siderophores may affect kaolinite dissolution and iron content.     

Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H₂O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as ≈1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, C_HCl^b, (3 × 10³ to 1.1 × 10⁴ ppm) is ≈40 ppm (by weight) and is independent of the HCl concentration under those conditions. For C_HCl^b of 1.1 × 10⁴ to 4.0 × 10⁴ ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log C_Au^b = [2.2 · log C_HCl^b] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations (< 1.1 × 10⁴ ppm), but that gold-chloride complexes, possibly AuCl₂H, are important at elevated concentrations of HCl (> 1.1 × 10⁴ ppm). The calculated Nernst partition coefficient (D_Au^b/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 10³ to 1.1 × 10⁴ ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents.     

Solubility of tin in (Cl, F)-bearing aqueous fluids at 700 °C, 140 MPa: A LA-ICP-MS study on synthetic fluid inclusions

Synthetic fluid inclusions in quartz were grown from cassiterite-saturated fluids in cold-seal pressure vessels at and subsequently analyzed by laser ablation-ICP-MS. Most inclusions were synthesized using a new technique that allows entrapment of fluid that had no immediate contact to the capsule walls, such that potential disequilibrium effects due to alloying could be avoided. Measured Sn solubilities increase with increasing ligand concentration in the fluid, ranging from 100 to 800 ppm in NaCl-bearing fluids (5-35 wt% NaCl), from 70 to 2000 ppm in HF-bearing fluids (0.5-3.2 m HF), and from 0.8 to 11 wt% in HCl-bearing fluids (0.5-4.4 m HCl). Two runs performed with the in-situ cracking method after 1 week of pre-equilibration demonstrate that the speed of hydrogen diffusion through the capsule wall relative to that of fluid inclusion formation is a critical factor in f_O2-dependent solubility studies. Graphical evaluation of the solubility data suggests that Sn may have been dissolved as Sn(OH)Cl in the NaCl-bearing fluids, as Sn(OH)Cl and SnCl₂ in the HCl-bearing fluids, and as SnF₂ in the HF-bearing fluids. Experiments with NaF-bearing fluids produced an additional melt phase with an approximate composition of 53 wt% SiO₂, 25 wt% H₂O, 14 wt% NaF and 8 wt% SnO, which caused the composition of the coexisting fluid to be buffered at 0.5 wt% NaF and 150 ppm Sn. Fluorine-rich, peralkaline melts may therefore serve as important transport media for Sn in the final crystallization stages of tin granites. Based on the available cassiterite-solubility data in fluids and melts, in natural granite systems is estimated to be in the order of 0.1-4 (depending on their aluminosity), suggesting that Sn is not easily mobilized by magmatic-hydrothermal fluids. This interpretation is in accordance with the high degrees of Sn-enrichment commonly observed in highly fractionated melt inclusions. is primarily controlled by the HCl concentration in the fluid, which in turn is a function of the aluminum saturation index of the magma. Compared to HCl, the effect of fluorine on is subordinate.     

Gold and copper partitioning in magmatic-hydrothermal systems at 800 °C and 100 MPa

Porphyry-type ore deposits sometimes contain fluid inclusion compositions consistent with the partitioning of copper and gold into vapor relative to coexisting brine at the depositional stage. However, this has not been reproduced experimentally at magmatic conditions. In an attempt to determine the conditions under which copper and gold may partition preferentially into vapor relative to brine at temperatures above the solidus of granitic magmas, we performed experiments at 800 °C, 100 MPa, oxygen fugacity () buffered by Ni-NiO, and fixed at either 3.5 × 10⁻² by using intermediate solid solution-pyrrhotite, or 1.2 × 10⁻⁴ by using intermediate solid solution-pyrrhotite-bornite. The coexisting vapor (∼3 wt.% NaCl eq.) and brine (∼68 wt.% NaCl eq.) were composed initially of NaCl + KCl + HCl + H₂O, with starting HCl set to <1000 μg/g in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped at run conditions and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our experiments demonstrate that copper and gold partitioned strongly into the magmatic volatile phase(s) (MVP) (i.e., vapor or brine) relative to a silicate melt over the entire imposed range of . Nernst style partition coefficients between coexisting brine (b) and melt (m), D^b/m (±1σ), range from 3.6(±2.2) × 10¹ to 4(±2) × 10² for copper and from 1.2(±0.6) × 10² to 2.4(±2.4) × 10³ for gold. Partition coefficients between coexisting vapor (v) and melt, D^v/m range from 2.1 ± 0.7 to 18 ± 5 and 7(±3) × 10¹ to 1.6(±1.6) × 10² for copper and gold, respectively. Partition coefficients for all experiments between coexisting brine and vapor, D^b/v (±1σ), range from 7(±2) to 1.0(±0.4) × 10² and 1.7(±0.2) to 15(±2) for copper and gold, respectively. Observed average D^b/v at an of 1.2 × 10⁻⁴ were elevated, 95(±5) and 15 ± 1 for copper and gold, respectively, relative to those at the higher of 3.5 × 10⁻² where D^b/v were 10(±5) for copper and 7(±6) for gold. Thus, there is an inverse relationship between the and the D^b/v for both copper and gold with increasing resulting in a decrease in the D^b/v signifying increased importance of the vapor phase for copper and gold transport. This suggests that copper and gold may complex with volatile S-species as well as Cl-species at magmatic conditions, however, none of the experiments of our study at 800 °C and 100 MPa had a D^b/v ? 1. We did not directly determine speciation, but infer the existence of some metal-sulfur complexes based on the reported data. We suggest that copper and gold partition preferentially into the brine in most instances at or above the wet solidus. However, in most systems, the mass of vapor is greater than the mass of brine, and vapor transport of copper and gold may become more important in the magmatic environment at higher , lower , or near the critical point in a salt-water system. A D^b/v ? 1 at subsolidus hydrothermal conditions may also occur in response to changes in temperature, , , and/or acidity.Additionally, both copper and gold were observed to partition into intermediate solid solution and bornite much more strongly than into vapor, brine or silicate melt. This suggests that, although vapor and brine are both efficient at removing copper and gold from a silicate melt, the presence of Cu-Fe sulfides can sequester a substantial portion of the copper and gold contained within a silicate melt if the Cu-Fe sulfides are abundant.     

Stability of chloridogold(I) complexes in aqueous solutions from 300 to 600°C and from 500 to 1800 bar

The solubility of gold has been measured in the system H₂O+H₂+HCl+NaCl+NaOH at temperatures from 300 to 600°C and pressures from 500 to 1800 bar in order to determine the stability and stoichiometry of chloride complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave system. This approach permitted the independent determination of the concentrations of all critical aqueous components in solution for the determination of the stability and stoichiometry of gold(I) complexes. The solubilities (i.e. total dissolved gold) were in the range 9.9 × 10⁻⁹ to 3.26 × 10⁻⁵ mol kg⁻¹ (0.002-6.42 mg kg⁻¹) in solutions of total dissolved chloride between 0.150 and 1.720 mol kg⁻¹, total dissolved sodium between 0.000 and 0.975 mol kg⁻¹ and total dissolved hydrogen between 4.34 × 10⁻⁶ and 7.87 × 10⁻⁴ mol kg⁻¹. A nonlinear least squares treatment of the data demonstrates that the solubility of gold in chloride solutions is accurately described by the reactions,

     

Kaolinite dissolution and precipitation kinetics at 22 °C and pH 4

Dissolution and precipitation rates of low defect Georgia kaolinite (KGa-1b) as a function of Gibbs free energy of reaction (or reaction affinity) were measured at 22 °C and pH 4 in continuously stirred flowthrough reactors. Steady state dissolution experiments showed slightly incongruent dissolution, with a Si/Al ratio of about 1.12 that is attributed to the re-adsorption of Al on to the kaolinite surface. No inhibition of the kaolinite dissolution rate was apparent when dissolved aluminum was varied from 0 and 60 μM. The relationship between dissolution rates and the reaction affinity can be described well by a Transition State Theory (TST) rate formulation with a Temkin coefficient of 2

     

Calcite solubility and speciation in supercritical NaCl-HCl aqueous fluids

The solubility of calcite in NaCl-H₂O and in HCl-H₂O fluids was measured using an extraction-quench hydrothermal apparatus. Experiments were conducted at 2 kbar, between 400° C and 600° C. Measurements in NaCl-H₂O were conducted in two ways: 1) at constant pressure and NaCl concentration, as a function of temperature; and 2) at constant pressure and temperature, as a function of NaCl concentration. In both the NaCl-H₂O and the HCl-H₂O systems, the solubility of calcite increases with increasing chlorine concentrations. For example, the log calcium molality in equilibrium with calcite increases from –3.75 at 2 kbar and 500° C, in pure H₂O to –3.10 at 2 kbar and 500° C at log NaCl molality=–1.67. At fixed pressure and NaCl molality, the solubility of calcite is almost constant from 400° C to 550° C, but increases somewhat at higher temperatures. The results can be used to determine the dominant calcium species in the experimental solutions as a function of NaCl concentration and to obtain values for the second dissociation constant of CaCl_2(aq). At 2 kbar, 400° C, 500° C, and 600° C, we calculate values for the log of the dissociation constant of CaCl⁺ of –2.1, –3.2, and –4.3, respectively. The 400° C and 500° C values are consistent with those obtained by Frantz and Marshall (1982) using electrical conductance techniques. However, our 600° C value is 0.8 log units higher than that reported by Frantz and Marshall. The calcite solubilities in the NaCl-H₂O and HCl-H₂O systems are inconsistent with the solubilities of calcite in pure H₂O reported by Walther and Long (1986). They are, however, consistent with the measurements of calcite solubilities in pure H₂O presented in this study. These results allow for the calculation of the solubilities of calcium silicates and carbonates in fluids that contain CO₂ and NaCl.     

Gold(I) complexing in aqueous sulphide solutions to 500°C at 500 bar

The solubility of gold has been measured in aqueous sulphide solutions from 100 to 500°C at 500 bar in order to determine the stability and stoichiometry of sulphide complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through system. The solubilities, measured as total dissolved gold, were in the range 3.6 × 10⁻⁸ to 6.65 × 10⁻⁴ mol kg⁻¹ (0.007-131 mg kg⁻¹), in solutions of total reduced sulphur between 0.0164 and 0.133 mol kg⁻¹, total chloride between 0.000 and 0.240 mol kg⁻¹, total sodium between 0.000 and 0.200 mol kg⁻¹, total dissolved hydrogen between 1.63 × 10⁻⁵ and 5.43 × 10⁻⁴ mol kg⁻¹ and a corresponding pH_{T, p} of 1.5 to 9.8. A non-linear least squares treatment of the data demonstrates that the solubility of gold in aqueous sulphide solutions is accurately described by the reactions

Au(s)+H₂S(aq)=AuHS(aq)+0.5H₂(g) K_s,100     

Speciation of reduced C-O-H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4 GPa and 800 °C

The structure of silicate melts in the system Na₂O·4SiO₂ saturated with reduced C-O-H volatile components and of coexisting silicate-saturated C-O-H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800 °C) and pressure (up to 1435 MPa). Redox conditions in the HDAC were controlled with the reaction, Mo + H₂O = MoO₂ + H₂, which is slightly more reducing than the Fe + H₂O = FeO + H₂ buffer at 800 °C and less.The dominant species in the fluid are CH₄ + H₂O together with minor amounts of molecular H₂ and an undersaturated hydrocarbon species. In coexisting melt, CH₃ - groups linked to the silicate melt structure via Si-O-CH₃ bonding may dominate and possibly coexists with molecular CH₄. The abundance ratio of CH₃ - groups in melts relative to CH₄ in fluids increases from 0.01 to 0.07 between 500 and 800 °C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400 °C and 730 MPa, respectively. A schematic solution mechanism is, Si-O-Si + CH₄?Si-O-CH₃+H-O-Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C-O-H) components will, therefore, affect melt properties in a manner resembling dissolved H₂O.     

The hydrolysis of gold(I) in aqueous solutions to 600°c and 1500 bar

The solubility of gold has been measured in aqueous solutions at temperatures between 300 and 600°C and pressures from 500 to 1500 bar to determine the stability and stoichiometry of the hydroxy complexes of gold(I) in hydrothermal solutions. The experiments were carried out using a flow-through autoclave system. The solubilities, measured as total dissolved gold, were in the range 1.2 × 10⁻⁸ to 2.0 × 10⁻⁶ mol kg⁻¹ (0.002 to 0.40 mg kg⁻¹), in solutions of total dissolved sodium between 0.0 and 0.5 mol kg⁻¹, and total dissolved hydrogen between 4.0 × 10⁻⁶ and 4.0 × 10⁻⁴ mol kg⁻¹. At constant hydrogen molality, the solubility of gold increases with increasing temperature and decreases with increasing pressure. The solubilities were found to be independent of pH but increased with decreasing hydrogen molality at constant temperature and pressure. Consequently, gold dissolves in aqueous solutions of acidic to alkaline pH according to the reactionAu(s)+H₂O(l)=AuOH(aq)+0.5H₂(g) K_s,1The solubility constant, logK_s,1, increases with increasing temperature from a minimum of −8.76 (±0.18) at 300°C and 500 bar to a maximum of −7.50 (±0.11) at 500°C and 1500 bar and decreases to −7.61 (±0.08) at 600°C and 1500 bar. From the equilibrium solubility constant and the redox potential of gold, the formation constant to form AuOH(aq) was calculated. At 25°C the complex formation is characterised by an exothermic enthalpy and a positive entropy. With increasing temperature and decreasing pressure, the formation reaction becomes endothermic and is accompanied by a large positive entropy, indicating a greater electrostatic interaction between Au⁺ and OH⁻.     

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H₂O ± NaCl ± HCl ± H₂S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As^(III) and As^(V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As^(III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As^(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P_sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H₃AsO₃) between an aqueous solution (pure H₂O) and its saturated vapor (K = mAs_vapor /mAs_liquid) were found to be independent of As^(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)₃(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H₂S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As^(III)-H₂O (±NaCl) solutions up to 500°C demonstrate that the As(OH)₃ complex dominates As speciation both in dense H₂O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As^(III) measured in the H₂O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic^(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As₂O₃ solid is consistent with the formation, in the vapor phase, of both As₄O₆ and As(OH)₃. Arsenic^(V) (arsenic acid, H₃AsO₄) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)₃ in the vapor phase.The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)₃(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)₃(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H₂O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.     

Arsenous acid ionisation in aqueous solutions from 25 to 300 °C

The ultraviolet spectra of dilute, aqueous arsenic (III)-containing solutions have been measured from 25 to 300 °C at the saturated vapour pressure. From these measurements, the equilibrium constant was obtained for the reaction

     

An integrated thermodynamic mixing model for sphalerite geobarometry from 300 to 850°C and up to 1 GPa

An asymmetric, Margules-type, solid solution model was used to model the mixing behavior of Fe-Zn sphalerites. The model is based on an analysis of experimental results from fifteen independent data sources. After a careful, stepwise, analysis of the available runs, the solid solution model was constrained using a refined experimental database of 279 experiments which were simultaneously regressed to obtain the excess parameters and a general geobarometric equation. The model indicates that, when pressures are low, the value of γ_FeS^Sph, which is always greater than one, is higher at low FeS contents and an increase in temperature causes it to decline. However, for certain compositions γ_FeS^Sph values might be slightly less at low T than at high T. This behavior is corrected when pressure increases, regardless of the composition. The excess Gibbs free energy has positive values at any P&T while it is asymmetric. Pressure increases the value of the excess free energy. On the other hand, the Gibbs free energy of mixing is always negative, with a single minimum that tends to move towards Fe-poorer compositions as the pressure goes up. An increase in temperature leads to a more negative Gibbs free energy mixing function suggesting that increasingly Fe-poorer sphalerite would be expected at high temperatures and pressures. The application of the solid solution model to a selection of case-studies provided results which are consistent with independent pressure estimates. However, the pressure determinations for sphalerite + pyrite + pyrrhotite and sphalerite + pyrrhotite assemblages are very sensitive to uncertainties in the composition of the phases involved and, to a lesser extent, to temperature. The results of the application of the model to a field case (scheelite-mineralized Hercynian veins from the Central Pyrenees) were also consistent when compared with independent pressure-constraining silicate assemblages. Thus, the solid solution model described in this paper provides a workable framework with which to compute the pressures of the formation of rocks over a wide range of geological conditions.