Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO₃⁻ and CO₃²⁻ concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO₃⁻, CO₃²⁻, and CO_2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO₃⁻, CO₃²⁻, and H₂O, NaHCO₃ solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl₂ was added to the NaHCO₃ solutions. This resulted in immediate BaCO₃ precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO₃⁻ and H₂O as a function of temperature is governed by the equation:

     

Mechanisms of equilibrium and kinetic oxygen isotope effects in synthetic aragonite at 25 °C

Aragonite was precipitated in the laboratory at 25 °C in isotopic equilibrium with Na-Ca-Mg-Cl-CO₃ solutions at two different pH values (i.e., pH = ∼8.2 and ∼10.8) by the constant addition method. On the basis of the oxygen isotope composition of the aragonite precipitates, it was demonstrated that the equilibrium aragonite-water fractionation factor is independent of the pH of the parent solution and equal to:

1000lnα_{(aragonite-H2O)}=29.12±0.09     

Platinum solubility in a haplobasaltic melt at 1250°C and 0.2 GPa: The effect of water content and oxygen fugacity

Haplobasaltic melts with a 101 kPa dry eutectic composition (An₄₂Di₅₈) and varying water contents were equilibrated with their platinum capsule at 1523 K and 200 MPa in an internally heated pressure vessel (IHPV) equipped with a rapid quench device. Experimental products were inclusion-free glasses representative of the Pt-saturated silicate melts at the experimental conditions. Platinum concentrations were determined using an isotope dilution multicollector inductively coupled plasma mass spectrometer and water contents and distribution by Karl Fischer titration and Fourier transform infrared spectroscopy, respectively.The water content of the melt has no intrinsic effect on platinum solubility, for concentrations between 0.9 wt.% and 4.4 wt.% H₂O (saturation). Platinum solubility increases with increasing water content, but this effect is an indirect effect because increasing water content at fixed f_H2 (imposed by the IHPV) increases the oxygen fugacity of the experiment.The positive oxygen fugacity dependence of Pt solubility in a hydrous silicate melt at 200 MPa is identical to that in anhydrous melts of the same composition determined in previous studies at 101 kPa. This study extends the range of platinum solubilities to oxygen fugacities lower than was previously possible. Combining the data of this and previous studies, Pt solubility is related to oxygen fugacity (in bar) at 1523 K by the equation:

[Pt]_total(ppb)=1389×f_O2+7531×(f_O2)^1/2     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

The kinetics of the dissolution of chlorite as a function of pH and at 25°C

Far from equilibrium, quasi-steady state dissolution rates of an iron rich chlorite (Mg_2.76Fe²⁺_1.90Fe³⁺_0.07Al_0.97)[Si_2.48Al_1.52O₁₀](OH)₈, have been measured as a function of H⁺ concentration for the pH range 3 to 10.5 and at 25°C. The rates were determined using a single pass flow through cell and with a time frame for observing the steady state condition of between 10 to 50 days. Rates are independent of the buffers used to control the pH, sample preparation, experimental methodology and chlorite composition. The results were collated with literature values allowing the rate to be expressed as a function of H⁺ as;

     

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.     

The effect of fluoride on the dissolution rates of natural glasses at pH 4 and 25°C

Far-from-equilibrium, steady-state dissolution rates at pH 4 of a suite of natural glasses, ranging from basaltic to rhyolitic in composition, have been determined as a function of aqueous fluoride concentrations up to 1.8 × 10⁻⁴ mol/kg in mixed-flow reactors. Dissolution rates of each of these glasses increase monotonically with increasing aqueous fluoride concentration. Measured dissolution rates are found to be consistent with both the Furrer and Stumm (1986) surface coordination model and the Oelkers (2001) multi-oxide dissolution model. Application of the latter model yields the following equation that can describe all measured rates as a function of both glass and aqueous solution composition:

     

The solubility of calcite,strontianite and witherite in NaCl solutions at 25°C

The solubility of CaCO₃ (calcite), SiCO₃ (strontianite), and BaCO₃ (witherite) has been determined in NaCl solutions from 0.1 to 6 m at 25°C. Activity coefficients estimated from Pitzer's equations with higher order interaction terms (θ and Ψ) were used to extrapolate the results to infinite dilution. Thermodynamic values of $\text{pK}{}_{\mathrm{sp}}\text{= 8.46 ± 0.03,9.13 ± 0.03}$ and $\text{8.56 ± 0.04}$ were found, respectively, for CaCO₃, SrCO₃ and BaCO₃ at 25°C. These results are in reasonable agreement with literature data. Since Pitzer parameters for the interactions of CO₃^2? with Ca²⁺, Sr²⁺ and Ba²⁺ were not used, our results indicate that they are not necessary at low values of $\text{P}{}_{\mathrm{co2}}$.     

Heat capacities of aqueous sodium hydroxide/aluminate mixtures and prediction of the solubility constant of boehmite up to 300 °C

A modified commercial (Setaram C80) calorimeter has been used to measure the isobaric volumetric heat capacities of concentrated alkaline sodium aluminate solutions at ionic strengths from 1 to 6 mol kg⁻¹, with up to 40 mol.% substitution of hydroxide by aluminate, at temperatures from 50 to 300 °C and a pressure of 10 MPa. Apparent molar heat capacities for the mixtures, C_p?, derived from these data were found to depend linearly on the aluminate substitution level, i.e., they followed Young’s rule. These quantities were used to estimate the apparent molar heat capacities of pure, hypothetical sodium aluminate solutions, C_p? (‘NaAl(OH)₄’(aq)). Slopes of the Young’s rule plots were invariant with ionic strength at a given temperature but depended linearly on temperature. The heat capacities of ternary aqueous sodium hydroxide/aluminate mixtures could therefore be modelled using only two parameters in addition to those needed for the correlation of C_p? (NaOH(aq)) reported previously from these laboratories. An assessment of the standard thermodynamic quantities for boehmite, gibbsite and the aluminate ion yielded a set of recommended values that, together with the present heat capacity data, accurately predicts the solubility of gibbsite and boehmite at temperatures up to 300 °C.     

Effect of pH and organic ligands on the kinetics of smectite dissolution at 25 °C

Forward dissolution rates of Na-Montmorillonite (Wyoming) SWy-2 smectite (Ca_0.06Na_0.56)[Al_3.08Fe(III)_0.38Mg_0.54] [Si_7.93 Al_0.07]O₂₀(OH)₄ were measured at 25 °C in a mixed-flow reactor equipped with interior dialysis compartment (6-8 kDa membrane) as a function of pH (1-12), dissolved carbonate (0.5-10 mM), phosphate (10⁻⁵ to 0.03 M), and nine organic ligands (acetate, oxalate, citrate, EDTA, alginate, glucuronic acid, 3,4-dihydroxybenzoic acid, gluconate, and glucosamine) in the concentration range from 10⁻⁵ to 0.03 M. In organic-free solutions, the Si-based rates decrease with increasing pH at 1 ? pH ? 8 with a slope close to −0.2. At 9 ? pH ? 12, the Si-based rates increase with a slope of ∼0.3. In contrast, non-stoichiometric Mg release weakly depends on pH at 1 ? pH ? 12 and decreases with increasing pH. The empirical expression describing Si-release rates [R, mol/cm²/s] obtained in the present study at 25 °C, I = 0.01 M is given by

     

The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350 °C and pressures up to 160 bars

The solubility and speciation of the assemblage MoO₂-MoO₃ in water vapour were investigated in experiments conducted at 350 °C, P_total from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10⁻⁴ to 6.5 × 10⁻² bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction

(A1)     

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH)₂) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H₂PO₄⁻ > catechol ≥ HCO₃⁻ > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO₄³⁻, CO₃²⁻, F⁻, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO₄²⁻ and B(OH)₄⁻ with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation.The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01-0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.     

The dissolution rates of natural glasses as a function of their composition at pH 4 and 10.6, and temperatures from 25 to 74°C

Far-from-equilibrium dissolution rates of a suite of volcanic glasses that range from basaltic to rhyolitic in composition were measured in mixed flow reactors at pH 4 and 10.6, and temperatures from 25 to 74°C. Experiments performed on glasses of similar composition suggest that dissolution rates are more closely proportional to geometric surface areas than their BET surface areas. Measured geometric surface area normalized dissolution rates (r_+,geo) at 25°C were found to vary exponentially with the silica content of the glasses. For pH 4 solutions this relation is given by:

(A1)     

Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25 °C

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 25 °C in batch and stirred flow-through reactors over the pH range of 1-13.5, in KNO₃ solutions. Dissolution rates were obtained based on the release of Si and Al at steady-state under far from equilibrium conditions. Dissolution was non-stoichiometric between pH 5 and 10, due to adsorption/precipitation of Al. Dissolution rates computed from batch and flow-through experiments were consistent, irrespective of the Si and Al concentrations. Sample pre-treatment and the interlayer cation do not affect the steady-state dissolution rate or stoichiometry of cation release. The rate dependence on pH can be described by:

     

Solubility of metallic mercury in octane, dodecane and toluene at temperatures between 100°C and 200°C

The solubility of metallic mercury in dodecane, octane and toluene has been investigated experimentally at temperatures up to 200°C and pressures up to 6 bars (toluene). The equilibrium Hg concentrations are very similar in octane and dodecane, reaching values of 821 ppm and 647 ppm, respectively at 200°C, whereas they are significantly lower in toluene (e.g., 280 ppm at 200°C). The behavior of Hg in toluene is nevertheless similar to that in the alkanes. There is a strong prograde dependence of Hg concentration on temperature in both types of solvent, which can be described by the following experimentally determined relationships:

     

Experimental measurement of the solubility of bismuth phases in water vapor from 220 °C to 300 °C: Implications for ore formation

Preliminary measurements were carried out of the solubility of the O_2-buffering assemblage bismuth + bismite (Bi₂O₃) in aqueous liquid–vapor and vapor-only systems at temperatures of 220, 250 and 300 °C. All experiments were carried out in Ti reaction vessels and were designed such that the Bi solids were contained in a silica tube that prevented contact with liquid water at any time during the experiment. Two blank (no Bi solids present) liquid–vapor experiments at 220 °C yielded Bi concentrations (±1σ) in the condensed liquid of 0.22 ± 0.02 mg/L, whereas the solubility measurements at this temperature yielded an average value of approximately 6 ± 9 mg/L, with replicate experiments ranging from 0.3 to 26 mg/L. Although the 6 mg/L value is associated with a considerable degree of uncertainty, the experiments do indicate transport of Bi through the vapor phase. Measured Bi concentrations in the condensed liquid at 250 °C were in the same range as those at 220 °C, whereas those at 300 °C were significantly lower (i.e., all below the blank value). Vapor-only experiments necessarily contained much smaller initial volumes of water, thereby making the results more susceptible to contamination. Single blank runs at 220 and 300 °C yielded Bi concentrations of 82 and 16 mg/L, respectively. Measured concentrations (±1σ) of Bi in the vapor-only solubility experiments at 220 °C were 235 ± 78 mg/L for an initial water volume of 0.5 mL, and at 300 °C were 56 ± 30 mg/L and 33 ± 21 for initial water volumes of 1 and 2 mL, respectively, suggesting strong preferential partitioning of Bi into the vapor. The results indicate a negative dependence of Bi solubility on temperature, but are inconclusive with respect to the dependence of Bi solubility on water density or fugacity.     

Kinetics and mechanism of natural fluorapatite dissolution at 25 °C and pH from 3 to 12

The dissolution rates of natural fluorapatite (FAP), Ca₁₀(PO₄)₆F₂, were measured at 25 °C in mixed-flow reactors as a function of pH from 3.0 to 11.7, and aqueous calcium, phosphorus, and fluoride concentration. After an initial preferential Ca and/or F release, stoichiometric Ca, P, and F release was observed. Measured FAP dissolution rates decrease with increasing pH at 3 ? pH ? 7, FAP dissolution rates are pH independent at 7 ? pH ? 10, and FAP dissolution rates again decrease with increasing pH at pH ? 10. Measured FAP dissolution rates are independent of aqueous Ca, P, and F concentration at pH ≈ 3 and pH ≈ 10.Apatite dissolution appears to be initiated by the relatively rapid removal from the near surface of F and the Ca located in the M1 sites, via proton for Ca exchange reactions. Dissolution rates are controlled by the destruction of this F and Ca depleted surface layer. The destruction of this layer is facilitated by the adsorption/penetration of protons into the surface at acidic conditions, and by surface hydration at neutral and basic conditions. Taking into account these two parallel mechanisms, measured fluorapatite forward dissolution rates can be accurately described using

     

UV-Vis spectrophotometric and XAFS studies of ferric chloride complexes in hyper-saline LiCl solutions at 25-90 °C

The speciation and thermodynamic properties of ferric chloride complexes in hydrothermal solutions and hypersaline brines are still poorly understood, despite the importance of this element as a micronutrient and ore-component. Available experimental data are limited to room temperature and relatively low chloride concentrations. This paper reports results of UV-Vis spectrophotometric and synchrotron XAFS experiments of ferric chloride complexes in chloride concentrations up to 15 m and at temperatures of 25-90 °C. Qualitative interpretation of the UV-Vis spectra shows that FeCl²⁺, FeCl₂⁺, FeCl_3(aq) and FeCl₄⁻ were present in the experimental solutions. As chloride concentrations increase, higher ligand number complexes become important with FeCl₄⁻ predominating in solutions containing more than 10 m at 25 °C. The predominance fields of FeCl_3(aq) and FeCl₄⁻ expand to lower Cl concentrations with increasing T. Both XANES and UV-Vis spectra reveal a major change in the geometry of the complex between FeCl₂⁺ and FeCl_3(aq). EXAFS data confirm that the number of chloride ligands increases with increasing chloride concentration and show that Fe³⁺, FeCl²⁺ and FeCl₂⁺ share an octahedral geometry. FeCl_3(aq) could be either tetrahedral or trigonal dipyramidal, while FeCl₄⁻ is expected to be tetrahedral. EXAFS data support a tetrahedral geometry for FeCl₄⁻, especially at 90 °C, but do not allow to distinguish between a tetrahedral or trigonal dipyramidal geometry for FeCl_3(aq) because of similar Fe-Cl distances. At room temperature, EXAFS data suggest that FeCl_3(aq) may be a mixture of octahedral and tetrahedral or trigonal dipyramidal forms.The room temperature formation constants for three ferric chloride complexes (FeCl₂⁺, FeCl_3(aq) and FeCl₄⁻) determined from the UV data are generally in good agreement with previous studies. Calculations based on the properties extrapolated to 300 °C show that hematite solubility is much higher than previously estimated, and that the high orders complexes FeCl_3(aq) and FeCl₄⁻ are important at high temperatures even in solutions with low chloride concentrations. The accuracy of these properties is limited by a poor understanding of activity-composition relationships in concentrated electrolytes, and by limitations in the available experimental techniques and extrapolation algorithms; however, the inclusion of higher order complexes in numerical models of ore transport and deposition allows for a more accurate qualitative prediction of Fe behaviour in hydrothermal and hypersaline systems.     

Controls on gold solubility in arc magmas: An experimental study at 1000 °C and 4 kbar

In order to (1) explain the worldwide association between epithermal gold-copper-molybdenum deposits and arc magmas and (2) test the hypothesis that adakitic magmas would be Au-specialized, we have determined the solubility of Au at 4 kbar and 1000 °C for three intermediate magmas (two adakites and one calc-alkaline composition) from the Philippines. The experiments were performed over a fO₂ range corresponding to reducing (∼NNO−1), moderately oxidizing (∼NNO+1.5) and strongly oxidizing (∼NNO+3) conditions as measured by solid Ni-Pd-O sensors. They were carried out in gold containers, the latter serving also as the source of gold, in presence of variable amounts of H₂O and, in a few additional experiments, of S. Concentrations of Au in glasses were determined by LA-ICPMS. Gold solubility in melt is very low (30-240 ppb) but increases with fO₂ in a way consistent with the dissolution of gold as both Au¹⁺ and Au³⁺ species. In the S-bearing experiments performed at ∼NNO−1, gold solubility reaches much higher values, from ∼1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and the non-adakitic compositions investigated. Oxygen fugacity and the sulfur concentration in melt are the main parameters controlling the incorporation and concentration of gold in magmas. Certain adakitic and non-adakitic magmas have high fO₂ and magmatic S concentrations favorable to the incorporation and transport of gold. Therefore, the cause of a particular association between some arc magmas and Au-Cu-Mo deposits needs to be searched in the origin of those specialized magmas by involvement of Au- and S-rich protoliths. The subducted slab, which contains metal-rich massive sulfides, may constitute a potentially favorable protolith for the genesis of magmas specialized with respect to gold.