A simple predictive model of quartz solubility in water-salt-CO2 systems at temperatures up to 1000 °C and pressures up to 1000 MPa

Knowledge of the solubility of quartz over a broad spectrum of aqueous fluid compositions and T-P conditions is essential to our understanding of water-rock interaction in the Earth’s crust. We propose an equation to compute the molality of aqueous silica, m_SiO2(aq), mol·(kg H₂O)⁻¹, in equilibrium with quartz and water-salt-CO₂ fluids, as follows:

     

Experimental determination of the solubility of natural wollastonite in pure water up to pressures of 5 GPa and at temperatures of 400-800 °C

The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO₃) has been determined at temperatures from 400 to 800 °C and pressures between 0.8 and 5 GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure-temperature range investigated. The solubility of CaSiO₃ varies between 0.175 and 13.485 wt% and increases systematically with both temperature and pressure up to 3.0 GPa. Above 3.0 GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61 wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is then

log(m_woll)=2.2288-3418.23×T^-1+671386.84×T^-2+logρ_H2O×(5.4578+2359.11×T^-1).     

Dissolution kinetics of fosteritic olivine at 90-150 °C including effects of the presence of CO2

The steady state dissolution rate of San Carlos olivine [Mg_1.82Fe_0.18 SiO₄] in dilute aqueous solutions was measured at 90, 120, and 150 °C and pH ranging from 2 to 12.5. Dissolution experiments were performed in a stirred flow-through reactor, under either a nitrogen or carbon dioxide atmosphere at pressures between 15 and 180 bar. Low pH values were achieved either by adding HCl to the solution or by pressurising the reactor with CO₂, whereas high pH values were achieved by adding LiOH. Dissolution was stoichiometric for almost all experiments except for a brief start-up period. At all three temperatures, the dissolution rate decreases with increasing pH at acidic to neutral conditions with a slope of close to 0.5; by regressing all data for 2 ? pH ? 8.5 and 90 °C ? T ? 150 °C together, the following correlation for the dissolution rate in CO₂-free solutions is obtained:

     

Microbial dissolution of calcite at T = 28 °C and ambient pCO2

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO₂ over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH₄⁺ as an N source, and H₂PO₄⁻ as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H₂O-CO₂-CaCO₃ system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH₄⁺ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H₂CO₃ generated by dissolution of atmospheric CO₂ (H₂CO₃ + CaCO₃ → Ca²⁺ + 2HCO₃⁻) and H⁺ released during NH₄⁺ uptake (H⁺ + CaCO₃ → Ca²⁺ + HCO₃⁻). Reaction with H₂CO₃ and H⁺ supplied ∼45% and 55% of the total Ca²⁺ and ∼60% and 40% of the total HCO₃⁻, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH₄⁺ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H₂CO₃. In lactate bearing reactors, most H⁺ generated by NH₄⁺ uptake reacted with HCO₃⁻ produced by lactate oxidation to yield CO₂ and H₂O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H₂CO₃ at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH₄⁺ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.     

Fugacities and free energies of CO2 at high pressures and temperatures

Fugacity and free-energy values have been calculated from p-v-t data for CO₂ in the temperature range from 50 to 1000°C, at pressures from 25 to 1400 bars.     

An experimental study of the solubility of molybdenum in H2O and KCl-H2O solutions from 500 °C to 800 °C, and 150 to 300 MPa

The solubility of molybdenum (Mo) was determined at temperatures from 500 °C to 800 °C and 150 to 300 MPa in KCl-H₂O and pure H₂O solutions in cold-seal experiments. The solutions were trapped as synthetic fluid inclusions in quartz at experimental conditions, and analyzed by laser ablation inductively coupled plasma mass spectrometry (LA ICPMS).Mo solubilities of 1.6 wt% in the case of KCl-bearing aqueous solutions and up to 0.8 wt% in pure H₂O were found. Mo solubility is temperature dependent, but not pressure dependent over the investigated range, and correlates positively with salinity (KCl concentration). Molar ratios of ∼1 for Mo/Cl and Mo/K are derived based on our data. In combination with results of synchrotron X-ray absorption spectroscopy of individual fluid inclusions, it is suggested that Mo-oxo-chloride complexes are present at high salinity (>20 wt% KCl) and ion pairs at moderate to low salinity (<11 wt% KCl) in KCl-H₂O aqueous solutions. Similarly, in the pure H₂O experiments molybdic acid is the dominant species in aqueous solution. The results of these hydrothermal Mo experiments fit with earlier studies conducted at lower temperatures and indicate that high Mo concentrations can be transported in aqueous solutions. Therefore, the Mo concentration in aqueous fluids seems not to be the limiting factor for ore formation, whereas precipitation processes and the availability of sulfur appear to be the main controlling factors in the formation of molybdenite (MoS₂).     

Experimental investigation of the solubility of albite and jadeite in H2O, with paragonite + quartz at 500 and 600 °C, and 1-2.25 GPa

The solubilities of the assemblages albite + paragonite + quartz and jadeite + paragonite + quartz in H₂O were determined at 500 and 600 °C, 1.0-2.25 GPa, using hydrothermal piston-cylinder methods. The three minerals are isobarically and isothermally invariant in the presence of H₂O, so fluid composition is uniquely determined at each pressure and temperature. A phase-bracketing approach was used to achieve accurate solubility determinations. Albite + quartz and jadeite + quartz dissolve incongruently in H₂O, yielding residual paragonite which could not be retrieved and weighed. Solution composition fixed by the three-mineral assemblage at a given pressure and temperature was therefore bracketed by adding NaSi₃O_6.5 glass in successive experiments, until no paragonite was observed in run products. Solubilities derived from experiments bounding the appearance of paragonite thus constrain the equilibrium fluid composition. Results indicate that, at a given pressure, Na, Al, and Si concentrations are higher at 600 °C than at 500 °C. At both 500 and 600 °C, solubilities of all three elements increase with pressure in the albite stability field, to a maximum at the jadeite-albite-quartz equilibrium. In the jadeite stability field, element concentrations decline with continued pressure increase. At the solubility maximum, Na, Al, and Si concentrations are, respectively, 0.16, 0.05, and 0.48 molal at 500 °C, and 0.45, 0.27, and 1.56 molal at 600 °C. Bulk solubilities are 3.3 and 10.3 wt% oxides, respectively. Observed element concentrations are everywhere greater than those predicted from extrapolated thermodynamic data for simple ions, monomers, ion pairs, and the silica dimer. The measurements therefore require the presence of additional, polymerized Na-Al-Si-bearing species in the solutions. The excess solubility is >50% at all conditions, indicating that polymeric structures are the predominant solutes in the P-T region studied. The solubility patterns likely arise from combination of the large solid volume change associated with the albite-jadeite-quartz equilibrium and the rise in Na-Al-Si polymerization with approach to the hydrothermal melting curves of albite + quartz and jadeite + quartz. Our results indicate that polymerization of Na-Al-Si solutes is a fundamental aspect of fluid-rock interaction at high pressure. In addition, the data suggest that high-pressure metamorphic isograds can impose unexpected controls on metasomatic mass transfer, that significant metasomatic mass transfer prior to melting should be considered in migmatitic terranes, and that polymeric complexes may be an important transport agent in subduction zones.     

Boron speciation in aqueous fluids at 22 to 600°C and 0.1 MPa to 2 GPa

The speciation of boron in H₂O+H₃BO₃±NaCl and H₂O+Na₂B₄O₇ fluids was studied in situ at temperatures between 22 and 600°C and pressures from 0.1 MPa to ∼2 GPa using Raman spectroscopy and a hydrothermal diamond anvil cell. Additionally, we determined the frequency shifts of the 877 cm⁻¹ Raman line of [B(OH)₃]⁰ in aqueous fluids with temperature (∂ν₈₇₇/∂T)_{p = 0.1 MPa} = −0.02532 cm⁻¹K⁻¹ and pressure (∂ν₈₇₇/∂P)_{T = 22°C} = 4.06 cm⁻¹GPa⁻¹. The observed species in acidic fluids were [B(OH)₃]⁰ and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO₂(aq). The ratio of this B^[4]-O species to [B(OH)₃]⁰ increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)₃]⁰ and [B(OH)₄]⁻ were the only remaining detectable species at 500 and 600°C. The Raman spectra showed an increase of [B(OH)₃]⁰ relative to [B(OH)₄]⁻ with temperature and an increase of [B(OH)₄]⁻ relative to [B(OH)₃]⁰ with pressure.The general trend in the boron speciation is a higher stability of simpler complexes with temperature. The experimental observations strongly indicate that planar three-coordinated [B(OH)₃]⁰ is the predominant boron species in the aqueous phase over a wide range of P-T-pH conditions. This supports the validity of previous assumptions on boron coordination in crustal and mantle wedge fluids.     

Molecular dynamics simulation of the CH4 and CH4-H2O systems up to 10 GPa and 2573 K

Based on our previous study of the intermolecular potential for pure H₂O and the strict evaluation of the competitive potential models for pure CH₄ and the ab initio fitting potential surface across CH₄-H₂O molecules in this study, we carried out more than two thousand molecular dynamics simulations for the PVTx properties of pure CH₄ and the CH₄-H₂O mixtures up to 2573 K and 10 GPa. Comparison of 1941 simulations with experimental PVT data for pure CH₄ shows an average deviation of 0.96% and a maximum deviation of 2.82%. The comparison of the results of 519 simulations of the mixtures with the experimental measurements reveals that the PVTx properties of the CH₄-H₂O mixtures generally agree with the extensive experimental data with an average deviation of 0.83% and 4% in maximum, which is equivalent to the experimental uncertainty. Moreover, the maximum deviation between the experimental data and the simulation results decreases to about 2% as temperature and pressure increase, indicating that the high accuracy of the simulation is well retained in the high temperature and pressure region.After the validation of the simulation method and the intermolecular potential models, we systematically simulated the PVTx properties of this binary system from 673 K and 0.05 GPa to 2573 K and 10 GPa. In order to integrate all the simulation results and the experimental data for the calculation of thermodynamic properties, an equation of state (EOS) is developed for the CH₄-H₂O system covering 673-2573 K and 0.01-10 GPa. Isochores for compositions <4 mol% CH₄ up to 773 K and 600 MPa are also determined in this paper. The program for the EOS can be downloaded from www.geochem-model.org/programs.htm.     

Coupled phase and aqueous species equilibrium of the H2O-CO2-NaCl-CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite

A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H₂O-CO₂-NaCl-CaCO₃ system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor-liquid-solid (calcite, halite) equilibrium together with the chemical equilibrium of H⁺, Na⁺, Ca²⁺, , Ca(OH)⁺, OH⁻, Cl⁻, , , CO_2(aq) and CaCO_3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO_2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T-P-m range, hence calcite solubility, CO₂ gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data.One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO₂ can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility.The functionality of pH value, alkalinity, CO₂ gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl_(aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

Kinetics of the reaction MgO + Al2O3 → MgAl2O4 and Al-Mg interdiffusion in spinel at 1200 to 2000°C and 1.0 to 4.0 GPa

The rate of spinel (MgAl₂O₄) growth at the interface between MgO and Al₂O₃ was investigated systematically at temperatures of 1200° to ∼2000°C and pressures between 1.0 and 4.0 GPa with a solid-media, piston-cylinder apparatus. As reported in previous 1-atm studies, the thickness (ΔX) of the spinel layer increases linearly with the square root of time for experiments differing only in duration, irrespective of pressure-temperature (P-T) conditions. The reaction rate constant (k = ΔX²/2t) is log-linear in 1/T and also in pressure. The apparent activation energy of 410 kJ/mol is independent of pressure; the apparent activation volume increases systematically with increasing temperature. Electron microprobe traverses across the spinel layer reveal a significant Al excess and charge-compensating Mg deficit near the spinel/corundum interface. This nonstoichiometry is promoted by high temperatures (>1500°C), suppressed by high pressures and varies linearly across the spinel to a near-stoichiometric composition at the interface with periclase. The Al and Mg composition gradients can be used to extract interdiffusion coefficients for Al ↔ Mg exchange through the spinel, which are described by D?=2.5×10⁻⁶ exp(−28200/T) m²sThese diffusivities differ substantially from the reaction rate constant k, reflecting the fact that k is a combination of the diffusivity and the reaction potential as indicated by the difference in spinel composition across the spinel layer (i.e., coexisting with corundum vs. coexisting with periclase). A simple model can be used to separate the two effects and show that the reaction potential (i.e., the MgO-Al₂O₃ phase diagram) is sensitive to changes in both temperature and pressure, whereas the governing diffusivity depends only on temperature.     

A thermodynamic model for the prediction of phase equilibria and speciation in the H2O-CO2-NaCl-CaCO3-CaSO4 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to halite saturation

A thermodynamic model is developed for the calculation of both phase and speciation equilibrium in the H₂O-CO₂-NaCl-CaCO₃-CaSO₄ system from 0 to 250 °C, and from 1 to 1000 bar with NaCl concentrations up to the saturation of halite. The vapor-liquid-solid (calcite, gypsum, anhydrite and halite) equilibrium together with the chemical equilibrium of H⁺,Na⁺,Ca²⁺, , , and CaSO_4(aq) in the aqueous liquid phase as a function of temperature, pressure and salt concentrations can be calculated with accuracy close to the experimental results.Based on this model validated from experimental data, it can be seen that temperature, pressure and salinity all have significant effects on pH, alkalinity and speciations of aqueous solutions and on the solubility of calcite, halite, anhydrite and gypsum. The solubility of anhydrite and gypsum will decrease as temperature increases (e.g. the solubility will decrease by 90% from 360 K to 460 K). The increase of pressure may increase the solubility of sulphate minerals (e.g. gypsum solubility increases by about 20-40% from vapor pressure to 600 bar). Addition of NaCl to the solution may increase mineral solubility up to about 3 molality of NaCl, adding more NaCl beyond that may slightly decrease its solubility. Dissolved CO₂ in solution may decrease the solubility of minerals. The influence of dissolved calcite on the solubility of gypsum and anhydrite can be ignored, but dissolved gypsum or anhydrite has a big influence on the calcite solubility. Online calculation is made available on www.geochem-model.org/model.     

The system H2O-NaCl. Part I: Correlation formulae for phase relations in temperature-pressure-composition space from 0 to 1000 °C, 0 to 5000 bar, and 0 to 1 XNaCl

Realistic simulations of fluid flow in geologic systems have severely been hampered by the lack of a consistent formulation for fluid properties for binary salt-water fluids over the temperature-pressure-composition ranges encountered in the Earth’s crust. As the first of two companion studies, a set of correlations describing the phase stability relations in the system H₂O-NaCl is developed. Pure water is described by the IAPS-84 equation of state. New correlations comprise the vapor pressure of halite and molten NaCl, the NaCl melting curve, the composition of halite-saturated liquid and vapor, the pressure of vapor + liquid + halite coexistence, the temperature-pressure and temperature-composition relations for the critical curve, and the compositions of liquid and vapor on the vapor + liquid coexistence surface. The correlations yield accurate values for temperatures from 0 to 1000 °C, pressures from 0 to 5000 bar, and compositions from 0 to 1 X_NaCl (mole fraction of NaCl). To facilitate their use in fluid flow simulations, the correlations are entirely formulated as functions of temperature, pressure and composition.     

Characterization of elemental release during microbe-granite interactions at T = 28 °C

This study used batch reactors to characterize the mechanisms and rates of elemental release (Al, Ca, K, Mg, Na, F, Fe, P, Sr, and Si) during interaction of a single bacterial species (Burkholderia fungorum) with granite at T = 28 °C for 35 days. The objective was to evaluate how actively metabolizing heterotrophic bacteria might influence granite weathering on the continents. We supplied glucose as a C source, either NH₄ or NO₃ as N sources, and either dissolved PO₄ or trace apatite in granite as P sources. Cell growth occurred under all experimental conditions. However, solution pH decreased from ∼7 to 4 in NH₄-bearing reactors, whereas pH remained near-neutral in NO₃-bearing reactors. Measurements of dissolved CO₂ and gluconate together with mass-balances for cell growth suggest that pH lowering in NH₄-bearing reactors resulted from gluconic acid release and H⁺ extrusion during NH₄ uptake. In NO₃-bearing reactors, B. fungormum likely produced gluconic acid and consumed H⁺ simultaneously during NO₃ utilization.Over the entire 35-day period, NH₄-bearing biotic reactors yielded the highest release rates for all elements considered. However, chemical analyses of biomass show that bacteria scavenged Na, P, and Sr during growth. Abiotic control reactors followed different reaction paths and experienced much lower elemental release rates compared to biotic reactors. Because release rates inversely correlate with pH, we conclude that proton-promoted dissolution was the dominant reaction mechanism. Solute speciation modeling indicates that formation of Al-F and Fe-F complexes in biotic reactors may have enhanced mineral solubilities and release rates by lowering Al and Fe activities. Mass-balances further reveal that Ca-bearing trace phases (calcite, fluorite, and fluorapatite) provided most of the dissolved Ca, whereas more abundant phases (plagioclase) contributed negligible amounts. Our findings imply that during the incipient stages of granite weathering, heterotrophic bacteria utilizing glucose and NH₄ only moderately elevate silicate weathering reactions that consume atmospheric CO₂. However, by enhancing the dissolution of non-silicate, Ca-bearing trace minerals, they could contribute to high Ca/Na ratios commonly observed in granitic watersheds.     

Characterization of elemental release during microbe-basalt interactions at T = 28 °C

This study used batch reactors to characterize the rates and mechanisms of elemental release during the interaction of a single bacterial species (Burkholderia fungorum) with Columbia River Flood Basalt at T = 28 °C for 36 days. We primarily examined the release of Ca, Mg, P, Si, and Sr under a variety of biotic and abiotic conditions with the aim of evaluating how actively metabolizing bacteria might influence basalt weathering on the continents. Four days after inoculating P-limited reactors (those lacking P in the growth medium), the concentration of viable planktonic cells increased from ∼10⁴ to 10⁸ CFU (Colony Forming Units)/mL, pH decreased from ∼7 to 4, and glucose decreased from ∼1200 to 0 μmol/L. Mass-balance and acid-base equilibria calculations suggest that the lowered pH resulted from either respired CO₂, organic acids released during biomass synthesis, or H⁺ extrusion during uptake. Between days 4 and 36, cell numbers remained constant at ∼10⁸ CFU/mL and pH increased to ∼5. Purely abiotic control reactors as well as control reactors containing inert cells (∼10⁸ CFU/mL) showed constant glucose concentrations, thus confirming the absence of biological activity in these experiments. The pH of all control reactors remained near-neutral, except for one experiment where the pH was initially adjusted to 4 but rapidly rose to 7 within 2 days. Over the entire 36 day period, P-limited reactors containing viable bacteria yielded the highest Ca, Mg, Si, and Sr release rates. Release rates inversely correlate with pH, indicating that proton-promoted dissolution was the dominant reaction mechanism. Both biotic and abiotic P-limited reactors displayed low P concentrations. Chemical analyses of bacteria collected at the end of the experiments, combined with mass-balances between the biological and fluid phases, demonstrate that the absence of dissolved P in the biotic reactors resulted from microbial P uptake. The only P source in the basalt is a small amount of apatite (∼1.2%), which occurs as needles within feldspar grains and glass. We therefore conclude that B. fungorum utilized apatite as a P source for biomass synthesis, which stimulated elemental release from coexisting mineral phases via pH lowering. The results of this study suggest that actively metabolizing bacteria have the potential to influence elemental release from basalt in continental settings.     

Dissociation constants and speciation in aqueous Li2SO4 and K2SO4 from measurements of electrical conductance to 673 K and 29 MPa

The electrical conductivities of aqueous solutions of Li₂SO₄ and K₂SO₄ have been measured at 523-673 K at 20-29 MPa in dilute solutions for molalities up to 2 × 10⁻² mol kg⁻¹. These conductivities have been fitted to the conductance equation of Turq, Blum, Bernard, and Kunz with a consensus mixing rule and mean spherical approximation activity coefficients. In the temperature interval 523-653 K, where the dielectric constant, ε, is greater than 14, the electrical conductance data can be fitted by a solution model which includes ion association to form , , and , where M is Li or K. The adjustable parameters of this model are the first and second dissociation constants of the M₂SO₄. For the 673 K and 300 kg m⁻³ state point where the Coulomb interactions are the strongest (dielectric constant, ε = 5), models with more extensive association give good fits to the data. In the case of the Li₂SO₄ model, including the multi-ion associate, , gave an extremely good fit to the conductance data.     

Equation of state of the H2O, CO2, and H2O-CO2 systems up to 10 GPa and 2573.15 K: Molecular dynamics simulations with ab initio potential surface

Based on our previous development of the molecular interaction potential for pure H₂O and CO₂ [Zhang, Z.G., Duan, Z.H. 2005a. Isothermal-isobaric molecular dynamics simulations of the PVT properties of water over wide range of temperatures and pressures. Phys. Earth Planet Interiors149, 335-354; Zhang, Z.G., Duan, Z.H. 2005b. An optimized molecular potential for carbon dioxide. J. Chem. Phys.122, 214507] and the ab initio potential surface across CO₂-H₂O molecules constructed in this study, we carried out more than one thousand molecular dynamics simulations of the PVTx properties of the CO₂-H₂O mixtures in the temperature-pressure range from 673.15 to 2573.15 K up to 10.0 GPa. Comparison with extensive experimental PVTx data indicates that the simulated results generally agree with experimental data within 2% in density, equivalent to experimental uncertainty. Even the data under the highest experimental temperature-pressure conditions (up to 1673 K and 1.94 GPa) are well predicted with the agreement within 1.0% in density, indicating that the high accuracy of the simulation is well retained as the temperature and pressure increase. The consistent and stable predictability of the simulation from low to high temperature-pressure and the fact that the molecular dynamics simulation resort to no experimental data but to ab initio molecular potential makes us convinced that the simulation results should be reliable up to at least 2573 K and 10 GPa with errors less than 2% in density. In order to integrate all the simulation results of this study and previous studies [Zhang and Duan, 2005a, 2005b] and the experimental data for the calculation of volumetric properties (volume, density, and excess volume), heat properties, and chemical properties (fugacity, activity, and possibly supercritical phase separation), an equation of state (EOS) is laboriously developed for the CO₂, H₂O, and CO₂-H₂O systems. This EOS reproduces all the experimental and simulated data covering a wide temperature and pressure range from 673.15 to 2573.15 K and from 0 to 10.0 GPa within experimental or simulation uncertainty.     

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 partitioning of molybdenum(VI) between aqueous liquid and vapour at temperatures up to 370 °C

We have conducted experiments to evaluate the vapour-liquid fractionation of Mo(VI) in the system MoO₃-NH₃-H₂O at 300-370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH ? 4. Molybdenum speciation in the vapour is described by a single complex, MoO₃H₂O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO₄⁻) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO₄²⁻) and molybdic acid (H₂MoO₄⁰) are present in smaller quantities. As vapour-liquid fractionation occurs between neutral species, it is governed by the reaction H₂MoO₄⁰(aq) = MoO₃ · H₂O(g). Fractionation is therefore controlled by the concentration of H₂MoO₄⁰ in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of , it cannot be used to describe Mo fractionation in aqueous vapour-liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H₂MoO₄⁰ in the liquid. This constant may be applied to aqueous vapour-liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite-microcline reaction boundary.