Solubility of silica polymorphs in electrolyte solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary Paris Basin

Activity coefficient for aqueous silica in saline waters and brines from the Paris Basin was calculated using Pitzer's specific interaction model. Quartz and chalcedony are the only reported authigenic silica minerals in the Dogger aquifer of the Paris Basin (France). However, the measured silica concentrations fall between those of these two phases. The silica concentrations measured in Dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains. This hypothesis is supported by other studies reported in the literature. The proposed model, which incorporates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the deep sedimentary solutions of the Dogger aquifers. In the Keuper brines, silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz. Another application of the present work is shown by an example, where we examined the importance of precisely evaluating the activity coefficient in basin characterisation, as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.     

Amorphous silica solubilities—I. Behavior in aqueous sodium nitrate solutions; 25–300°C, 0–6 molal

The solubility of amorphous silica was obtained in aqueous sodium nitrate solutions up to six molal and at temperatures from 25 to 300°C. It was expected that solubilities in aqueous sodium chloride solutions would be similar. At 25°C, the solubility of amorphous silica is lowered from that in water to 0.00086 m in 6.12 m sodium nitrate, or a decrease of 60%. At 300°C, the corresponding decrease is only 27% from a solubility of 0.0269 m in H₂O. From the change in solubility with temperature at a given constant molality of sodium nitrate, the molal heat of solution over the range, 100 to 300°C, increases from + 2.93 kcal mol^?1 in water to + 3.64 kcal mol^?1 in 6m sodium nitrate. The value approaches a constant of +3.8 kcal mol^?1 as sodium nitrate approaches saturation at 10.8 molal.     

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

     

Amorphous silica solubilities—III. Activity coefficient relations and predictions of solubility behavior in salt solutions, 0–350°C

Activity coefficients of amorphous silica in added salt solutions, obtained from solubility measurements, were calculated on the basis of concentration (molarity) and composition (molality). The logarithm of the activity coefficients based on concentration when plotted against molarity of added salt, as previous observations would dictate, yielded straight line relations even to 6 M. The use of composition units produced instead curves at the highest molalities and caused divergences in behavior from the single straight lines for 1-1 salts of a common cation. Introduction of the activity of water also caused divergences in behavior. Thus, it might appear that amorphous silica is fully hydrated at 25°C as a solid in equilibrium with solution. Lack of densities of aqueous sodium nitrate necessitated using units of molality in interpretations at high temperatures. Nevertheless, approximately straight line behavior of log activity coefficient vs m was still observed to reasonably high molalities, and allowed a simple equation to describe activity coefficient behavior as a function of both molality of sodium nitrate and temperature. The simplicity of the relation, with some assumptions, might be used to predict solubility of amorphous silica at high temperatures in other added aqueous salt solutions.     

Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar

The solubility of the high grade pelite assemblage albite+K-feldspar+andalusite+quartz at 650 °C and 2 kbar was determined in aqueous solutions over a total chloride range of 0.01–3 m_Cl^tot using rapid-quench hydrothermal technique. The concentration of Na, K, Si, and Al was determined in the fluid phase after quench. The K/Na ratio was determined by approaching the equilibrium from below and above. It is 0.34 at low chloride concentrations and decreases slightly to 0.31 with increasing total chloride. Silica and aluminum concentrations were determined only from undersaturation. The silica solubility is found to be independent of chloride concentration and is 0.13 molal. Aluminum is nearly independent of chloride concentration decreasing only slightly from 0.0015 to 0.0007 molal. Comparison of the experimental data with thermodynamic model calculations demonstrates that the silica concentrations are well predicted, while significant differences exist between individual databases for Al speciation and its total concentration. Al concentrations are underestimated by up to 10 to 15 orders of magnitude using the SUPCRT92 database. Predicted K/Na ratios are underestimated by up to 30%. The best predictions achieved for this simplified high-grade pelite assemblage are those using the SUPCRT92 database with revised thermodynamic data for feldspars and K- and Na-species (J. Phys. Chem. Ref. Data 24 (1995) 1401) and additional Al-species (Am. J. Sci. 295 (1995) 1255; Geochim. Cosmochim. Acta 61 (1997) 2175). The use of ideal mixing for neutral complexes in combination with the extended Debye–Hückel activity model for the charged species yields the most compatible speciation model.     

Surface chemistry of kaolinite and Na-montmorillonite in aqueous electrolyte solutions at 25 and 60 °C: Experimental and modeling study

The aqueous interfacial chemistry of kaolinite and Na-montmorillonite samples was investigated by potentiometric measurements using acid/base continuous titrations and batch experiments at 25 and 60 °C. Using the batch experimental method, a continuous drift of pH was observed reflecting the mineral dissolution. Consequently, the continuous titration method appears to be the best way of studying solid surface reactions. For each clay mineral, the net proton surface excess/consumption was calculated as a function of pH and ionic strength (0.025, 0.1 and 0.5 M). At 25 °C, and according to the literature data, the pH corresponding to zero net proton consumption for montmorillonite appears to depend on ionic strength, whereas the value for kaolinite is constant and close to 5. Similar results are obtained at 60 °C, which suggests that the point of zero net proton consumption for clay minerals does not depend on temperature, at least up to 60 °C. On the other hand, the temperature rise induces a slight increase of the net proton surface excess. Finally, the diffuse double layer formalism (DDLM) is used to model the experimental data. The model involves two processes: the protonation/deprotonation of two types of edge sites (aluminol and silanol) and H⁺/Na⁺ exchange reactions on basal surfaces, while a tiny proportion of the negative structural charge remains uncompensated. This last process maintains a negative surface potential whatever the pH of the solution, which is in agreement with electrokinetic data.     

Mechanism of chalcopyrite formation from iron monosulphides in aqueous solutions (< 100°C, pH 2–4.5)

In low-temperature aqueous solutions (< 100°C, pH 2–4.5), chalcopyrite (CuFeS₂) does not form through direct precipitation from solution. The pathway is exclusively via precursor iron sulphides and dissolved Cu salts. The reaction of dissolved Cu (II) salts with natural hexagonal pyrrhotite (Fe_0.9S) is diffusion controlled. The initial stage has an apparent activation energy of 11.4 ± 1.8 kJ mol⁻¹ and the rate (in units of mol dm⁻³s⁻¹ cm⁻²) is independent of the solid reactant surface area. The reaction proceeds through a series of metastable Cu-Fe-sulphide intermediaries. These phases form a series of ephemeral layers penetrating into the pyrrhotite surface. The first phase formed has the stoichiometry Cu_0.1Fe_0.9S. No Fe is released into the solution during its formation and this, together with the extremely low apparent activation energy and the stoichiometry, suggest that it is formed by stuffing of electron holes in the pyrrhotite structure with Cu ions. The transformation from the hexagonal close-packed arrangement of the pyrrhotite structure to the essentially cubic packing in chalcopyrite proceeds through a series of intermediaries, approximating in composition to members of the cubanite group. The rate of formation of these phases is controlled by the coupled diffusion of Fe (II), Fe (III), Cu (I) and Cu (II) species through the surface reaction zone, although the process as a whole can be approximated by steady-state diffusion of total Cu into a semi-infinite medium. Experiments with metastable precursor iron monosulphide phases, including amorphous FeS and synthetic mackinawite indicate similar reaction pathways.

The results suggest that chalcopyrite formation in low-temperature natural systems may be significantly constrained by kinetic factors. Chalcopyrite is, at least, a diagenetic mineral since its formation requires the prior formation of iron sulphides. However, at ambient temperatures its formation is probably limited to very early diagenesis.     

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,

     

The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C

Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from 250 to 700 °C was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional–viscous transition) at 280±30 °C. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between 280 and 400 °C, subgrain rotation recrystallization (SGR) in the 400–500 °C interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at 500 °C. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630±30 °C, the textures also include submaxima, which are indicative of combined basal a- and prism [c] slip.     

An experimental study of the solubility and speciation of the Rare Earth Elements (III) in fluoride- and chloride-bearing aqueous solutions at temperatures up to 300 °C

The solubility of REE(III) fluoride solids was determined in fluoride- and chloride-bearing solutions at 150, 200 and 250 °C and saturated water vapor pressure. These experimental data, together with experimental data from previously published studies, were used to evaluate formation constants for chloride- and fluoride-bearing aqueous species of the entire REE(III) group at temperatures up to 300 °C. The data show that the stability of these species differs significantly from that predicted theoretically. For example, contrary to the theoretical predictions, LREEF²⁺ species are more stable than HREEF²⁺ species at elevated temperature. The behavior of the chloride-bearing species is similar. Parameters for the Helgeson–Kirkham–Flowers (HKF) equation of state were determined for REEF²⁺, REECl²⁺ and REECl₂⁺ complexes using these experimental data and permit calculation of formation constants of these species at conditions not investigated experimentally. These data now permit the mobility of all REE in fluoride- and chloride-bearing hydrothermal systems to be reliably evaluated at intermediate temperatures and pressures.     

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.     

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).     

An in situ XAS study of copper(I) transport as hydrosulfide complexes in hydrothermal solutions (25-592 °C, 180-600 bar): Speciation and solubility in vapor and liquid phases

Chloride and hydrosulfide are the principal ligands assumed to govern transport of copper in hydrothermal fluids. Existing solubility experiments suggest that Cu(I)-hydrosulfide complexes are dominant compared to chloride complexes at low salinities in alkaline solutions (H₂S_(aq)/HS⁻ pH buffer), and may be important in transporting Cu in low density magmatic vapors, potentially controlling the liquid-vapor partitioning of Cu. This study provides the first in situ evidence of the solubility of copper sulfides and the nature and structure of the predominant Cu species in sulfur-containing fluids at temperatures up to 592 °C and pressures of 180-600 bar. XANES and EXAFS data show that at elevated T (?200 °C), Cu solubility occurs via a linear Cu complex. At 428 °C in alkaline solutions, Cu is coordinated by two sulfur atoms in a distorted linear coordination (angle ∼150-160°). This geometry is consistent with the species predicted by earlier solubility studies. In addition, in situ measurements of the solubility of chalcocite in 2 m NaHS solutions performed in this study are in remarkably good agreement with the solubilities calculated using available thermodynamic data for Cu(I)-hydrosulfide complexes, also supporting the interpretation of speciation in these studies and validating the extrapolation of low-T thermodynamic properties for to high P-T. Data on phase separation for the 2 m NaHS solution show that while significant amounts of copper can be partitioned into the vapor phase, there is no indication for preferential partitioning of Cu into the vapor. This is consistent with recent partitioning experiments conducted in autoclaves by Pokrovski et al. (2008a) and Simon et al. (2006). XANES data suggest that the species present in the low density phase is very similar to that present in the high density liquid, i.e., , although Cu(HS)(H₂S)⁰ cannot be excluded on the basis of XAS data.     

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.