Solubility of CO2 in a Ca-rich leucitite: effects of pressure,temperature, and oxygen fugacity

The solubility of carbon dioxide in a Ca-rich leucitite has been investigated as a function of pressure (0.1–2.0 GPa), temperature (1200–1600°C), and oxygen fugacity. The experiments were done in a rapid-quench internally-heated pressure vessel (0.1 GPa) and a piston cylinder (0.5–2.0 GPa). The leucitite glass, previously equilibrated at NNO, and silver oxalate were loaded in Fe-doped Pt capsules (oxidized conditions) and graphitelined Pt capsules (reduced conditions). Secondary Ion Mass Spectrometry and bulk carbon analyses were used to determine the amount of dissolved carbon. Speciation of carbon was characterized by Fourier transform microinfrared spectroscopy. At oxidized conditions, only CO₃ ^2- is observed as a dissolved species. The solubility is high with CO₂ contents in the melt attaining 6.2 wt% at 2.0 GPa and 1350°C. The solubility increases with pressure and shows a significant negative temperature dependence. An excellent correlation is obtained when the data are fit to a model, based on the simplified solubility reaction CO₂ ^(vapor)+O^2-(melt)CO₃ ^2-(melt), which describes the solubility of CO₂ as a function of pressure, temperature and fCO₂. At reduced conditions, the amount of carbon dissolved is significantly lower, and CO₃ ^2- is still the only species present in the melt. If the solubility model established at oxidized conditions is applied, the carbon dissolved appears to be essentially a function of fCO₂ alone although divergence increases in a consistent manner with pressure and temperature. This could suggest a low but significant solubility of CO with a positive temperature dependence or a departure of the calculated fluid compositions determined by the equation of state from the actual ones. The strong preferential solubility of carbon in its oxidized C⁴⁺ form, even at reduced conditions, implies that ascending melts with high CO₂ solubility can experience significant oxidation through degassing. This could reconcile the oxidized nature of some Ca-rich alkaline magmas with more reduced mantle source regions.     

CO2 solubility in aqueous solutions containing Na+, Ca2+, Cl−, SO42− and HCO3-: The effects of electrostricted water and ion hydration thermodynamics

Dissolution of CO₂ into deep subsurface brines for carbon sequestration is regarded as one of the few viable means of reducing the amount of CO₂ entering the atmosphere. Ions in solution partially control the amount of CO₂ that dissolves, but the mechanisms of the ion's influence are not clearly understood and thus CO₂ solubility is difficult to predict. In this study, CO₂ solubility was experimentally determined in water, NaCl, CaCl₂, Na₂SO_4, and NaHCO₃ solutions and a mixed brine similar to the Bravo Dome natural CO₂ reservoir; ionic strengths ranged up to 3.4 molal, temperatures to 140 °C, and CO₂ pressures to 35.5 MPa. Increasing ionic strength decreased CO₂ solubility for all solutions when the salt type remained unchanged, but ionic strength was a poor predictor of CO₂ solubility in solutions with different salts. A new equation was developed to use ion hydration number to calculate the concentration of electrostricted water molecules in solution. Dissolved CO₂ was strongly correlated (R² = 0.96) to electrostricted water concentration. Strong correlations were also identified between CO₂ solubility and hydration enthalpy and hydration entropy. These linear correlation equations predicted CO₂ solubility within 1% of the Bravo Dome brine and within 10% of two mixed brines from literature (a 10 wt % NaCl + KCl + CaCl₂ brine and a natural Na⁺, Ca²⁺, Cl⁻ type brine with minor amounts of Mg²⁺, K⁺, Sr²⁺ and Br⁻).     

Na2O solubility in CaO-MgO-SiO2 melts

The sodium solubility in silicate melts in the CaO-MgO-SiO₂ (CMS) system at 1400 °C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na_(g) evaporation from a Na₂O-xSiO₂ melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa₂O_(sample) = aNa₂O_(source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na₂O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO₂ < 100; in wt%), we found that Na₂O solubility is conveniently modeled as a linear function of the optical basicity (Λ) calculated on a Na-free basis melt composition. In our experiments, γNa₂O_(sample) ranges from 7 × 10⁻⁷ to 5 × 10⁻⁶, indicating a strongly non-ideal behavior of Na₂O solubility in the studied CMS melts (γNa₂O_(sample) ? 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na₂O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na₂O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q⁴.     

The system NaAlSi3O8−H2O−H2 (1200° C, 2 kbar): the solubility and interaction mechanism of fluid species with melt

The H₂O and H₂ solubilities in an albite melt at 1200° C and 2 kbar over the entire range of gas phase composition, from pure hydrogen to pure water were studied in gas-media pressure vessels. The water solubility initially increases with increasing hydrogen content until a maximum of 9.19 wt% H₂O atXH ₂ ^v =0.1 is reached, withXH ₂ ^v >0.1 the water solubility decreases. The hydrogen solubility curve has a maximum atXH ₂ ^v =0.42 where the concentration reaches 0.206 wt% H₂O. Over the entire compositional range¹H NMR (nuclear magnetic resonance) spectra show distinct absorption lines due to protons bound to OH groups and to isolated firmly bound water molecules. In NMR and Raman spectra there were no bands attributable to the H–H vibrations of molecular hydrogen. The X-ray photo-electronic spectra of hydrogen-bearing glasses show the Si2p (99 eV) band which corresponds to the zero-valency silicon. The formation of OH groups and molecular water during interaction between hydrogen-bearing fluids and melts under reducing conditions has a qualitative effect, the same as for water dissolution. Another point of interest is that hydrogen-bearing melts undergo more depolymerization than do hydrous melts.     

CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO₂ in dacitic melts equilibrated with H₂O-CO₂ fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO₂ is dissolved in dacitic glasses as molecular CO₂ and carbonate. The quantification of total CO₂ in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm⁻¹ can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO_2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO₂ species concentrations from the MIR spectra we have measured total CO₂ contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO₂ solubility in dacitic melts on x^fluid_CO2,total shows a strong positive deviation from linearity with almost constant CO₂ solubility at x_CO2fluid > 0.8 (maximum CO₂ solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO₂. A similar nonlinear variation of CO₂ solubility with x_CO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO₂ (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO₂ molecules.A thermodynamic model describing the dependence of the CO₂ solubility in hydrous rhyolitic and dacitic melts on T, P, f_CO2 and the mol fraction of water in the melt (x_water) has been developed. An exponential variation of the equilibrium constant K₁ with x_water is proposed to account for the nonlinear dependence of x_CO2,totalmelt on x_CO2fluid. The model reproduces the CO₂ solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x_CO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO₂ at low pressures (a Henrian behavior of the CO₂ solubility is observed at low pressure and low H₂O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x_CO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO₂ is much lower in rhyolitic melts (31.7 cm³/mol) than in dacitic melts (46.6 cm³/mol). The dissolution enthalpy for CO₂ in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO₂ solubility in silicic melts.     

Temperature dependence of CO2 solubility in high pressure quenched glasses of diopside composition

The temperature dependence of carbon dioxide solubility in glasses of diopside composition, quenched from 20 kbar, has been investigated using a combination of high-temperature mass spectrometry and Raman spectroscopy.CO₂-charged diopside glasses were synthesized in a piston-cylinder apparatus. Because of diffusion of hydrogen through the platinum capsules, significant amounts of H₂O, CH₄ and CO were detected along with CO: in the diopside glasses. All three carbon species show a bimodal release pattern in the mass pyrograms. The CO₂ solubility shows a linear and negative temperature dependence. We do not observe any maxima in the solubility curve as was reported previously (Mysen and virgo, 1980a).None of the additional bands observed in Raman spectra of CO₂-charged diopside glasses compared to those in the spectrum of diopside glass can be assigned to molecular CO₂. These bands are caused by CO^?2₃ ions and indicate that the physical solubility of molecular carbon dioxide is negligible. The bimodal release pattern observed for CO₂ in the mass pyrograms, is consistent with the Raman data which strongly suggests that CO^?2₃ ions are present in at least two distinct sites in the glass.     

The influence of Al2O3 on the H2O content in periclase and ferropericlase at 25 GPa

In this paper I present results of IR spectroscopic measurements of water solubility in Al-bearing periclase and ferropericlase (Mg# = 88) synthesized at 25 GPa and 1400–2000 °C. The IR spectra of their crystals show narrow absorption peaks at 3299, 3308, and 3474 cm^?1. The calculated H₂O contents are 11–25 ppm in periclase (Al₂O₃ = 0.9–1.2 wt.%) and 14–79 ppm in ferropericlase (Al₂O₃ = 0.9–2.9 wt.%). Ferropericlase contains more H₂O and Al₂O₃ than periclase at 1800–2000 °C. I suggest that addition of Al₂O₃ does not influence the solubility of water in ferropericlase but can favor the additional incorporation of Fe₂O₃ into the structure. The incorporation of Fe³⁺ into ferropericlase increases water solubility as a result of iron reduction to Fe²⁺. It is shown that water has limited solubility in ferropericlase from mantle peridotite; therefore, ferropericlase cannot be considered an important hydrogen-bearing mineral in the lower mantle.     

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.     

Behavior of molybdenum in alkali silicate melts: Effects of excess SiO2 and CO2

The effects of excess SiO₂ and CO₂ on the solubility of molybdenite in hydrous sodium disilicate melts were experimentally determined at 680 bars and 650°C. The molybdenite solubility decreases with increasing SiO₂ and CO₂. Under the experimental conditions, the MoS₂ content of the vapor-saturated liquid decreases from 10 wt.% to 2.5 wt.% at SiO₂ saturation. In the presence of CO₂, the solubility decreases to 4.6 wt.% MoS₂ and becomes negligible at high P_CO2. These results are explained as deriving from the increased polymerization and hence decreased NBO/Si ratio of the melt with increasing SiO₂ content and CO₂, respectively. Sulfur dissolves principally as SO₄^?2 at the relatively high fo₂ of the experiments. Consequently, the effect of sulfur is to lower the Mo solubility by effectively decreasing the NBO/Si ratio of the melt. Sulfur saturation is, therefore, likely to be a limiting factor in the Mo content of alkali silicate melts because of the chalcophile affinities of molybdenum.     

CO2 solubility and speciation in intermediate (andesitic) melts: the role of H2O and composition

We determined total CO₂ solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H₂O, OH⁻, molecular CO₂, and CO₃²⁻. CO₂ solubility was determined in hydrous andesite glasses and we found that H₂O content has a strong influence on C-O speciation and total CO₂ solubility. In anhydrous andesite melts with ∼60 wt.% SiO₂, total CO₂ solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO₂ solubility increases by about 0.06 wt.% per wt.% of total H₂O. As total H₂O increases from ∼0 to ∼3.4 wt.%, molecular CO₂ decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO₃²⁻ increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO₂ increases as the calculated mole fraction of CO₂ in the fluid increases, showing Henrian behavior. In contrast, CO₃²⁻ decreases as the calculated mole fraction of CO₂ in the fluid increases, indicating that CO₃²⁻ solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO₂ degassing. If substantial H₂O is present, total CO₂ solubility is higher and CO₂ will degas at relatively shallow levels compared to a drier melt. Total CO₂ solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H₂O contents (<1 wt.%). We found that total CO₂ solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO₂ is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.     

Rhondonite solubility and thermodynamic properties of aqueous MnCl2 in the system MnOSiO2HClH2O

The solubility of rhodonite, represented by the reaction MnSiO₃ (rhodonite) + 2HCl⁰ = MnCl₂⁰ + SiO₂ (quartz) + H₂O, was investigated experimentally in the temperature range 400°–700°C at 1 and 2 kbar by rapid-quench hydrothermal techniques and the Ag-AgCl buffer methods. Variations in the molalities of associated hydrogen chloride (m_HCl⁰) as a function of the molalities of total Mn indicate that Mn in the fluid in equilibrium with the assemblage rhodonite + quartz is predominantly associated as MnCl₂⁰. The Mn:Cl in the fluid ?2, indicating that Mn⁺² is the dominant oxidation state.The solubility data were used to calculate the equilibrium constant of the above reaction as a function of temperature, pressure, and the difference in Gibbs free energy of formation between MnCl₂⁰ and HCl⁰. The equilibrium constants of solubility for Mn minerals for which thermochemical data are available were also calculated. Calculated mineral solubilities were used in conjunction with the data of Frantz et al. (1981) to calculate the composition of supercritical fluids in equilibrium with Mn-bearing phases and assemblages. At 400°C and 1000 bars, supercritical fluids in equilibrium with olivines of compositions similar to those present in MORB tend to be enriched in Mn, despite the low mole fraction of tephroite in the olivine. Supercritical fluids in equilibrium with the assemblage quartz-hematite-rhodonite at 500° and 400°C and 1000 bars show high concentrations of Mn relative to Fe. Manganese concentrations in the fluids increase with decrease in the mole fraction of H, whereas Fe concentrations decrease. The data indicate that H fugacity plays a significant role in the separation of Mn from Fe in chloride-bearing hydrothermal fluids at supercritical temperatures.     

The solubility of fluorite in hydrothermal solutions,an experimental study

The solubility of fluorite in NaCl solutions increases with increasing temperature at all ionic strengths up to about 100°C. Above this temperature, the solubility passes through a maximum and possibly a minimum with increasing temperature at NaCl concentrations of 1.0M or less, and increases continuously with increasing temperature at NaCl concentrations above 1.0M. At any given temperature, the solubility of fluorite increases with increasing salt concentration in NaCl, KCl and CaCl₂ solutions. The solubility follows Debye-Hückel theory for KCl solutions. In NaCl and CaCl₂ solutions, the solubility of fluorite increases more rapidly than predicted by Debye-Hückel theory: the excess solubility is due to the presence of NaF^c, CaF⁺, and possibly of Na₂F⁺. The solubility of fluorite in NaCl-CaCl₂ and in NaCl-CaCl₂-MgCl₂ solutions is controlled by the common ion effect and by the presence of NaF^c, CaF⁺, and MgF⁺. The solubility of fluorite in NaCl-HCl solutions increases rapidly with increasing initial HCl concentration; the large solubility increase is due to the presence of HF^c. It seems likely that complexes other than those identified in this study rarely play a major role in fluoride transport and fluorite deposition at temperatures below 300°C.     

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.     

The solubility of calcite and aragonite in seawater of 35%. salinity at 25°C and atmospheric pressure

The solubilities of synthetic, natural and biogenic aragonite and calcite, in natural seawater of 35%. salinity at 25°C and 1 atm pressure, were measured using a closed system technique. Equilibration times ranged up to several months. The apparent solubility constant determined for calcite of 4.39(±0.20) × 10^?7 mol² kg^?2 is in good agreement with other recent solubility measurements and is constant after 5 days equilibration. When we measured aragonite solubility we observed that it decreased with increasing time of equilibration. The value of 6.65(±0.12) × 10^?7 mol² kg^?2, determined for equilibration times in excess of 2 months, is significantly less than that found in other recent measurements, which employed equilibration times of only a few hours to days. No statistically significant difference was found among the synthetic, natural and biogenic material. Solid to solution ratio, contamination of aragonite with up to 10 wt% calcite and recycling of the aragonite made no statistically significant difference in solubility when long equilibration times were used.Measured apparent solubility constants of aragonite and calcite are respectively 22( ± 3)% and 20( ± 2)% less than apparent solubility constants calculated from thermodynamic equilibrium constants and seawater total activity coefficients. These large differences in measured and calculated apparent solubility constants may be the result of the formation of surface layers of lower solubility than the bulk solid.     

Experimental determination of the solubility constant for magnesium chloride hydroxide hydrate (Mg3Cl(OH)5·4H2O, phase 5) at room temperature, and its importance to nuclear waste isolation in geological repositories in salt formations

In this study, the solubility constant of magnesium chloride hydroxide hydrate, Mg₃Cl(OH)₅·4H₂O, termed as phase 5, is determined from a series of solubility experiments in MgCl₂-NaCl solutions. The solubility constant in logarithmic units at 25 °C for the following reaction,

Mg₃Cl(OH)₅·4H₂O+5H⁺=3Mg²⁺+9H₂O(l)+Cl^-     

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 combined effects of ƒO2 and melt composition on SnO2 solubility and tin diffusivity in haplogranitic melts

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO₂ solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log ƒ_O2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn²⁺ species, whereas at oxidized conditions SnO₂ is much less soluble, and tin is present dominantly as a Sn⁴⁺ species. There is a strong melt composition control on SnO₂ solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log ƒ_O2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO₂ for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO₂ solubility is 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn0₂ solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn⁴⁺/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from 0.4 at log ƒ_O2 of FMQ + 3.49 to 0.01 at FMQ - 0.57. Over the same log foZ range the Sn⁴⁺/ΣSn ratio for the A.S.I. 0.6 composition ranges from 0.98 to 0.4 and for the A.S.I. 1.25 composition, from 0.8 to 0.02.Tin diffusivity is dependent on both f_O2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10^−7.5 cm²/sec for the peraluminous compositions and 10^−8.2 cm²/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10^−8.2 and 10^−9.0 cm²/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn²⁺ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn⁴⁺ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn²⁺ and Sn⁴⁺ is the same, but the bond strengths are different. At fixed f_O2 the faster diffusivity in the peraluminous compositions reflects the lower Sn⁴⁺/Sn²⁺ ratio. The fact the Sn⁴⁺/Sn²⁺ ratio in melts varies greatly with ƒ_O2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.     

Experimental determination of argon solubility in silicate melts: An assessment of the effects of liquid composition and temperature

The argon solubility of 38 liquids in the system Na₂O-CaO-MgO-Al₂O₃-SiO₂ (NCMAS) has been determined at 1873 K and 1 bar, the argon concentration of presaturated glasses being measured using a static mass spectrometer. For compositions in the subsystem diopside (CaMgSi₂O₆), nepheline (NaAlSiO₄), albite (NaAlSi₃O₈), anorthite (CaAl₂Si₂O₈), argon solubility is generally a linear function of the relative proportion of each end member, solubility being lowest in diopside melt (1.53 10⁻⁵ cm³ STP · g⁻¹ · bar⁻¹) and highest in albite melt (2.88 10⁻⁴ cm³ STP · g⁻¹ · bar⁻¹). For the tectosilicate joins studied (SiO₂-Na₂Al₂O₄, SiO₂-CaAl₂O₄, SiO₂-MgAl₂O₄) solubility decreases with decreasing silica content in all cases, being highest for Na-bearing liquids and lowest for Mg-bearing liquids at constant molar silica content. Where comparison is possible our results are in good agreement with data from the literature. When our data are considered in isolation we find that argon solubility shows an excellent correlation with calculated ionic porosity. The covariation of argon solubility and liquid density is also reasonable, that with molar volume less convincing and that with polymerization state (as defined by the ratio of the number of nonbridging oxygens and tetrahedral network forming cations; NBO/T) nonexistent. However, when our data are combined with those from the literature no well constrained correlation between argon solubility and ionic porosity is apparent. Based upon this observation and consideration of the temperature dependence of noble gas solubility it is concluded that ionic porosity is not a universally applicable parameter which may be used to predict noble gas solubility as a function of composition, temperature and pressure. Two new models for calculating argon solubility are proposed, both employing the notion of partial molar argon solubilities. The first uses oxide components, for which partial molar argon solubility is directly proportional to partial molar ionic porosity calculated at 1873 K, irrespective of the temperature of experimental equilibration. The second model, which offers the best fit to the available data, employs tetrahedral units rather than oxides as the proposed melt components. This latter model successfully accounts for reported argon solubilities in simple Al-free systems, in simple Al-bearing systems and in natural liquids. This is interpreted to infer that argon is incorporated in large sites in the liquid structure (such as the space within rings of n-tetrahedra) although further work is required to understand the quantitative links between melt structure and noble gas solubility.     

Dissolution sequestration mechanism of CO<Subscript>2</Subscript> at the Shiqianfeng saline aquifer in the Ordos Basin,northwestern China

This study focused on the target injection layers of deep saline aquifers in the Shiqianfeng Fm. in the Carbon Capture and Sequestration (CCS) Demonstration Projects in the Ordos Basin, northwestern China. The study employed a combination method of experiments and numerical simulation to investigate the dissolution mechanism and impact factors of CO₂ in these saline aquifers. The results showed (1) CO₂ solubility in different types of water chemistry were shown in ascending order: MgCl₂-type water < CaCl₂-type water < Na₂SO₄-type water < NaCl-type water < Na₂CO₃-type water < distilled water. These results were consistent with the calculated results undertaken by TOUGHREACT with about 5% margin of error. CO₂ solubility of Shiqianfeng Fm. saline was 1.05 mol/L; (2) compared with distilled water, the more complex the water’s chemical composition, the greater the increase in HCO₃ ^?concentration. While the water’s composition was relatively simple, the tested water’s HCO₃ ^?concentrations were in close accord with the calculated value undertaken by the TOUGHREACT code, and the more complex the water’s composition, the poorer the agreement was, probably due to the complex and unstable HCO₃ ^? complicating matters when in an aqueous solution system including both tested HCO₃ ^?concentration and calculated HCO₃ ^?concentration; (3) the CO₂ solubility in the saline at the temperature conditions of 55 °C and 70 °C were 1.17 and 1.02 mol/L. When compared with the calculated value of 1.20 and 1.05 mol/L, they were almost the same with only 1 and 3% margin of error; concentrations of HCO₃ ^? were 402.73 mg/L (0.007 mol/L) and 385.65 mg/L (0.006 mol/L), while the simulation results were 132.16 mg/L (0.002 mol/L) and 128.52 mg/L (0.002 mol/L). From the contrast between the tested data and the calculated data undertaken by the TOUGHREACT code, it was shown that TOUGHRACT code could better simulate the interaction between saline and CO₂ in the dissolution sequestration capacity. Therefore, TOUGHREACT code could be used for the inter-process prediction of CO₂ long-term geological storage of CO₂; (4) The Ca²⁺ concentration and SO₄ ^2?concentration in saline water had less effect on the solubility of CO₂ and HCO₃ ^?concentration. In addition, TDS and pH values of saline affected not only the solubility of CO₂, but also the conversion of CO₂ to HCO₃ ^? due to that they can affect the activity and acid-base balance. So in fact, we just need to consider that the TDS and pH values are main impact factors in the dissolution sequestration capacity of CO₂ geological sequestration in deep saline aquifers.     

Solubility of water in the α, β and γ phases of (Mg,Fe)2SiO4

 The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)₂SiO₄ was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/10⁶Si (1200 wt ppm H₂O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/10⁶Si (24,000 wt ppm H₂O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/10⁶Si (27,000 wt ppm H₂O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation C _OH = A(T)f nH₂Oexp(−PΔV/RT), where A(T) = 1.1 H/10⁶Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10^–6 m³/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone. Received: 10 February 1995 / Accepted: 23 October 1995