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1.
Bjorn Mysen 《Geochimica et cosmochimica acta》2010,74(14):4123-170
The structure of H2O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na2O•4SiO2-Na2O•4(NaAl)O2-H2O (5 and 10 mol% Al2O3, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H2O at nominal densities (from PVT properties of pure H2O) of 0.85 g/cm3 (NA10 experiments) and 0.86 g/cm3 (NA5 experiments) with the aluminosilicate + H2O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H2O (H2O°) and OH groups that form bonds with cations exist in all three phases. The OH/H2O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H2O, fH2O) with (OH/H2O°)melt > (OH/H2O°)fluid at all pressures and temperatures. Structural units of Q3, Q2, Q1, and Q0 type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q0 and Q1 increases and Q2 and Q3 decrease with fH2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of fH2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to fH2O.The melt structural data are used to describe relationships between activity of H2O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qn-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H2O content. 相似文献
2.
Robert C. Newton 《Geochimica et cosmochimica acta》2007,71(21):5191-5202
The solubility and stability of synthetic grossular were determined at 800 °C and 10 kbar in NaCl-H2O solutions over a large range of salinity. The measurements were made by evaluating the weight losses of grossular, corundum, and wollastonite crystals equilibrated with fluid for up to one week in Pt capsules and a piston-cylinder apparatus. Grossular dissolves congruently over the entire salinity range and displays a large solubility increase of 0.0053 to 0.132 molal Ca3Al2Si3O12 with increasing NaCl mole fraction (XNaCl) from 0 to 0.4. There is thus a solubility enhancement 25 times the pure H2O value over the investigated range, indicating strong solute interaction with NaCl. The Ca3Al2Si3O12 mole fraction versus NaCl mole fraction curve has a broad plateau between XNaCl = 0.2 and 0.4, indicating that the solute products are hydrous; the enhancement effect of NaCl interaction is eventually overtaken by the destabilizing effect of lowering H2O activity. In this respect, the solubility behavior of grossular in NaCl solutions is similar to that of corundum and wollastonite. There is a substantial field of stability of grossular at 800 °C and 10 kbar in the system CaSiO3-Al2O3-H2O-NaCl. At high Al2O3/CaSiO3 bulk compositions the grossular + fluid field is limited by the appearance of corundum. Zoisite appears metastably with corundum in initially pure H2O, but disappears once grossular is nucleated. At XNaCl = 0.3, however, zoisite is stable with corundum and fluid; this is the only departure from the quaternary system encountered in this study. Corundum solubility is very high in solutions containing both NaCl and CaSiO3: Al2O3 molality increases from 0.0013 in initially pure H2O to near 0.15 at XNaCl = 0.4 in CaSiO3-saturated solutions, a >100-fold enhancement. In contrast, addition of Al2O3 to wollastonite-saturated NaCl solutions increases CaSiO3 molality by only 12%. This suggests that at high pH (quench pH is 11-12), the stability of solute Ca chloride and Na-Al ± Si complexes account for high Al2O3 solubility, and that Ca-Al ± Si complexes are minor. The high solubility and basic dissolution reaction of grossular suggest that Al may be a very mobile component in calcareous rocks in the deep crust and upper mantle when migrating saline solutions are present. 相似文献
3.
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 NaSi3O6.5 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. 相似文献
4.
Christomir Christov 《Geochimica et cosmochimica acta》2007,71(14):3557-3569
The isopiestic method has been used to determine the osmotic coefficients of the binary solutions NaBr-H2O (from 0.745 to 5.953 mol kg−1) and KBr-H2O (from 0.741 to 5.683 mol kg−1) at the temperature t = 50 °C. Sodium chloride solutions have been used as isopiestic reference standards. The isopiestic results obtained have been combined with all other experimental thermodynamic quantities available in literature (osmotic coefficients, water activities, bromide mineral’s solubilities) to construct a chemical model that calculates solute and solvent activities and solid-liquid equilibria in the NaBr-H2O, KBr-H2O and Na-K-Br-H2O systems from dilute to high solution concentration within the 0-300 °C temperature range. The Harvie and Weare [Harvie C., and Weare J. (1980) The prediction of mineral solubilities in naturalwaters: the Na-K-Mg-Ca-Cl-SO4-H2O system from zero to high concentration at 25 °C. Geochim. Cosmochim. Acta44, 981-997] solubility modeling approach, incorporating their implementation of the concentration-dependent specific interaction equations of Pitzer [Pitzer K. (1973) Thermodynamics of electrolytes. I. Theoretical basis and general equations. J. Phys. Chem.77, 268-277] is employed. The model for binary systems is validated by comparing activity coefficient predictions with those given in literature, and not used in the parameterization process. Limitations of the mixed solutions model due to data insufficiencies are discussed. This model expands the variable temperature sodium-potassium model of Greenberg and Moller [Greenberg J., and Moller N. (1989) The prediction of mineral solubilities in natural waters: a chemical equilibrium model for the Na-K-Ca-Cl-SO4-H2O system to high concentration from 0 to 250 °C. Geochim. Cosmochim. Acta53, 2503-2518] by evaluating Br− pure electrolyte and mixing solution parameters and the chemical potentials of three bromide solid phases: NaBr-2H2O (cr), NaBr (cr) and KBr (cr). 相似文献
5.
Weihua Liu Stacey J. Borg Denis Testemale Barbara Etschmann Jean-Louis Hazemann Joël Brugger 《Geochimica et cosmochimica acta》2011,75(5):1227-1248
Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl42−, while a lower order tetrahedral complex, most likely CoCl2(H2O)2(aq), predominates at low salinity (Cl:Co ratios ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H2O)62+ (octCo-O = 2.075(19) Å), tetrahedral CoCl42− (tetCo-Cl = 2.252(19) Å) and tetrahedral CoCl2(H2O)2(aq) (tetCo-O = 2.038(54) Å and tetCo-Cl = 2.210(56) Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr42−. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl42− and CoCl2(H2O)2(aq) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl42− is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids. 相似文献
6.
Robert C. Newton 《Geochimica et cosmochimica acta》2006,70(22):5571-5582
Solubilities of corundum (Al2O3) and wollastonite (CaSiO3) were measured in H2O-NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H2O and NaCl at pressure (P) and temperature (T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al2O3 molality (mAl2O3) of 0.0013(1) (1σ error) in pure H2O and then leveling off to a maximum of ∼0.015 at halite saturation (XNaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H2O, , passes through a maximum at XNaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H2O at this P and T(mCaSiO3=0.0167(6)). It undergoes great enhancement, with a maximum solubility relative to that in H2O at XNaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H2O-NaCl-SiO2, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from mSiO2=1.248 in pure H2O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H2O reproduces the data and indicates that hydrogen bonding of molecular H2O to dissolved silica species is thermodynamically negligible. The maxima in for corundum and wollastonite indicate that the solute products include hydrates and Na+ and/or Cl− species produced by molar ratios of reactant H2O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism. 相似文献
7.
Kirsten U. Rempel Anthony E. Williams-Jones Artas A. Migdisov 《Geochimica et cosmochimica acta》2009,73(11):3381-6609
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. 相似文献
8.
The ultraviolet spectra of dilute aqueous solutions of antimony (III) have been measured from 25 to 300 °C at the saturated vapour pressure. From these measurements, equilibrium constants were obtained for the following reactions:
H3SbO30 ? H+ + H2SbO3− 相似文献
9.
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. 相似文献
10.
Göril Möschner Barbara Lothenbach Andrea Ulrich Ruben Kretzschmar 《Geochimica et cosmochimica acta》2008,72(1):1-18
The solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca4[Fe(OH)6]2(SO4) · 6H2O) and Fe-monocarbonate (Ca4[Fe(OH)6]2(CO3) · 6H2O) are formed. The solubilities of these hydrates at 25 °C are: 相似文献
11.
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. 相似文献
12.
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. 相似文献
13.
High-pressure liquids in the MgO-SiO2-H2O (MSH) system have been investigated at 11 and 13.5 GPa and between 1000 and 1350 °C. A bulk composition more magnesian than the tie-line forsterite-H2O was employed for the study. Rocking multi-anvil experiments were combined with a diamond trap set-up. After termination of the experiments, the liquid trapped in the diamond layer was analysed by laser ablation ICP-MS using the ‘freezing’ technique. At 11 GPa, liquids coexist with one or two of phase A, clinohumite, chondrodite, and forsterite. A marked discontinuity in the evolution of liquid compositions near 1100 °C is observed at 11 GPa. A step of ∼13 wt% H2O and 13 wt% MgO is interpreted to result from overstepping the fluid-saturated solidus reaction mass balanced to 1.00(18) phase A + 1.07(4) fluid = 0.63(15) chondrodite + 1.44(2) melt. At 13.5 GPa liquids coexist with one or two of hydrous wadsleyite, clinohumite, superhydrous B, phase B, and forsterite. The discontinuity in liquid composition is no longer present, indicating that the second critical endpoint of the solidus has been overstepped. Thus, hydrous melts in the Mg-rich part of the MSH system (molar bulk Mg/Si > 2) are chemically distinct from aqueous fluids at pressure up to 11 GPa. Convergence of fluid and melt compositions along the solidus resulting in a supercritical liquid occurs between 11 and 13.5 GPa, at which pressure the entire MSH system becomes supercritical. 相似文献
14.
Carbon dioxide- and salt-bearing solutions are common in granulite, ore-forming and magmatic environments. The presence of CO2 affects mineral solubilities, fluid miscibility, and viscosity and wetting properties, and is expected to affect salt speciation. EXAFS measurements of RbBr-H2O-CO2 fluids contained in corundum-osed synthetic fluid inclusions (SFLINCs) have been used to investigate the effect of CO2 on salt speciation at temperatures to 579 °C and pressures to around 0.26 GPa.Forward modelling indicates that solute dehydration is difficult to distinguish from up to around 40% of Rb-Br ion-pairing, so results refer to the total number of nearest neighbours, which are likely to be mostly O present in waters of hydration, but may also include Br, if ion pairing is present. Additionally, results relate to the number of well-ordered neighbours in the first shell, because nearest neighbours with a high degree of disorder may be present but contribute minimally to the EXAFS signal. Analysis of the EXAFS results at the Rb edge for the CO2-free solution is consistent with previous work and shows that the number of nearest neighbours for Rb in CO2-free solutions decreases from 6 ± 0.6 to 1.4 ± 0.1 as temperature increases from 20 to 534 °C. The decrease is accompanied by a decrease in Rb-x bondlengths of 0.05 Å, where x is the first shell scatterer. Results for the CO2-bearing solution are different to those for the CO2-free solution. The number of nearest neighbours is 16 and 22% less than for the CO2-bearing solution at 312 and 445 °C respectively. Changes in the numbers of nearest neighbours correlate well with calculated changes in the bulk solution dielectric constant; CO2-bearing and CO2-free solutions lie on the same trend, which suggests that it may be possible to calculate the number of nearest neighbours from dielectric constant. Rb-x bondlengths for the CO2-bearing solution are statistically indistinguishable to those for the CO2-free inclusions. Results for Br are worse quality than for Rb so EXAFS analysis could not be completed, however XANES spectra for CO2-free and CO2-bearing solutions are consistent with solute dehydration similar to that recorded by the Rb spectra. The conclusions of this study provide support for the notion that CO2 has a fundamental effect on the mechanics of solubility, and that these effects should be incorporated into conceptual and quantitative thermodynamic models. 相似文献
15.
E. Curti K. Fujiwara J. Tits A. Kitamura W. Müller 《Geochimica et cosmochimica acta》2010,74(12):3553-5422
High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl2, Na2SO4, NaCl and NaHCO3) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with 133Ba or 226Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both 133Ba and 226Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of 133Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The 133Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of 133Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10−11 to 1.9 × 10−10 mol m−2 s−1 (2.4-16 μmol m−2 d−1), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10−10 mol m−2 s−1 or ∼49 μmol m−2 d−1) were determined.Radium uptake by barite was determined by monitoring the decrease of 226Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (WAB = 3720-6200 J/mol, a0 = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (WAB = 1240 J/mol, a0 = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings. 相似文献
16.
The position of the Raman methane (CH4) symmetric stretching band (ν1) over the range 1-650 bar and 0.3-22 °C has been determined using a high-pressure optical cell mounted on a Raman microprobe. Two neon emission lines that closely bracket the CH4 band were collected simultaneously with each CH4 spectrum. The peak position was determined after least squares fitting using a summed Gaussian-Lorentzian method, resulting in a precision of ≈±0.02 cm−1 in peak position determination. The CH4ν1 band position shifts to lower wave number with increasing pressure. At a given pressure, the band shifts to lower wave number with decreasing temperature, and the magnitude of the temperature shift increases with increasing pressure. The relationship between the Raman CH4ν1 band position and temperature and pressure determined here may be used to estimate the internal pressure in natural or synthetic CH4-bearing fluid inclusions. This information, in turn, may be used to determine the density of pure CH4 fluid inclusions and the salinity of CH4-bearing aqueous inclusions. 相似文献
17.
K.U. Rempel A.E. Williams-Jones A.A. Migdisov 《Geochimica et cosmochimica acta》2008,72(13):3074-3083
The solubility and speciation of the assemblage MoO2-MoO3 in water vapour were investigated in experiments conducted at 350 °C, Ptotal from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10−4 to 6.5 × 10−2 bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction
(A1) 相似文献
18.
The steady state dissolution rate of San Carlos olivine [Mg1.82Fe0.18 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:
19.
The behavior of ammonium, NH4+, in aqueous systems was studied based on Raman spectroscopic experiments to 600 °C and about 1.3 GPa. Spectra obtained at ambient conditions revealed a strong reduction of the dynamic three-dimensional network of water with addition of ammonium chloride, particularly at small solute concentrations. The differential scattering cross section of the ν1-NH4+ Raman band in these solutions was found to be similar to that of salammoniac.The Raman band of silica monomers at ∼780 cm−1 was present in all spectra of the fluid at high temperatures in hydrothermal diamond-anvil cell experiments with H2O ± NH4Cl and quartz or the assemblage quartz + kyanite + K-feldspar ± muscovite/tobelite. However, these spectra indicated that dissolved silica is less polymerized in ammonium chloride solutions than in comparable experiments with water. Quantification based on the normalized integrated intensity of the H4SiO40 band showed that the silica solubility in experiments with H2O + NH4Cl was significantly lower than that in equimolal NaCl solutions. This suggests that ammonium causes a stronger decrease in the activity of water in chloridic solutions than sodium.The Raman spectra of the fluid also showed that a significant fraction of ammonium was converted to ammonia, NH3, in all experiments at temperatures above 300 °C. This indicates a shift towards acidic conditions for experiments without a buffering mineral assemblage. The estimated pH of the fluid was ∼2 at 600 °C, 0.26 GPa, 6.6 m initial NH4Cl, based on the ratio of the integrated ν1-NH3 and ν1-NH4+ intensities and the HCl0 dissociation constant. The NH3/NH4+ ratio increased with temperature and decreased with pressure. This implies that more ammonium should be retained in K-bearing minerals coexisting with chloridic fluids upon high-P low-T metamorphism. At 500 °C, 0.73 GPa, ammonium partitions preferentially into the fluid, as constrained from infrared spectroscopy on the muscovite and from mass balance.The conversion of K-feldspar to muscovite proceeded much faster in experiments with NH4Cl solutions than in comparable experiments with water. This is interpreted as being caused by enhancement of the rate-limiting alumina solubility, suggesting complexation of Al with NH4. Nucleation and growth of mica at the expense of K-feldspar and NH4+/K+ exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH4+ into K-feldspar was distinctly faster than K-feldspar consumption. 相似文献
20.
Charles A. Geiger Edgar Dachs Gilberto Artioli 《Geochimica et cosmochimica acta》2010,74(18):5202-5215
Armenite, ideal formula BaCa2Al6Si9O30·2H2O, and its dehydrated analog BaCa2Al6Si9O30 and epididymite, ideal formula Na2Be2Si6O15·H2O, and its dehydrated analog Na2Be2Si6O15 were studied by low-temperature relaxation calorimetry between 5 and 300 K to determine the heat capacity, Cp, behavior of their confined H2O. Differential thermal analysis and thermogravimetry measurements, FTIR spectroscopy, electron microprobe analysis and powder Rietveld refinements were undertaken to characterize the phases and the local environment around the H2O molecule.The determined structural formula for armenite is Ba0.88(0.01)Ca1.99(0.02)Na0.04(0.01)Al5.89(0.03)Si9.12(0.02)O30·2H2O and for epididymite Na1.88(0.03)K0.05(0.004)Na0.01(0.004)Be2.02(0.008)Si6.00(0.01)O15·H2O. The infrared (IR) spectra give information on the nature of the H2O molecules in the natural phases via their H2O stretching and bending vibrations, which in the case of epididymite only could be assigned. The powder X-ray diffraction data show that armenite and its dehydrated analog have similar structures, whereas in the case of epididymite there are structural differences between the natural and dehydrated phases. This is also reflected in the lattice IR mode behavior, as observed for the natural phases and the H2O-free phases. The standard entropy at 298 K for armenite is S° = 795.7 ± 6.2 J/mol K and its dehydrated analog is S° = 737.0 ± 6.2 J/mol K. For epididymite S° = 425.7 ± 4.1 J/mol K was obtained and its dehydrated analog has S° = 372.5 ± 5.0 J/mol K. The heat capacity and entropy of dehydration at 298 K are Δ = 3.4 J/mol K and ΔSrxn = 319.1 J/mol K and Δ = −14.3 J/mol K and ΔSrxn = 135.7 J/mol K for armenite and epididymite, respectively. The H2O molecules in both phases appear to be ordered. They are held in place via an ion-dipole interaction between the H2O molecule and a Ca cation in the case of armenite and a Na cation in epididymite and through hydrogen-bonding between the H2O molecule and oxygen atoms of the respective silicate frameworks. Of the three different H2O phases ice, liquid water and steam, the Cp behavior of confined H2O in both armenite and epididymite is most similar to that of ice, but there are differences between the two silicates and from the Cp behavior of ice. Hydrogen-bonding behavior and its relation to the entropy of confined H2O at 298 K is analyzed for various microporous silicates.The entropy of confined H2O at 298 K in various silicates increases approximately linearly with increasing average wavenumber of the OH-stretching vibrations. The interpretation is that decreased hydrogen-bonding strength between a H2O molecule and the silicate framework, as well as weak ion-dipole interactions, results in increased entropy of H2O. This results in increased amplitudes of external H2O vibrations, especially translations of the molecule, and they contribute strongly to the entropy of confined H2O at T < 298 K. 相似文献