Standard State Gibbs Free Energy Research Articles

Linear Correlations of Gibbs Free Energy of REE Phosphates (Monazite, Xenotime, and Rhabdophane) and Internally Consistent Binary Mixing Properties

Rare Earth Elements (REE) phosphates (monazite, xenotime, and rhabdophane) are critical REE-bearing minerals typically formed in hydrothermal and magmatic ore deposits. The thermodynamic properties of those REE minerals are crucial to understanding the solubility, speciation, and transport of REE complexes. However, the standard-state Gibbs free energy of formation (∆G°f) values reported for these minerals in the literature vary by up to 25 kJ mol−1. Here, we present linear free energy relationships that allow the evaluation and estimation of the ∆G°f values at 25 °C and 1 bar for the three minerals from the ionic radius (rREE3+) and the non-solvation Gibbs free energy contribution to the REE3+ aqua ion (∆G°n, REE3+): ∆G°f,monazite − 399.71 rREE3+ = 1.0059 ∆G°n,REE3+ − 2522.51; ∆G°f,xenotime − 344.08 rREE3+ = 0.9909 ∆G°n,REE3+ − 2451.53; and ∆G°f,rhabdophane − 416.17 rREE3+ = 1.0067 ∆G°n, REE3+ − 2688.86. Moreover, based on the new dataset derived for REE end-members, we re-fitted the binary Margules parameter (W) from previous theoretical calculations into linear correlations: W + 0.00204 ∆G°’n,monazite = 39.3549 ∆V + 0.0641; W + 0.00255 ∆G°’n,xenotime = 25.4885 ∆V − 0.0062. The internally consistent thermodynamic properties of these REE phosphates are incorporated into the computer program Supcrtbl, which is available online at Zhu’s research website.

Expanding the taxonomic and environmental extent of an underexplored carbon metabolism-oxalotrophy.

Oxalate serves various functions in the biological processes of plants, fungi, bacteria, and animals. It occurs naturally in the minerals weddellite and whewellite (calcium oxalates) or as oxalic acid. The environmental accumulation of oxalate is disproportionately low compared to the prevalence of highly productive oxalogens, namely plants. It is hypothesized that oxalotrophic microbes limit oxalate accumulation by degrading oxalate minerals to carbonates via an under-explored biogeochemical cycle known as the oxalate-carbonate pathway (OCP). Neither the diversity nor the ecology of oxalotrophic bacteria is fully understood. This research investigated the phylogenetic relationships of the bacterial genes oxc, frc, oxdC, and oxlT, which encode key enzymes for oxalotrophy, using bioinformatic approaches and publicly available omics datasets. Phylogenetic trees of oxc and oxdC genes demonstrated grouping by both source environment and taxonomy. All four trees included genes from metagenome-assembled genomes (MAGs) that contained novel lineages and environments for oxalotrophs. In particular, sequences of each gene were recovered from marine environments. These results were supported with marine transcriptome sequences and description of key amino acid residue conservation. Additionally, we investigated the theoretical energy yield from oxalotrophy across marine-relevant pressure and temperature conditions and found similar standard state Gibbs free energy to "low energy" marine sediment metabolisms, such as anaerobic oxidation of methane coupled to sulfate reduction. These findings suggest further need to understand the role of bacterial oxalotrophy in the OCP, particularly in marine environments, and its contribution to global carbon cycling.

Hydrogen abstraction of alkyl radicals from polycyclic aromatic hydrocarbons and heterocyclic aromatic hydrocarbons

To understand the condensation during heavy oil thermal processing, the formation of aromatic carbon radicals by H-abstraction of methyl radicals (CH3) or ethyl radicals (C2H5) from polycyclic aromatic hydrocarbons (PAHs) or heterocyclic aromatic hydrocarbons (hetero-PAHs) was studied by density functional calculation. The H-abstraction from different PAHs all has a positive standard state Gibbs free energy change, while increasing reaction temperature or radical concentration promotes the reaction to proceed spontaneously. CH3 mainly contributes to the formation of aromatic carbon radicals on PAHs, and the activation entropy of H-abstraction determines the difference in reaction rate constants. The particularity of H-abstraction from hetero-PAHs is reflected at the α-site of heterocycles. The H-abstraction from N-containing hetero-PAHs occurs preferentially at the α-site of heterocycles, and both CH3 and C2H5 could participate in abstracting the α-H. The H-abstraction from S-containing hetero-PAHs occurs preferentially on coupled aromatic rings, showing reaction behavior similar to the H-abstraction on PAHs.

Uncovering the Role of Key Active-Site Side Chains in Catalysis: An Extended Brønsted Relationship for Substrate Deprotonation Catalyzed by Wild-Type and Variants of Triosephosphate Isomerase.

We report results of detailed empirical valence bond simulations that model the effect of several amino acid substitutions on the thermodynamic (ΔG°) and kinetic activation (ΔG⧧) barriers to deprotonation of dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde 3-phosphate (GAP) bound to wild-type triosephosphate isomerase (TIM), as well as to the K12G, E97A, E97D, E97Q, K12G/E97A, I170A, L230A, I170A/L230A, and P166A variants of this enzyme. The EVB simulations model the observed effect of the P166A mutation on protein structure. The E97A, E97Q, and E97D mutations of the conserved E97 side chain result in ≤1.0 kcal mol–1 decreases in the activation barrier for substrate deprotonation. The agreement between experimental and computed activation barriers is within ±1 kcal mol–1, with a strong linear correlation between ΔG⧧ and ΔG° for all 11 variants, with slopes β = 0.73 (R2 = 0.994) and β = 0.74 (R2 = 0.995) for the deprotonation of DHAP and GAP, respectively. These Brønsted-type correlations show that the amino acid side chains examined in this study function to reduce the standard-state Gibbs free energy of reaction for deprotonation of the weak α-carbonyl carbon acid substrate to form the enediolate phosphate reaction intermediate. TIM utilizes the cationic side chain of K12 to provide direct electrostatic stabilization of the enolate oxyanion, and the nonpolar side chains of P166, I170, and L230 are utilized for the construction of an active-site cavity that provides optimal stabilization of the enediolate phosphate intermediate relative to the carbon acid substrate.

Electron carriers in microbial sulfate reduction inferred from experimental and environmental sulfur isotope fractionations.

Dissimilatory sulfate reduction (DSR) has been a key process influencing the global carbon cycle, atmospheric composition and climate for much of Earth's history, yet the energy metabolism of sulfate-reducing microbes remains poorly understood. Many organisms, particularly sulfate reducers, live in low-energy environments and metabolize at very low rates, requiring specific physiological adaptations. We identify one such potential adaptation-the electron carriers selected for survival under energy-limited conditions. Employing a quantitative biochemical-isotopic model, we find that the large S isotope fractionations (>55‰) observed in a wide range of natural environments and culture experiments at low respiration rates are only possible when the standard-state Gibbs free energy (ΔG'°) of all steps during DSR is more positive than -10 kJ mol-1. This implies that at low respiration rates, only electron carriers with modestly negative reduction potentials are involved, such as menaquinone, rubredoxin, rubrerythrin or some flavodoxins. Furthermore, the constraints from S isotope fractionation imply that ferredoxins with a strongly negative reduction potential cannot be the direct electron donor to S intermediates at low respiration rates. Although most sulfate reducers have the genetic potential to express a variety of electron carriers, our results suggest that a key physiological adaptation of sulfate reducers to low-energy environments is to use electron carriers with modestly negative reduction potentials.

Characterization of calmodulin-Fas death domain interaction: an integrated experimental and computational study.

The Fas death receptor-activated death-inducing signaling complex (DISC) regulates apoptosis in many normal and cancer cells. Qualitative biochemical experiments demonstrate that calmodulin (CaM) binds to the death domain of Fas. The interaction between CaM and Fas regulates Fas-mediated DISC formation. A quantitative understanding of the interaction between CaM and Fas is important for the optimal design of antagonists for CaM or Fas to regulate the CaM–Fas interaction, thus modulating Fas-mediated DISC formation and apoptosis. The V254N mutation of the Fas death domain (Fas DD) is analogous to an identified mutant allele of Fas in lpr-cg mice that have a deficiency in Fas-mediated apoptosis. In this study, the interactions of CaM with the Fas DD wild type (Fas DD WT) and with the Fas DD V254N mutant were characterized using isothermal titration calorimetry (ITC), circular dichroism spectroscopy (CD), and molecular dynamics (MD) simulations. ITC results reveal an endothermic binding characteristic and an entropy-driven interaction of CaM with Fas DD WT or with Fas DD V254N. The Fas DD V254N mutation decreased the association constant (Ka) for CaM–Fas DD binding from (1.79 ± 0.20) × 106 to (0.88 ± 0.14) × 106 M–1 and slightly increased a standard state Gibbs free energy (ΔG°) for CaM–Fas DD binding from −8.87 ± 0.07 to −8.43 ± 0.10 kcal/mol. CD secondary structure analysis and MD simulation results did not show significant secondary structural changes of the Fas DD caused by the V254N mutation. The conformational and dynamical motion analyses, the analyses of hydrogen bond formation within the CaM binding region, the contact numbers of each residue, and the electrostatic potential for the CaM binding region based on MD simulations demonstrated changes caused by the Fas DD V254N mutation. These changes caused by the Fas DD V254N mutation could affect the van der Waals interactions and electrostatic interactions between CaM and Fas DD, thereby affecting CaM–Fas DD interactions. Results from this study characterize CaM–Fas DD interactions in a quantitative way, providing structural and thermodynamic evidence of the role of the Fas DD V254N mutation in the CaM–Fas DD interaction. Furthermore, the results could help to identify novel strategies for regulating CaM–Fas DD interactions and Fas DD conformation and thus to modulate Fas-mediated DISC formation and thus Fas-mediated apoptosis.

Investigation of Structure, Adsorption Free Energy, and Overcharging Behavior of Trivalent Yttrium Adsorbed at the Muscovite (001)–Water Interface

The speciation, coverage, and thermodynamics of trivalent Y3+ adsorbed at a negatively charged muscovite (001)–water interface were measured to investigate electrostatic ion–ion correlations in a strong coupling regime. In situ specular crystal truncation rod and resonant anomalous X-ray reflectivity data show that Y3+ adsorbs as three distinct species (inner-sphere, adsorbed outer-sphere, and extended outer-sphere) among which the fractional coverage of the inner-sphere species (∼10%) is the smallest. Uptake measurements show that the maximum Y3+ coverage is higher by 50% than the amount needed to satisfy the muscovite surface charge in the absence of background electrolyte, but this overcharging is largely suppressed in the presence of 0.1 m NaCl. The measured intrinsic standard-state Gibbs free energy for Y3+ adsorption (−36.9 ± 0.9 kJ/mol) is smaller than that expected solely on the basis of its charge, and this discrepancy can be partly explained by the difference in adsorbed cation speciation at the charged interface.

Calculation of the Gibbs free energy of solvation and dissociation of HCl in water via Monte Carlo simulations and continuum solvation models

The Gibbs free energy of solvation and dissociation of hydrogen chloride in water is calculated through a combined molecular simulation/quantum chemical approach at four temperatures between T = 300 and 450 K. The Gibbs free energy is first decomposed into the sum of two components: the Gibbs free energy of transfer of molecular HCl from the vapor to the aqueous liquid phase and the standard-state Gibbs free energy of acid dissociation of HCl in aqueous solution. The former quantity is calculated using Gibbs ensemble Monte Carlo simulations using either Kohn-Sham density functional theory or a molecular mechanics force field to determine the system's potential energy. The latter Gibbs free energy contribution is computed using a continuum solvation model utilizing either experimental reference data or micro-solvated clusters. The predicted combined solvation and dissociation Gibbs free energies agree very well with available experimental data.

A high-temperature high-pressure calorimeter for determining heats of solution up to 623 K

A high-temperature high-pressure isoperibol calorimeter for determining the heats of solution and reaction of very dilute substances in water (10(-4) m) at temperatures up to 623 K is described. The energies of vaporization of water at steam saturation pressure were measured as a function of temperature and the results agree with the corresponding values from steam tables to better than 0.08+/-0.18%. The novelties of the present instrument relative to flow type heat capacity calorimeters are that measurements can be made at orders of magnitude lower concentrations and that measurement of heat of reaction involving solids or gases or in the presence of high concentrations of supporting electrolytes, acids, and bases is possible. Furthermore, the advantage of using enthalpy data over heat capacity data for calculations of the standard state Gibbs free energies of electrolytes is that the experimental heat data of this research need only be integrated once to derive higher temperature free energy data from lower temperatures. The derived heat capacities can be used mathematically to obtain free energies by double integration. However, the resulting errors are much smaller than if experimental aqueous heat capacities were used for the integrations.

Thermodynamic Properties of Autunite, Uranyl Hydrogen Phosphate, and Uranyl Orthophosphate from Solubility and Calorimetric Measurements

In this study, we use solubility and drop-solution calorimetry measurements to determine the thermodynamic properties of the uranyl phosphate phases autunite, uranyl hydrogen phosphate, and uranyl orthophosphate. Conducting the solubility measurements from both supersaturated and undersaturated conditions and under different pH conditions rigorously demonstrates attainment of equilibrium and yields well-constrained solubility product values. We use the solubility data and the calorimetry data, respectively, to calculate standard-state Gibbs free energies of formation and standard-state enthalpies of formation for these uranyl phosphate phases. Combining these results allows us also to calculate the standard-state entropy of formation for each mineral phase. The results from this study are part of a combined effort to develop reliable and internally consistent thermodynamic data for environmentally relevant uranyl minerals. Data such as these are required to optimize and quantitatively assess the effect of phosphate amendment remediation technologies for uranium contaminated systems.

Estimation of equilibrium surface precipitation constants for trivalent metal sorption onto hydrous ferric oxide and calcite

Equilibrium constants for modeling surface precipitation of trivalent metal cations ( M 3 + ) onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energies of formation, ( Δ G f , MvX ( ss ) 0 ) of the surface precipitates. The surface precipitation reactions were derived from Farley et. al. [K.J. Farley, D.A. Dzombak, F.M.M. Morel, J. Colloid Interface Sci. 106 (1985) 226] surface precipitation model, which are based on surface complexation model coupled with solid solution representation for surface precipitation on the solid surface. The Δ G f , MvX ( ss ) 0 values were correlated through the following linear free energy relations Δ G f , M ( OH ) 3 ( ss ) 0 - 791.70 r M 3 + = 0.1587 Δ G n , M 3 + 0 - 1273.07 and Δ G f , M 2 ( CO 3 ) 3 ( ss ) 0 - 197.241 r M 3 + = 0.278 Δ G n , M 3 + 0 - 1431.27 where ‘ss’ stands for the end-member solid component of surface precipitate, Δ G f , MvX ( ss ) 0 is in kJ/mol, r M 3 + is the Shannon-Prewitt radius of M 3 + in a given coordination state (nm), and Δ G n , M 3 + 0 is the non-solvation contribution to the Gibbs free energy of formation of the aqueous M 3 + ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.

Energy Transfer from Adenosine Triphosphate: Quantitative Analysis and Mechanistic Insights

The ATP-ADP thermodynamic cycle is the fundamental mode of energy exchange in oxidative phosphorylation, photophosphorylation, muscle contraction, and intracellular transport by various molecular motors and is therefore of vital importance in biological energy transduction and storage. Following a recent suggestion in the pages of this journal (Ross, J. J. Phys. Chem. B 2006, 110, 6987-6990), we have carried out a simple quantitative analysis of a direct molecular mechanism of energy transfer from adenosine triphosphate (ATP). The simulation provides new insights into the mechanistic events following terminal phosphorus-oxygen bond cleavage during ATP hydrolysis. This approach also allows for the division of the energy-transfer process into elementary steps and for the prediction of the distribution of the standard-state Gibbs free energy of the overall ATP hydrolysis process among the various steps of substrate binding, bond cleavage, and product release in the enzymatic cycle, which had proved very difficult to specify previously. These predictions are consistent with available experimental data on ATP hydrolysis by protein biomolecular machines. The fundamental biological implications arising from our results are also discussed in detail. The aspects considered in this work enable us to look at the entire process of ATP synthesis/hydrolysis and energy transduction and storage in various biological molecular machines in a logically consistent and unified way.

Uranophane dissolution and growth in CaCl 2–SiO 2(aq) test solutions

The dissolution and growth of uranophane [Ca(UO 2) 2(SiO 3OH) 2·5H 2O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10 −2 M CaCl 2 and ∼10 −3 M SiO 2(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction ( log K sp o = 11.01 ± 0.09 ) and standard state Gibbs free energy of formation for uranophane ( Δ G f o = - 6196.1 ± 0.5 kJ mol −1).

Solubility measurements of the uranyl oxide hydrate phases metaschoepite, compreignacite, Na–compreignacite, becquerelite, and clarkeite

The mobility of uranium under oxidizing conditions can only be modeled if the thermodynamic stabilities of the secondary uranyl minerals are known. Toward this end, we synthesized metaschoepite (UO 3(H 2O) 2), becquerelite (Ca(UO 2) 6O 4(OH) 6(H 2O) 8), compreignacite (K 2(UO 2) 6O 4(OH) 6(H 2O) 7), sodium compreignacite (Na 2(UO 2) 6O 4(OH) 6(H 2O) 7), and clarkeite (Na(UO 2)O(OH)) and performed solubility measurements from both undersaturation and supersaturation under controlled-pH conditions. The solubility measurements rigorously constrain the values of the solubility products for these synthetic phases, and consequently the standard-state Gibbs free energies of formation of the phases. The calculated lg solubility product values (lg K sp), with associated 1 σ uncertainties, for metaschoepite, becquerelite, compreignacite, sodium compreignacite, and clarkeite are (5.6 −0.2/+0.1), (40.5 −1.4/+0.2), (35.8 −0.5/+0.3), (39.4 −1.1/+0.7), and (9.4 −0.9/+0.6), respectively. The standard-state Gibbs free energies of formation, with their 2 σ uncertainties, for these same phases are (−1632.2 ± 7.4) kJ · mol −1, (−10305.6 ± 26.5) kJ · mol −1, (−10107.3 ± 21.8) kJ · mol −1, (−10045.6 ±24.5) kJ · mol −1, and (−1635.1 ± 23.4) kJ · mol −1, respectively. Combining our data with previously measured standard-state enthalpies of formation for metaschoepite, becquerelite, sodium compreignacite, and clarkeite yields calculated standard-state entropies of formation, with associated 2 σ uncertainties, of (−532.5 ± 8.1) J · mol −1 · K −1, (−3634.5 ± 29.7) J · mol −1 · K −1, ( −2987.6 ± 28.5) J · mol −1 · K −1, and (−300.5 ± 23.9) J · mol −1 · K −1, respectively. The measurements and associated calculated thermodynamic properties from this study not only describe the stability and solubility at T = 298 K, but also can be used in predictions of uranium mobility through extrapolation of these properties to temperatures and pressures of geologic and environmental interest.

Nucleoside Phosphorylation by Phosphate Minerals

In the presence of formamide, crystal phosphate minerals may act as phosphate donors to nucleosides, yielding both 5'- and, to a lesser extent, 3'-phosphorylated forms. With the mineral Libethenite the formation of 5'-AMP can be as high as 6% of the adenosine input and last for at least 10(3) h. At high concentrations, soluble non-mineral phosphate donors (KH(2)PO(4) or 5'-CMP) afford 2'- and 2':3'-cyclic AMP in addition to 5'-and 3'-AMP. The phosphate minerals analyzed were Herderite Ca[BePO(4)F], Hureaulite Mn(2+)(5)(PO(3)(OH)(2)(PO(4))(2)(H(2)O)(4), Libethenite Cu(2+)(2)(PO(4))(OH), Pyromorphite Pb(5)(PO(4))(3)Cl, Turquoise Cu(2+)Al(6)(PO(4))(4)(OH)(8)(H(2)O)(4), Fluorapatite Ca(5)(PO(4))(3)F, Hydroxylapatite Ca(5)(PO(4))(3)OH, Vivianite Fe(2+)(3)(PO(4))(2)(H(2)O)(8), Cornetite Cu(2+)(3)(PO(4))(OH)(3), Pseudomalachite Cu(2+)(5)(PO(4))(2)(OH)(4), Reichenbachite Cu(2+)(5)(PO(4))(2)(OH)(4), and Ludjibaite Cu(2+)(5)(PO(4))(2)(OH)(4)). Based on their behavior in the formamide-driven nucleoside phosphorylation reaction, these minerals can be characterized as: 1) inactive, 2) low level phosphorylating agents, or 3) active phosphorylating agents. Instances were detected (Libethenite and Hydroxylapatite) in which phosphorylation occurs on the mineral surface, followed by release of the phosphorylated compounds. Libethenite and Cornetite markedly protect the beta-glycosidic bond. Thus, activated nucleic monomers can form in a liquid non-aqueous environment in conditions compatible with the thermodynamics of polymerization, providing a solution to the standard-state Gibbs free energy change (DeltaG degrees ') problem, the major obstacle for polymerizations in the liquid phase in plausible prebiotic scenarios.

Thermodynamic properties of soddyite from solubility and calorimetry measurements

The release of uranium from geologic nuclear waste repositories under oxidizing conditions can only be modeled if the thermodynamic properties of the secondary uranyl minerals that form in the repository setting are known. Toward this end, we synthesized soddyite ((UO 2) 2(SiO 4)(H 2O) 2), and performed solubility measurements from both undersaturation and supersaturation. The solubility measurements rigorously constrain the value of the solubility product of synthetic soddyite, and consequently its standard-state Gibbs free energy of formation. The log solubility product (lg K sp) with its error (1 σ) is (6.43 + 0.20/−0.37), and the standard-state Gibbs free energy of formation is (−3652.2 ± 4.2 (2 σ)) kJ mol −1. High-temperature drop solution calorimetry was conducted, yielding a calculated standard-state enthalpy of formation of soddyite of (−4045.4 ± 4.9 (2 σ)) kJ · mol −1. The standard-state Gibbs free energy and enthalpy of formation yield a calculated standard-state entropy of formation of soddyite of (−1318.7 ± 21.7 (2 σ)) J · mol −1 · K −1. The measurements and associated thermodynamic calculations not only describe the T = 298 K stability and solubility of soddyite, but they also can be used in predictions of repository performance through extrapolation of these properties to repository temperatures.

Estimation of surface precipitation constants for sorption of divalent metals onto hydrous ferric oxide and calcite

Equilibrium constants for modeling surface precipitation of divalent metals, M 2+, onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energy of formation, Δ G f o, of the surface precipitates. The experimental Δ G f o of the surface precipitates was derived from the surface precipitation model of Farley et al. [J. Colloid Interface Sci. 106 (1985) 226], which in turn was based on a surface complexation model coupled with solid solution representation for precipitation on the surface. The Δ G f o values are correlated through the relations ΔG f, M(OH) 2( s) o −77.210 r M 2+ =1.03266 ΔG n, M 2+ o −305.368 and ΔG f, MCO 3( s) o −83.991 r M 2+ =0.915 ΔG n, M 2+ o −343.331 where ‘s’ stands for the end-member component of the solid solution, Δ G f o is in kcal/mol, r represents the Shannon–Prewitt radius of M 2+ in a given coordination state (Å), and Δ G n, M 2+ o denotes the non-solvation contribution to the Gibbs free energy of formation of the aqueous M 2+ ion. The coefficients in the correlations were regressed from the aforementioned experimental Δ G f o. The statistically significant correlations ( R 2=0.99) allow approximate estimation of free energies and, hence, equilibrium constants of the surface precipitation reactions for Be 2+, Mg 2+, Ca 2+, Mn 2+, Co 2+, Ni 2+, Sr 2+, Sn 2+, Ba 2+, Eu 2+, Ra 2+, Pb 2+, Hg 2+, Cu 2+, and UO 2 2+.

Partitioning of F-Cl-OH between minerals and hydrothermal fluids

A thermodynamic analysis of F-Cl-OH partitioning between minerals and hydrothermal fluids has resulted in the retrieval of standard-state Gibbs free energies for fluormuscovite, fluorphlogopite, fluorannite, fluortremolite, fluortalc, hydroxyapatite, fluorapatite, chlorapatite, and chlorannite from hydrothermal experimental studies. Standard-state entropies, heat capacities, and volumes are either derived from experimental studies or estimated following Helgeson et al. (1978). The derived thermodynamic properties are internally consistent and consistent with the thermodynamic properties of minerals and aqueous species from Berman (1988, 1990), Sverjensky et al. (1991a), Shock and Helgeson (1988), and Shock et al. (1989), and therefore can be extrapolated over a wide range of temperatures and pressures for application to geochemistry, igneous and metamorphic petrology, and ore deposits. The derived standard-state thermodynamic properties for F and Cl endmember phases provide a basis for predicting the fluoride and chloride concentrations of former aqueous fluids from the measured F and Cl concentrations in minerals. Speciation and solubility calculations simulating F and Cl partitioning between minerals and hydrothermal fluids in the systems Na 2O-K 2OAl 2O 3-SiO 2-HF-H 2O and Na 2O-K 2O-Al 2O 3-SiO 2-HCl-H 2O, and systems containing apatites, show that the partitioning is a strong function of temperature, pressure, and fluid composition. Increase of temperature favors partitioning of F into fluids with respect to minerals, while it favors partitioning of Cl into annite. The decrease of both pressure and pH of fluids favors partitioning of Cl into annite with respect to fluids. In addition to predicting fluoride and chloride concentrations in hydrothermal fluids, the results of this study enable mass transfer calculations including both F-C1-OH partitioning and metal complexing of halides during water-rock interactions in a variety of geological systems.

Calculation of the thermodynamic properties of aqueous electrolytes to 1000.degree.C and 5000 bar from a semicontinuum model for ion hydration

A semicontinuum approach for prediction of standard-state Gibbs free energies of ionic hydration, {Delta}{sub h}G, at high temperature steam conditions has been revised to permit predictions at temperatures from 0 to 1000{degree}C and any pressure up to 5 kbar. The revised semicontinuum model provides for a more realistic identification of inner-shell and outer-shell contributions. The inner-shell term accounts for contributions to {Delta}{sub h}G from the first six H{sub 2}O molecules of hydration. Inner-shell contributions are calculated by using available experimental equilibrium constants for successive ionic hydration reactions and an empirical parameter that represents the effective volume increment per H{sub 2}O of hydration. The outer-shell term accounts for contributions to {Delta}{sub h}G from H{sub 2}O molecules outside the inner-shell region. Outer-shell contributions are obtained from the Born equation using the bulk solvent dielectric constant and an empirical parameter that represents the effective Born radius. The empirical parameters in the revised model are characterized in a manner that ensures reasonable consistency with experimental standard-state volumes and heat capacities for aqueous electrolytes. Calculations on alkali-metal halide and alkali-metal hydroxide electrolytes are considered, and the results for NaCl are used to illustrate the pressure and temperature behavior of standard-state properties. Evaluation of uncertainties inmore » predicted values for {Delta}{sub h}G supports application of the revised model for rough quantitative estimation of standard-state properties at supercritical conditions. The validity of the revised model at supercritical conditions is also supported by a comparison of calculated and experimental apparent molal heat capacities for NaCl.« less

Solubility and complexing of Ni in the system NiO-H 2O-HCl

The solubility of bunsenite (NiO) in Cl-bearing fluids in the range of 450°–700°C, 1–2 kbar was determined using the Ag + AgCl acid buffer technique. Based on the results of the experiments, it is concluded that the associated NiCl 0 2 complex is the dominant Ni species in the fluid over the entire temperature-pressure range investigated. The temperature dependence of the equilibrium constant for the reaction NiO( s) + 2 HCl 0( aq) = NiCl 0 2( aq) + H 2 O is given by log K = −4.17(±0.55) + 4629(±464)/ T( K) at 1 kbar, and log K = −4.75(±0.91) + 5933(±756)/ T( K) at 2 kbar. The calculated difference in standard state Gibbs free energy of formation between NiCl 0 2 and 2HCl 0 in kcal is G 0( NiCl 0 2) − 2 G 0( HCl 0) = −20.77(±2.22) + 0.03264(±0.0026) T( K), at 1 kbar and G 0( NiCl 0 2) − 2 G 0( HCl 0) = −25.01(±1.35) + 0.03264(±0.0016) T( K) at 2 kbar. Comparison of the solubilities of Ni end-member minerals with those of Ca, Mn, Fe, and Mg indicates that nickel minerals generally are the least soluble at a given temperature and pressure. The relatively low solubility of Ni end-member minerals, combined with the relatively low concentration of Ni in most rocks, should result in a quite low mobility of Ni in hydrothermal fluids.