An electron microscopy study of naturally occurring oxidation produced precipitates in iron-bearing olivines

Iron-bearing olivine grains naturally altered by oxidation were examined in the transmission electron microscope to determine the precipitate phases and their crystallographic and morphological relationships to the host. Precipitate complexes heterogeneously nucleated on dislocations were composed of Si-rich, Mg-rich/Si-rich, and Fe-rich regions corresponding to - tridymite, enstatite, and magnetite and/or hematite, respectively. The tridymite and magnetite (hematite) occurred as rod-like interleaved fingers, while the enstatite was more equidimensional. The crystal orientations of the precipitate phases with respect to the host structure, listed in Table 2, were well defined, but, in general, could not be simply related to the close packing of oxygen planes. Iron-rich (001) planar precipitates occasionally nucleated homogeneously in the host as well as heterogeneously on dislocations. Oxygen diffusion does not appear to be the rate-controlling process for precipitate nucleation and growth as precipitation kinetics in fresh olivine oxidized at 900 ° C would indicate an oxygen diffusivity of 10^–8 cm²/sec, a value 10 orders of magnitude faster than determined previously.     

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures

Solubility experiments were performed at 30 kbars in the system Mg₂SiO₄-SiO₂-H₂O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO₂ component from solids of appropriate SiO₂-rich compositions in the system Mg₂SiO₄-SiO₂-H₂O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO₂). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure.Comparison of the results with previous solubility studies in the system K₂O-MgO-Al₂O₃-SiO₂-H₂O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.     

Laboratory simulation of an oxidizing perturbation in a deep granite environment

An experiment designed to study oxidizing perturbations in deep crystalline rock, a potential host for nuclear waste disposal, was conducted. This experiment simulated a fracture surface in contact with circulating groundwater, in which dissolved oxygen was injected periodically. Major physicochemical and biological parameters were monitored during this 1-yr experiment. Modeling of the results indicates that the kinetics of oxygen uptake may be represented by a simple steady-state rate law combining enzymatic catalysis (Monod) and a first-order rate law. Combined chemical and biological data demonstrate the coupling of organic/inorganic processes during the uptake of dissolved oxygen and the progressive return to reducing conditions. Timescales for these stages are discussed. Experimental results also suggest that iron-reducing bacteria, which are robust and well-adapted microorganisms, play a key role in these interfacial processes. These results show that an operational definition of the “redox buffering capacity” in a granitic medium cannot ignore the effect of bacteria and therefore the controls on bacterial substrates (organic carbon, H₂, CH₄, CO₂).     

Structural and mineralogical evaluation of an experimentally produced impact crater in granite

A crater 30 cm in diameter and 4.4 cm in depth was produced upon impact of an aluminium sphere with a homogeneous granite target. The volume excavated was 748 cm³, the mass ejected 1933 g. The crater geometry is compared with previous laboratory experiments. Mineralogical investigations revealed that shock induced, microscopic fracturing is lowest in the direction of uniaxial compression, followed by a 45° profile. Due to reflections of stress waves at the free surface, the horizontal profile displayed the highest fracture index. Kinking of biotite was very common in samples close to the crater walls (≈ 50 kb). However it faded out at a distance which corresponds to approximately 10 kb. This seems to be the lower pressure limit for the formation of kink bands under shock conditions.     

Spatial and temporal distribution and characteristics of eukaryote-dominated microbial biofilms in an acid mine drainage environment: implications for development of iron-rich stromatolites

Spatial and temporal distribution and characteristics of three eukaryotic biofilms were monitored for an 18-month period in an acid mine drainage environment at the Green Valley coalmine in Indiana, USA. Each biofilm is dominated (>90 %) by a single eukaryotic microorganism based on enumeration: Euglena mutabilis, the diatom species Nitzschia tubicola, and a filamentous alga belonging to the genus Klebsormidium sp. The E. mutabilis-dominated biofilm occurs year round, covering up to 100 % of the channel bottom in spring and fall. The N. tubicola-dominated biofilm is less abundant, exists as small patches in spring and fall, expands from these patches to cover up to 50 % of the channel bottom in June, and is absent in winter. The Klebsormidium-dominated biofilm is restricted to small patches covering <5 % of the channel bottom from spring through fall and is absent in winter. Also present are floating microbial scum layers. The eukaryotic biofilms and scum layers contribute to the attenuation of precipitates and to the formation of organosedimentary structures, or stromatolites, by trapping and binding chemical precipitates via aerotaxis and phototaxis and by serving as a medium for passive accumulation of precipitates. Each stromatolite layer represents the morphological characteristics of each eukaryotic biofilm that served as the architect of the layer and the time of year the biofilm populated the channel. Processes involved in stromatolite formation also attenuate chemical sediments by binding them to the channel bottom and prior stromatolite surface rather than allowing them to be carried to the adjacent drainage system where they may become bioavailable to other forms of life.     

Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy

X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe²⁺ and Fe³⁺ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe²⁺-O bond distance, closely matching octahedral Fe²⁺-O bonds obtained from solid oxide model compounds. Solution Fe²⁺-O bond distances decrease with chloride/iron ratio, pH, and total FeCl₂ concentration. A slight intensification of the $\text{1s → 3d}$ transition with increasing FeCl₂ concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe³⁺ solutions show a pronounced increase in the $\text{1s → 3d}$ transition intensities between 1.0 M FeCl₃/7.8 M Cl^? to 1.0 M FeCl₃/ 15 M Cl^?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe³⁺-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H₂O)₆³⁺ or trans-Fe(H₂O)₄Cl₂ or both; Fe³⁺-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe³⁺-Cl bond distances of 2.25 Å].     

The chemistry of pyrite formation in aqueous solution and its relation to the depositional environment

Experimental investigations on pyrite synthesis indicate that before pyrite can be produced by a reaction involving ferrous iron, the disulphide ion must be formed; in experiments described the ion was obtained by the action of H₂S in aqueous solution on elemental sulphur. Conditions under which the experiments were conducted indicate that pyrite will not form above pH 6.0. The reaction to produce pyrite is fastest when oxygen is excluded and elemental sulphur is produced from the oxidation of H₂S by ferric iron. A reaction between FeS and elemental sulphur will yield pyrite at a much slower rate, although the same basic reaction is involved. An attempt has been made to relate the occurrence of pyrite in different sedimentary environments to this basic chemistry.

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Zusammenfassung Wie Versuche zeigen, ist die Voraussetzung der Pyrit-Bildung das Vorliegen von S ₂ ^2– -Ionen, die dann mit Fe^II reagieren. Die S ₂ ^2– -Ionen wurden durch Einwirken einer verdünnten H₂S-Lösung auf elementaren Schwefel erhalten. Pyrite entstehen in diesen Experimenten somit nur unterhalb pH 6. Pyrit erhält man am schnellsten, wenn Sauerstoff abwesend ist und der H₂S durch Fe^III oxidiert wird. Die Umsetzung von FeS mit elementarem Schwefel liefert Pyrit wesentlich langsamer, wenn auch die zugrunde liegenden Reaktionen sich entsprechen. Es wird versucht, sedimentäre Pyrit-Vorkommen entsprechend diesen Reaktionsabläufen zu deuten.

     

Modeling the effects of bond environment on equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: , , , FeCl₃(H₂O)₃, FeCl₃(H₂O)₂, , FeCl₅H₂O²⁻, , ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation.Relative to , all models show a nearly linear decrease in ⁵⁶Fe/⁵⁴Fe as the number of Cl⁻ ions per Fe³⁺ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl⁻ at 20 °C. At 20 °C, 1000 ln β (β = ⁵⁶Fe/⁵⁴Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for , 8.04-9.12‰ for , 7.61-8.73‰ for , 7.14-8.25‰ for , and 3.09-4.41‰ for . The fractionation between and ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe³⁺/Fe²⁺ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models.Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe-X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for , ,, respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl₃(H₂O)₂ vs. FeCl₃(H₂O)₃ and 5.25‰ vs. 4.94‰ for vs. FeCl₅H₂O²⁻ with the B3LYP/6-31G(d) model). As more Fe-Cl bonds substitute for Fe-OH₂ bonds (with a resulting decrease in β), the lengths of the Fe-Cl bonds and the Fe-O bonds increase.Preliminary modeling of shows an Fe³⁺/Fe²⁺ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H₂O molecules to models of improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8-1.0‰. An additional hydration sphere around increases 1000 ln β by only 0.1‰.Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record.     

An X-ray absorption fine structure and nuclear magnetic resonance spectroscopy study of gallium-silica complexes in aqueous solution

The influence of aqueous silica on gallium(III) hydrolysis in dilute (2 × 10⁻⁴ ≤ m_Ga ≤ 5 × 10⁻³) and moderately concentrated (0.02 ≤ m_Ga ≤ 0.3) aqueous solutions was studied at ambient temperature, using high resolution X-ray absorption fine structure (XAFS) and nuclear magnetic resonance (NMR) spectroscopies, respectively. Results show that, in Si-free acidic solutions (pH < 3), Ga is hexa-coordinated with oxygens of H₂O molecules and/or OH groups in the first coordination sphere of the metal. With increasing pH, these hydroxyl groups are progressively replaced by bridging oxygens (-O-), and polymerized Ga-hydroxide complexes form via Ga-O-Ga chemical bonds. In the 2.5-3.5 pH range, both XAFS and NMR spectra are consistent with the dominant presence of the Ga₁₃ Keggin polycation, which has the same local structure as A1₁₃. Under basic pH (pH > 8), Ga exhibits a tetrahedral coordination, corresponding to Ga(OH)₄⁻ species, in agreement with previous NMR and potentiometric studies. Major changes in Ga hydrolysis have been detected in the presence of aqueous silica. Ga is tetra-coordinated, both in basic and acid (i.e., at pH > 2.7) Si-bearing solutions (0.01 ≤ m_Si ≤ 0.2), and forms stable gallium-silicate complexes. In these species, Ga binds via bridging oxygen to 2 ± 1 silicons, with an average Ga-Si distance of 3.16 ± 0.05 Å, and to 2 ± 1 silicons, with an average Ga-Si distance of 3.39 ± 0.03 Å. These two sets of Ga-Si distances imply the formation of two types of Ga-silicate aqueous complex, cyclic Ga-Si_2-3 species (formed by the substitution of Si in its tri-, tetra- or hexa-cyclic polymers by Ga atoms), and chainlike GaSi_2-4 species (similar to those found for A1), respectively. The increase in the number of Si neighbors (a measure of the complex concentration and stability), in alkaline media, with increasing SiO₂(aq) content and decreasing pH is similar to that for A1-Si complexes found in neutral to basic solutions. At very acid pH and moderate silica concentrations, the presence of another type of Ga-Si complex, in which Ga remains hexa-coordinated and binds to the silicon tetrahedra via the GaO₆ octahedron corners, has also been detected. These species are similar to those found for Al³⁺ in acid solutions. Thus, as for aluminum, silicic acid greatly hampers Ga hydrolysis and enhances Ga mobility in natural waters via the formation of gallium-silicate complexes.     

Thermodynamic properties of Pd chloride complexes and the Pd2+(aq) ion in aqueous solutions

Based on the expert review of literature data on the thermodynamic properties of species in the Cl-Pd system, stepwise and overall stability constants are recommended for species of the composition [PdCl _n ]^{2 ? n} , and the standard electrode potential of the half-cell PdCl ₄ ^2? /Pd(c) is evaluated at E _298,15° = 0.646 ± 0.007 V, which corresponds to Δ _f G _298.15° = ?400.4 ± 1.4 kJ/mol for the ion PdCl ₄ ^2? (aq). Derived from calorimetric data, Δ _f H _298.15° PdCl ₄ ^2? (aq) = ?524.6 ± 1.6 kJ/mol and Δ _f H _298.15° Pd²⁺(aq) = 189.7 ± 2.6 kJ/mol. The assumed values of the overall stability constant of the PdCl ₄ ^2? ion and the standard electrode potential of the PdCl ₄ ^2? /Pd(c) half-cell correspond to Δ _f G _298.15° = 190.1 ± 1.4 kJ/mol and S _298.15° = ?94.2 ± 10 J/(mol K) for the Pd²⁺(aq) ion.     

Inorganic species in geologic fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes

Correlations among experimentally determined standard partial molal thermodynamic properties of inorganic aqueous species at 25 degrees C and 1 bar allow estimates of these properties for numerous monatomic cations and anions, polyatomic anions, oxyanions, acid oxyanions, neutral oxy-acid species, dissolved gases, and hydroxide complexes of metal cations. Combined with correlations among parameters in the revised Helgeson-Kirkham-Flowers (HKF) equation of state (Shock et al., 1992), these estimates permit predictions of standard partial molal volumes, heat capacities, and entropies, as well as apparent standard partial molal enthalpies and Gibbs free energies of formation to 1000 degrees C and 5 kb for hundreds of inorganic aqueous species of interest in geochemistry. Data and parameters for more than 300 inorganic aqueous species are presented. Close agreement between calculated and experimentally determined equilibrium constants for acid dissociation reactions and cation hydrolysis reactions supports the generality and validity of these predictive methods. These data facilitate the calculation of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth.     

Halocarbons in the environment: estimates of thermodynamic properties for aqueous chloroethylene species and their stabilities in natural settings

Standard partial molal thermodynamic parameters for the aqueous chlorinated-ethylene species, perchloroethylene (PCE), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), cis-1,2-dichloroethylene (cis-1,2-DCE), trans-1,2-dichloroethylene (trans-1,2,-DCE), and vinyl chloride (VC) have been estimated by using experimental gas-solubility data and correlation algorithms. The provided thermodynamic values may be used to calculate properties of reactions involving the aqueous chloroethylene species at a wide range of temperatures and pressures. Estimated values for the chloroethylenes were used, along with published values for minerals, gases, aqueous ions, and aqueous neutral organic species, to calculate the stability of chloroethylene species in equilibrium with the minerals magnetite, hematite, pyrite, and pyrrhotite in the subsurface. Estimated values for the aqueous chloroethylenes were also used to calculate reduction potentials for microbially-mediated reductive dechlorination half-reactions at elevated temperatures. Calculations indicate that all aqueous chloroethylene species are energetically favored to decompose to ethylene(aq) under a wide range of conditions in the subsurface, by both abiotic and biotic pathways. Anaerobic microbially mediated degradation is especially favored under conditions at least sufficiently reducing to promote sulfate-reduction, but not under conditions sufficient for microbial denitrification, pyrolusite reduction, or ferric-iron reduction.     

Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation

The local and geometrical structure around gold (III) e.g., Au³⁺ ions in aqueous solution with different OH⁻/Cl⁻ molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCl_n(OH)_4−n]⁻ solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl⁻ ligands by OH⁻ ligands. A square–planar geometry for [AuCl_n(OH)_4−n]⁻ with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH⁻/Cl⁻ molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.     

Reactions of aqueous iron-DFOB (desferrioxamine B) complexes with flavin mononucleotide in the absence of strong iron(II) chelators

The mechanisms controlling microbial uptake of Fe^III-siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the Fe^III center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of Fe^III-DFOB by various reducing agents in solutions containing Fe^II-chelating colorimetric agents for monitoring reaction progress, but the importance of these findings is unclear because the colorimetric agents themselves stabilize and enhance the reactions being monitored. This study examines the reduction of Fe^III complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMN_HQ) in the absence of strong Fe^II-chelating agents, and Fe redox cycling in solutions containing DFOB and oxidized and reduced FMN species. Experimental results demonstrate that the rate and extent of Fe^III-DFOB reduction is strongly dependent on pH and FMN_HQ concentration. At pH ? 5, incomplete Fe^III reduction is observed due to two processes that re-oxidize Fe^II, namely, the autodecomposition of Fe^II-DFOB complexes (Fe^II oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of Fe^II-DFOB complexes with the fully oxidized flavin mononucleotide product (FMN_OX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse Fe^II oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state Fe^II concentrations observed over a range of pH conditions in the presence of excess FMN_HQ and FMN_OX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of Fe^III/Fe^II and FMN_OX/FMN_HQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated Fe^III-DFOB and Fe^II-DFOB complexes, where protonation of one hydroxamate group opens up two Fe coordination positions. This finding suggests that ternary complex formation (FMN-Fe-DFOB) facilitates inner-sphere electron transfer reactions between the flavin and Fe center.     

Stability of chloridogold(I) complexes in aqueous solutions from 300 to 600°C and from 500 to 1800 bar

The solubility of gold has been measured in the system H₂O+H₂+HCl+NaCl+NaOH at temperatures from 300 to 600°C and pressures from 500 to 1800 bar in order to determine the stability and stoichiometry of chloride complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave system. This approach permitted the independent determination of the concentrations of all critical aqueous components in solution for the determination of the stability and stoichiometry of gold(I) complexes. The solubilities (i.e. total dissolved gold) were in the range 9.9 × 10⁻⁹ to 3.26 × 10⁻⁵ mol kg⁻¹ (0.002-6.42 mg kg⁻¹) in solutions of total dissolved chloride between 0.150 and 1.720 mol kg⁻¹, total dissolved sodium between 0.000 and 0.975 mol kg⁻¹ and total dissolved hydrogen between 4.34 × 10⁻⁶ and 7.87 × 10⁻⁴ mol kg⁻¹. A nonlinear least squares treatment of the data demonstrates that the solubility of gold in chloride solutions is accurately described by the reactions,

     

A spectrophotometric study of aqueous copper(I)-chloride complexes in LiCl solutions between 100 °C and 250 °C

Copper transport and deposition in highly saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I)-chloride complexes at elevated temperatures and in highly saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This paper presents the results of a study of copper(I)-chloride complexes at much higher chloride concentrations, 1.5 m to 9.1 m, using a UV spectrophotometric method. The UV spectra of copper(I)-bearing LiCl solutions were measured at temperatures between 100 °C and 250 °C at vapor-saturated pressures and quantitative interpretation of the spectra shows that CuCl₂⁻, CuCl₃²⁻, and CuCl₄³⁻ were present in the experimental solutions. The fitted logarithms of formation constants (log K) for CuCl₂⁻ are in good agreement with the previous results of solubility experiments reported by Xiao et al. (1998) and Liu et al. (2001). The log K values for CuCl₃²⁻ also agree with those of Liu et al. (2001) and theoretical estimates of Sverjensky et al. (1997). This study presents the first experimentally determined formation constants for CuCl₄³⁻, at temperatures greater than 25 °C, and indicates that this complex predominates at chloride concentrations greater than 5 m. Based on the new log K values generated from this study, the calculated chalcopyrite solubility in NaCl solutions indicates that in addition to cooling, fluid mixing (dilution of saline fluids) may be an important factor controlling the deposition of copper minerals from hydrothermal solutions.     

A critical evaluation of the thermodynamic data for boron ions,ion pairs,complexes, and polyanions in aqueous solution at 298.15 K and 1 bar

Energy related industrial development, municipal waste treatment processes and some natural sources, contribute to B (being present) in the environment in elevated concentrations. Boron forms numerous aqueous ion pairs and complexes, some of which would not normally exist in significant concentrations under typical baseline environmental conditions. A complete understanding of the aqueous chemistry of B depends on an accurate evaluation of the thermodynamic data for these species.Equilibrium constants for 29 inorganic complexes, ion pairs, and polyanions of B have been critically evaluated. Apparent equilibrium constants have been extrapolated to zero ionic strength, and the selected thermodynamic values are tabulated. Gibbs free energies of formation for the aqueous species have been computed; these are presented with entropy and enthalpy values where available     

Zinc complexation in aqueous sulfide solutions: Determination of the stoichiometry and stability of complexes via ZnS(cr) solubility measurements at 100 °C and 150 bars

The solubility of ZnS_(cr) was measured at 100 °C, 150 bars in sulfide solutions as a function of sulfur concentration (m(S_total) = 0.02-0.15) and acidity (pH_t = 2-11). The experiments were conducted using a Ti flow-through hydrothermal reactor enabling the sampling of large volumes of solutions at experimental conditions, with the subsequent concentration and determination of trace quantities of Zn. Prior to the experiments, a long-term in situ conditioning of the solid phase was performed in order to attain the reproducible Zn concentrations (i.e. solubilities). The ZnS_(cr) solubility product was monitored in the course of the experiment. The following species were found to account for Zn speciation in solution: Zn²⁺ (pH_t < 3), (pH_t 3-4.5), (pH_t 5-8), and ZnS(HS)⁻ (pH_t > 8) (pH_t predominance regions are given for m(S_total) = 0.1). Solubility data collected in this study at pH_t > 3 were combined with the ZnS_(cr) solubility product determined at lower pH to yield the following equilibrium constants (t = 100 °C, P = 150 bars):