Mica-feldspar equilibria in supercritical alkali chloride solutions

The relations between cordiente, hercynite and magnetite in specimens of the garnetiferous migmatites within the osumilite-in isograd from the high-grade polymetamorphic Precambrian of Rogaland, SW Norway, have been studied, using an electron microprobe. In most specimens the primary spinel of the M 2 stage of metamorphism is exsolved into hercynite and magnetite. The exsolution continued during subsequent cooling to moderate temperature conditions of the M 2 stage or even to retrogressive conditions of metamorphism of the M 4 stage. Fe-Mg distributions give evidence that cordierite and hercynite are in equilibrium at these mentioned conditions, while their textural relations belong to the high-grade metamorphic stage M 2. This implies that neither cordierite nor spinel are suitable for use in geothermometry of high-grade metamorphic stage M 2 in Rogaland. It is tentatively concluded that re-equilibration of Mg and Fe continues to lower retrograde temperatures in the Fe-rich cordierite-hercynite pairs than in Mg-rich pairs.     

Surface charge density on silica in alkali and alkaline earth chloride electrolyte solutions

The surface charge density of colloidal SiO₂ (Aerosil 380) was measured in alkali chloride (0.067 and 0.20 M LiCl, NaCl, and KCl) and alkaline earth chloride (0.067 M MgCl₂, CaCl₂, SrCl₂, BaCl₂) solutions. Measurements were conducted at 25°C by potentiometric titrations using the constant ionic medium method in a CO₂-free system. The experimental design measured surface charge for solutions with constant ionic strength as well as constant cation concentration. Alkali chloride solutions promote negative surface charge density in the order LiCl < NaCl < KCl to give the “regular” lyotropic behavior previously reported. In contrast, the alkaline earth chloride solutions exhibit a reversed lyotropic trend with increasing crystallographic radius where increasing negative charge is promoted in the order BaCl₂ < SrCl₂ < CaCl₂ < MgCl₂.The origin of the opposing affinity trends is probed by testing the hypothesis that this reversal is rooted in the differing solvent structuring characteristics of the IA and IIA cations at the silica-water interface. This idea arises from earlier postulations that solvent structuring effects increase entropy through solvent disordering and these gains must be much greater than the small, positive enthalpy associated with electrostatic interactions. By correlating measured charge density with a proxy for the solvent-structuring ability of cations, this study shows that silica surface charge density is maximized by those electrolytes that have the strongest effects on solvent structuring. We suggest that for a given solid material, solvation entropy has a role in determining the ionic specificity of electrostatic interactions and reiterate the idea that the concept of lyotropy is rooted in the solvent-structuring ability of cations at the interface.     

Mineral-solution equilibria—V. Solubilities of rock-forming minerals in supercritical fluids

The solubility constants of sixty-nine rock-forming minerals have been computed for temperatures between 400 and 600°C at 1000 and 2000 bar pressure using the free-energy data for aqueous solutes presented in Parts I through IV of this series combined with the thermodynamic properties of minerals from Helgesonet al. (1978). An example describing solution compositions in equilibrium with a spilite is discussed. A computer program for calculating solution compositions in equilibrium with mineral assemblages is included as an appendix.     

Solubility of gold in hydrothermal chloride solutions

The solubility of gold has been determined in chloride solutions in the temperature range 300–500°C corresponding to the inferred range for the formation of “hypothermal” gold deposits. The solutions were buffered with respect to HC1 by a K-feldspar-muscovite-quartz assemblage, and to oxygen by the assemblage haematite-magnetite. Solubilities increased rapidly with temperature from about 10 p.p.m. at 300°C, to 500 and 1000 p.p.m. at 500°C at 1000 and 2000 bar, respectively.These results are discussed in terms of possible solution species in this high-temperature region where molecular behaviour predominates in the solution equilibria. It is suggested that gold and other metals may be transported to the site of ore-deposition in undersaturated high-temperature solutions. Ore deposition may take place at lower temperatures where ionic gold chloride or sulfide species dominate the chemistry of the ore solutions.     

Mg-Fe partitioning between biotite and a supercritical chloride solution

An experimental study of the partitioning of Mg and Fe between synthetic biotite and an aqueous chloride solution in the supercritical region as a function of temperature, pressure and concentration of Mg and Fe is reported. In the temperature range 500°–700° C and the pressure range 25–200 MPa, the Mg-Fe distribution between biotite and the chloride solution can be described by distribution curves based on the ideal solution model within a data scattering of 8%. Mg is preferentially partitioned into biotite, and Fe prefers the solution. This tendency is enhanced with increasing temperature. The distribution constants for the Mg-Fe exchange reactions in the system K(Mg,Fe)₃AlSi₃O₁₀(OH)₂-(Mg,Fe)Cl₂-KCl-H₂O have been determined. The present data favor a model in which the activity of Fe and Mg in biotite is close to the mole fraction at temperatures above 500° C. Comparison of the Mg-Fe partitioning between biotite-chloride solution and olivine-chloride solution reveals a slight enrichment of Fe in olivine relative to biotite.     

The solubility of iron hydroxide in sodium chloride solutions

The solubility of iron(III) hydroxide as a function of pH was investigated in NaCl solutions at different temperatures (5–50°C) and ionic strengths (0–5 M). Our results at 25°C and 0.7 M in the acidic range are similar to the solubility in seawater. The results between 7.5 to 9 are constant (close to 10⁻¹¹ M) and are lower than those found in seawater (>10⁻¹⁰) in this pH range. The solubility subsequently increases as the pH increases from 9 to 12. The solubility between 6 and 7.5 has a change of slope that cannot be accounted for by changes in the speciation of Fe(III). This effect has been attributed to a solid-state transformation of Fe(OH)₃ to FeOOH. The effect of ionic strength from 0.1 to 5 M at a pH near 8 was quite small. The solubility at 5°C is considerably higher than at 25°C at neutral pH range. The effects of temperature and ionic strength on the solubility at low and high pH have been attributed to the effects on the solubility product and the formation of FeOH²⁺ and Fe(OH)₄⁻. The results have been used to determine the solubility products of Fe(OH)₃, K¹_Fe(OH)3 and hydrolysis constants, β¹₁, β¹₂, β¹₃, and β¹₄ as a function of temperature (T, K) and ionic strength (I):log K¹_Fe(OH)3 = −13.486 − 0.1856 I^0.5 + 0.3073 I + 5254/T (σ = 0.08)log β¹₁ = 2.517 − 0.8885 I^0.5 + 0.2139 I − 1320/T (σ = 0.03)log β¹₂ = 0.4511 − 0.3305 I^0.5 − 1996/T (σ = 0.1)log β¹₃ = −0.2965 − 0.7881 I^0.5 − 4086/T (σ = 0.6)log β¹₄ = 4.4466 − 0.8505 I^0.5 − 7980/T. (σ = 0.2)Both strong ethylenediaminetetraacetic acid and weak (HA) organic ligands greatly affect iron solubility. The additions of ethylenediaminetetraacetic acid and humic material were shown to increase the solubility near pH 8. The higher solubility of Fe(III) in seawater compared to 0.7 M NaCl may be caused by natural organic ligands.     

Iron control by equilibria between hydroxy-Green Rusts and solutions in hydromorphic soils

In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mössbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [Fe^II_1−xFe^III_x(OH)₂]^+x· [x/z A^−z]^−x, where compensating interlayer anions, A⁻, can be Cl⁻, SO₄²⁻, CO₃²⁻ or OH⁻, and where x ranges a priori from 0 to 1. In large ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fe_t, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mössbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log K_sp = 28.2 ± 0.8 for the reaction Fe₃(OH)₇ + e⁻ + 7 H⁺ = 3 Fe²⁺ + 7 H₂O; log K_sp = 25.4 ± 0.7 for the reaction Fe₂(OH)₅ + e⁻ + 5 H⁺ = 2 Fe²⁺ + 5 H₂O; log K_sp = 45.8 ± 0.9 for the reaction Fe₃(OH)₈ + 2e⁻ + 8 H⁺ = 3 Fe²⁺ + 8 H₂O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: Δ_fG° (Fe₃(OH)₇, cr, 282.15 K) = −1799.7 ± 6 kJ mol⁻¹, Δ_fG° (Fe₂(OH)₅, cr, 282.15 K) = −1244.1 ± 6 kJ mol⁻¹ and Δ_fG° (Fe₃(OH)₈, cr, 282.15 K) = −1944.3 ± 6 kJ mol⁻¹.     

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

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

A method of calculating quartz solubilities in aqueous sodium chloride solutions

The aqueous silica species that form when quartz dissolves in water or saline solutions are hydrated. Therefore, the amount of quartz that will dissolve at a given temperature is influenced by the prevailing activity of water. Using a standard state in which there are 1,000 g of water (55.51 moles) per 1,000 cm³ of solution allows activity of water in a NaCl solution at high temperature to be closely approximated by the effective density of water, p_e, in that solution, i.e. the product of the density of the NaCl solution times the weight fraction of water in the solution, corrected for the amount of water strongly bound to aqueous silica and Na⁺ as water of hydration. Generally, the hydration of water correction is negligible.The solubility of quartz in pure water is well known over a large temperature-pressure range. An empirical formula expresses that solubility in terms of temperature and density of water and thus takes care of activity coefficient and pressure-effect terms. Solubilities of quartz in NaCl solutions can be calculated by using that equation and substituting p_e, for the density of pure water. Calculated and experimentally determined quartz solubilities in NaCl solutions show excellent agreement when the experiments were carried out in non-reactive platinum, gold, or gold plus titanium containers. Reactive metal containers generally yield dissolved silica concentrations higher than calculated, probably because of the formation of metal chlorides plus NaOH and H₂. In the absence of NaOH there appears to be no detectable silica complexing in NaCl solutions, and the variation in quartz solubility with NaCl concentration at constant temperature can be accounted for entirely by variations in the activity of water.The average hydration number per molecule of dissolved SiO₂ in liquid water and NaCl solutions decreases from about 2.4 at 200°C to about 2.1 at 350°C. This suggests that H₄SiO₄ may be the dominant aqueous silica species at 350°C, but other polymeric forms become important at lower temperatures.     

Selective zinc-lead extraction from chloride solutions with long-chain amines

The zinc-lead separation from a chloride solution in the form of a synthetic leach liquor has been studied using tertiary aliphatic amines and tetra-alkylammonium salts. Larger extractions, and hence distribution coefficients, are observed with ammonium salts. However, the best separation factors occur when employing tertiary amines. The results show that it is possible to leave the lead in the aqueous solution whilst the zinc is transferred to the organic phase by a proper choice of the hydrochloric acid and extractant concentrations. An approach to coordination chemistry is suitable to explain the extraction mechanism.     

A spectrophotometric study of neodymium(III) complexation in chloride solutions

The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250°C and a pressure of 50 bars. The simple ion, Nd³⁺, is dominant at 25°C, whereas NdCl²⁺ and NdCl₂⁺ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions:Nd³⁺ + Cl⁻ = NdCl²⁺ β₁,Nd³⁺ + 2 · Cl⁻ = NdCl⁺₂ β₂.The values of β₁ were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of β₂ obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200°C.Theoretical estimates of β₁ and β₂ by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl²⁺ and NdCl₂⁺ at temperatures above 150°C than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for β₁ and β₂ similar to those obtained experimentally.Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of ∼2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth’s crust.     

Electrical conductivity of alkali feldspar solid solutions at high temperatures and high pressures

The electrical conductivities of alkali feldspar solid solutions ranging in chemical composition from albite (NaAlSi₃O₈) to K-feldspar (KAlSi₃O₈) were measured at 1.0 GPa and temperatures of 873–1,173 K in a multi-anvil apparatus. The complex impedance was determined by the AC impedance spectroscopy technique in the frequency range of 0.1–10⁶ Hz. Our experimental results revealed that the electrical conductivities of alkali feldspar solid solutions increase with increasing temperature, and the linear relationship between electrical conductivity and temperature fits the Arrhenius formula. The electrical conductivities of solid solutions increase with the increasing Na content at constant temperature. At 1.0 GPa, the activation enthalpy of solid solution series shows strong dependency on the composition, and there is an abrupt increase from the composition of Or₄₀Ab₆₀ to Or₆₀Ab₄₀, where it reaches a value of 0.96 eV. According to these results in this study, it is proposed that the dominant conduction mechanism in alkali feldspar solid solutions under high temperature and high pressure is ionic conduction. Furthermore, since the activation enthalpy is less than 1.0 eV for the alkali feldspar solid solutions, it is suggested to be a model where Na⁺ and K⁺ transport involves an interstitial mechanism for electrical conduction. The change of main charge carriers can be responsible for the abrupt increase in the activation energy for Or₆₀Ab₄₀. All electrical conductivity data were fitted by a general formula in order to show the dependence of activation enthalpy and pre-exponential factor on chemical composition. Combining our experimental results with the effective medium theory, we theoretically calculated the electrical conductivity of alkali feldspar granite, alkali feldspar quartz syenite, and alkali feldspar syenite with different mineral content and variable chemical composition of alkali feldspar at high temperatures at 1.0 GPa, and the calculated results are almost in agreement with previous experimental studies on silicate rocks.     

Leaching of an oxide gold ore with chloride/hypochlorite solutions

An oxide gold ore was subjected to chloride/hypochlorite leaching at room temperature. The effects of three factors, including Ca(OCl)₂ vs. NaOCl, OCl⁻ concentration, and HCl concentration on gold leaching performance were investigated. Due to formation of CaOCl⁺ complex in solution and hence less reactivity, calcium hypochlorite produces a sluggish gold leaching kinetics, taking twice the time (46 h) to achieve maximum gold recovery of 58% compared to sodium hypochlorite. 10 g/L of total initial hypochlorite species in solution produces reasonable gold recoveries. The amount of added HCl and hence the initial pH was found to have a major effect on gold leaching kinetics and maximum gold recovery. A high level of 9 g/L of added HCl causes HClO to be very reactive, producing very fast kinetics, reaching 67% gold extraction in 4 h. It also causes a faster consumption of hypochlorous acid, through catalytic decomposition (by NiO and CuO) and disproportionation. Hypochlorous acid reactions with sulfide and ferrous content of ore proceed very slowly in the pH range of 4–11. Gold–chloro complexes are strongly adsorbed on quartz component of ore. To minimize this undesirable adsorption of gold–chloro species, the aging time must be limited to a few hours only.     

Antimonous acid protonation/deprotonation equilibria in hydrothermal solutions to 300 °C

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⁻     

High pressure phase equilibria in the system CaTiO3-CaSiO3: stability of perovskite solid solutions

Phase relations in the system CaTiO₃-CaSiO₃ were experimentally examined at 5.3–14.7 GPa and 1200–1600 °C with a 6–8 type multianvil apparatus. As pressure increases, stability field of perovskite solid solution extends from CaTiO₃ to CaSiO₃, and the perovskite becomes stable for the entire composition range above about 12.3 GPa. The stability field of Ca(Ti_1?X, Si_X)₂O₅ (0.78<x≦1) titanite solid solution +Ca₂SiO₄ larnite exists in the CaSiO₃-rich composition range at 9.3–12.3 GPa and 1200 °C. Perovskite solid solutions containing CaSiO₃ component of 0 to 66 mol% could be quenched to 1 atm. The composition-molar volume relationship of perovskite solid solution showed that molar volume of perovskite solid solution linearly reduces from the value of CaTiO₃ to that of CaSiO₃.     

Intermobility of barium,strontium, and lead in chloride and sulfate leach solutions

Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2–0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t_50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t_50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.

     

Analcime equilibria

The stability fields of analcime and analcime+quartz have been investigated using conventional hydrothermal techniques, over the approximate range of conditions 160–600 °C and 500–5000 bars fluid pressure. The dehydration of analcime (Na₂Al₂Si_3·3O_11·6 · nH₂O) to albite, nepheline and H₂O occurs at temperatures of 492±5 °C at 500 bars, 538±5 °C at 1000 bars, 578±5 °C at 2000 bars and 598±5 °C at 3000 bars. In the presence of quartz, analcine dehydrates to highly disordered albite and H₂O at about 200 °C and 2000 bars, 196°±5 °C and 3000 bars, about 190 °C and 4000 bars, and 183±5 °C at 5000 bars P_fluid. The synthetic phase equilibria appear to be compatible with field observations that primary analcimes occur as phenocrysts or in groundmass in some volcanic and hypabyssal rocks and secondary analcimes in sedimentary, hydrothermally altered and low-grade metamorphic rocks.