Boris R. Tagirov

Experimental study of dissociation of HCl from 350 to 500°C and from 500 to 2500 bars: Thermodynamic properties of HCl°(aq)

New values of the dissociation constants of HCl° referring to low density supercritical solutions and near-critical temperatures of water (350–500°C and 500–2500 bars) have been obtained based on comparison of AgCl(s) solubility in NaCl or KCl solutions with Ag(s) solubility in HCl + NaCl or KCl solutions at controlled hydrogen fugacities. During the course of this study the thermodynamic properties of AgCls− were refined with the aid of the revised HKF equation of state (Tanger and Helgeson, 1988). The dissociation constants of HCl° obtained in the present work are higher than that found from electrical conductance measurements (Frantz and Marshall, 1984) by more than an order of magnitude at pressures of about 500 bars, but the difference becomes smaller as the pressure increases. Both sets of dissociation constants agree well at high pressures where the isothermal compressibility of water is less than about 1.42·10−4 bar−1. To reliably compare experimental data obtained by different methods, the Redlikh-Kwong equation of state was applied to available literature data as well as to the results of this study. Finally, the standard state thermodynamic properties and HKF parameters for HCl°(aq) were established. These results allow extrapolation of the thermodynamic properties of HCl°(aq) and consequently the HCl dissociation constant up to 700°C and 5000 bars.

Aluminum speciation in crustal fluids revisited

Aluminum speciation in crustal fluids is assessed by means of standard thermodynamic properties at 25°C, 1 bar, and revised Helgeson-Kirkham-Flowers (HKF) (Tanger J. C. IV and Helgeson H. C., “Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Revised equations of state for the standard partial molal properties of ions and electrolytes,” Am. J. Sci. 288, 19–98, 1988) equations of state parameters for aqueous species in the system Al-O-H-Na-Si-Cl-F-SO4 derived from recent experimental data with the help of isocoulombic reactions and correlations among parameters in the HKF model. In acidic to neutral hydrothermal solutions and for fluorine concentrations in excess of 1 ppm, the fluoride complexes AlFn3−n dominate Al speciation at temperature (T) 300°C) hydrothermal and metamorphic fluids, aluminum mobility is considerably enhanced by formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq) ion paired mixed species. NaAl(OH)2F20(aq) controls Al transport in granite-derived fluids and during greisenization. At alkaline pH, Al(OH)4−, Al(OH)3H3SiO4−, and the NaAl(OH)40(aq) ion-pair are the dominant Al species. Thermodynamic calculations show that as a result of strong interactions of Al(aq) with NaOH, NaF, HF, and SiO2(aq) present in crustal fluids, the concentrations of aluminum in equilibrium with Al-bearing minerals can be several orders of magnitude higher than those calculated assuming that only Al hydroxyde complexes are formed. Interactions with these components are likely to be responsible for aluminum mobility during hydrothermal and metamorphic reactions.

Experimental study of aluminum speciation in fluoride-rich supercritical fluids

The solubility of the albite-paragonite-quartz mineral assemblage was measured as a function of NaCl and fluorine concentration at 400°C, 500 bars and at 450°C, 500 and 1000 bars. Decreasing Al concentrations with increasing NaCl molality in F-free fluids of low salinity (mNaCl < 0.01) demonstrates that Al(OH)4− dominates Al speciation and is formed according to the reaction 0.5 NaAl3Si3O12H2(cr)+2 H2O = 0.5 NaAlSi3O8(cr)+Al(OH)4−+H+. Log K results for this reaction are −11.28 ± 0.10 and −10.59 ± 0.10 at 400°C, 500 bars and 450°C, 1000 bars, respectively. Upon further salinity increase, Al concentration becomes constant (at 400°C, 500 bars) or even rises (at 450°C, 1000 bars). The observed Al behavior can be explained by the formation of NaAl(OH)40(aq) or NaAl(OH)3Cl(aq)0. The calculated constant for the reaction Al(OH)4−+Na+=NaAl(OH)40(aq) expressed in log units is equal to 2.46 and 2.04 at 400°C, 500 bars and 450°C, 1000 bars, respectively. These values are in good agreement with the predictions given in Diakonov et al. (1996). Addition of fluoride at m(NaCl) = const = 0.5 caused a sharp increase in Al concentration in equilibrium with the albite-paragonite-quartz mineral assemblage. As fluid pH was also constant, this solubility increase indicates strong aluminum-fluoride complexation with the formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq), according to 0.5 NaAl3Si3O12H2(cr)+Na++HF(aq)0+H2O = 0.5 NaAlSi3O8(cr)+ NaAl(OH)3F(aq)0+H+, log K = −5.17 and −5.23 at 400°C and 450°C, 500 bars, respectively, and 0.5 NaAl3Si3O12H2(cr)+Na++2 HF(aq)0 = 0.5 NaAlSi3O8(cr)+NaAl(OH)2F20(aq)+H+, log K = −2.19 and −1.64 at the same P-T conditions. It was found that temperature increase and pressure decrease promote the formation of Na-Al-OH-F species. Stability of NaAl(OH)2F20(aq) in low-density fluids also increases relative to NaAl(OH)3F(aq)0. These complexes, together with Al(OH)2F(aq)0 and AlOHF20(aq), whose stability constants were calculated from the corundum solubility measured by Soboleva and Zaraisky (1990) and Zaraisky (1994), are likely to dominate Al speciation in metamorphic fluids containing several ppm of fluorine.

A potentiometric study of Eu3+ complexation with acetate ligand from 25 to 170°C at Psat

Abstract Thermodynamic properties for europium acetate (EuCH3COO2+ and Eu(CH3COO)2+) complexes were studied by potentiometry at temperatures from 25 to 170°C at the saturation pressure of water. The thermodynamic association constants (K1 and K2) for the reaction Eu 3+ + n Acite − = EuAc n 3−n . were determined by two different experimental approaches at 25 to 75°C and 75 to 170°C. Logarithms of measured association constants increase with increasing temperature, showing that Eu-Ac complexing increases as temperature rises. Log K1 for EuCH3COO2+ increases from 2.91 at 25°C to 4.25 at 170°C, whereas log K2 for Eu(CH3COO)2+ increases from 4.83 at 25°C to 7.39 at 170°C. Species distribution calculated in this study using the experimentally determined association constants suggests that in acetate-bearing solutions (≥0.05 m) Eu-acetate complexes dominate over the free ion above pH of 4 to temperatures of at least 200°C. At equal total acetate and carbonate concentrations and at 25°C, the relative stability of carbonate and acetate complexes is close. However, as the temperature rises the relative stability of carbonate versus acetate complexes is model dependent.

Experimental study of aluminum-fluoride complexation in near-neutral and alkaline solutions to 300 C

Abstract Solubility of boehmite (from 90 to 300 °C) and gibbsite (at 44.5 °C) was measured as a function of NaF concentration in alkaline NH 4 OH-NH 4 Cl-(NaCl) solutions at saturated vapor pressure. The marked increase of boehmite and gibbsite solubility in the presence of fluorine is explained by the formation of Al(OH) 2 F 2 − according to Al(OH) 4 − +2F − =Al(OH) 2 F 2 − +2OH − . Log K values regressed from these data at 44.5, 90, 150, 200, and 300 °C are −6.51±0.15, −5.06±0.10, −3.98±0.10, −3.49±0.15, and −2.86±0.20, respectively. The dependence of log K on temperature is precisely described (within ±0.07 log unit) by the function p K =−log K =7297.15/ T ( K )−81.53+11.29·ln T ( K ) whose differentiation with respect to T yields the following thermodynamic properties for this reaction: log K 298.15 =−7.26, Δ r H ° 298.15 =75.28 kJ/mol, and Δ r S ° 298.15 =113.4 J/mol K.

X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system

Abstract Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of “invisible” Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of “invisible” Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and löllingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of “invisible” Au in the synthesized löllingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High “invisible” Au contents were observed in hydrothermal pyrite (40–90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of löllingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 Å (As), 0.13 Å (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ∼4 Å. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > löllingite (−0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.