Robert Gout

Kinetically controlled variations of major components and carbon and oxygen isotopes in a calcite-precipitating spring

Abstract A calcite-depositing stream in the Pyrenees, south France, enabled us to study the equilibration of major components and carbon and oxygen isotopes during the formation of calcite and the degassing of CO 2 , and to examine the possible influence of aquatic flora on these processes. Calcite is not precipitated from the solution until a supersaturation of ∼ 10 is attained. In order to start the reaction the activity of CO 2− 3 has to be increased from ∼ 5 · 10 −6 to 22 · 10 −6 . This is caused by an increase of pH due to the loss of CO 2 to the atmosphere. The excess free energy provided by CO 2 degassing is ∼ −5 kJ mol −1 . If formed under equilibrium conditions, the calcite should differ isotopically by Δ 13 C = + 2.3‰ from the dissolved carbonate. However, the observed difference is virtually zero and demonstrates substantial disequilibrium with respect to the stable carbon isotopes. Isotopic disequilibrium is also indicated by oxygen. 13 O-temperatures differ on the average by ∼ + 5°C from the observed temperatures of calcite precipitation. In contrast to calcite formation carbon-isotope equilibrium is attained between the dissolved carbonate and the atmosphere on the degassing of CO 2 . The observed changes of δ 13 C are in agreement with theoretical expectations, unless the surface area of the water is large. The variations of the major components and the carbon and oxygen isotopes observed during diurnal cycles are small and demonstrate that metabolic effects are negligible in a system with a high supply rate of ∼ 25 g HCO − 3 s −1 .

Thermodynamic properties and stoichiometry of As (III) hydroxide complexes at hydrothermal conditions

The stoichiometry and thermodynamic properties of As (III) hydroxide complexes were determined from both solubility and Raman spectroscopic measurements. Arsenolite, claudetite, and orpiment solubilities were measured at temperatures to 250 and 300 °C, respectively, in acid solutions (pH < 6) at the saturated vapor pressure of the system. Raman spectroscopic measurements were performed on As2O3-H2O solutions (0.02 ≤ As ≤ 6 m; 0 ≤ pH ≤ 9) at temperatures from 20 to 275 °C. Results indicate that H3AsO30(aq) is the dominant As-bearing species at concentrations up to ~1 m over a wide range of pH (0–8) and temperature (20–300 °C). At higher As concentrations (≥1–2 m), a polymerization-dehydration of H3AsO30(aq) occurs via the formation of As-O-As bonds, leading to the formation of poly-As aqueous complexes. These experimental results were combined with corresponding properties for arsenolite, claudetite, and orpiment obtained in this study to generate H3AsO30(aq) thermodynamic properties within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988). Calculations carried out using these properties indicate that orpiment, realgar, and native As can control As concentration in epithermal fluids at T ≤ 150–200 °C. At higher temperatures (≥250 °C), it is shown that arsenopyrite in association with pyrite and pyrrhotite or cassiterite can control As deposition in hydrothermal environments.

Boehmite solubility and aqueous aluminum speciation in hydrothermal solutions (90–350°C): Experimental study and modeling

Abstract The solubility of a pure synthetic boehmite was measured in noncomplexing solutions over a wide range of pH (2–9) and temperatures (90–350°C). A least-squares linear regression of these data was used to determine the dissociation constants ( K ∗ s 0 to K ∗ s 4 ) of the mineral. Values obtained in basic solutions ( K ∗ s 4 ) are in close agreement with published data. In weakly acidic and neutral media, however, the values are lower than those reported previously in the literature. These differences are the result of the use of improved analytical techniques with lower detection limits and a more thermodynamically pure and stable solid material. The hydrolysis constants deduced in the present study indicate that, as temperature increases, aluminum hydrolysis becomes stronger, and the stability fields of Al(OH) − 4 and Al(OH) 0 3 expand at the expense of the other species. A density model (Franck, 1956; Anderson et al., 1991) was applied to extrapolate the experimental data to 25°C. This extrapolation permitted generation of a consistent set of thermodynamic data ( Δ 0 f ,298.15 and ΔH 0 f,298.15 ) for the aqueous aluminum monomer species. These data have been used to calculate the solubility of corundum at high temperature and pressure. The close agreement obtained with available experimental data indicates that the aluminate ion is dominant in most crustal fluids, in the absence of large amounts of complexing species.

AN EXPERIMENTAL AND COMPUTATIONAL STUDY OF SODIUM-ALUMINUM COMPLEXING IN CRUSTAL FLUIDS

Abstract The stoichiometry and stability constants of Na-aluminate (NaAl(OH) 4 0 ) ion pair were determined from both boehmite solubility and potentiometric measurements. Boehmite solubility was measured at temperatures from 125 to 350 °C at pressures corresponding to equilibrium between liquid and vapor ( P SAT ) in the system Na-NH 4 -Cl-OH. Potentiometric measurements were performed at temperatures from 75 to 200 °C in Na-Al-Cl-OH solutions, using a Na-selective glass electrode. NaAl(OH) 4 0 stability constants derived independently from these two methods are in excellent agreement and increase markedly with increasing temperature from 0.8 at 25 °C to 209 at 350 °C. Experimental results obtained in this study were combined with corresponding data reported by Castet et al. (1993) for boehmite solubility to generate, within the framework of the revised HKF model, the standard partial molal properties and equations of state parameters for Al(OH) 4 − , Al(OH) 3 0 , and NaAl(OH) 4 0 . The solubilities of gibbsite, boehmite, and corundum calculated in Na-rich solutions using the thermodynamic data for Al aqueous species generated in this study are in close agreement with their experimental counterparts. Thermodynamic calculations carried out at temperatures up to 800 °C and pressures up to 5 kbar indicate that the formation of NaAl(OH) 4 0 ion pairs can markedly increase the solubility of Al-bearing minerals and thus, Al mobility, both in sedimentary basin and metamorphic fluids at pHs > 4.

The surface chemistry and structure of acid-leached albite: New insights on the dissolution mechanism of the alkali feldspars

The surfaces of fresh albite crystals, and of albite crystals following their two hour leaching in an aqueous 0.3 molal HCl solution at a temperature of 200°C, were analyzed by Raman spectroscopy, scanning electron microscopy, and Energy Dispersive Spectroscopy (EDS). Scanning electron microscopy reveals that dissolution is nonuniform and reacted surfaces exhibit a range from fresh to extremely altered regions, the latter characterized by the presence of etch pits and crater-like structures. Energy dispersive spectroscopy indicates that the chemical composition of the near surface is heterogeneous; the most altered regions have significantly lower NaSi and AlSi ratios compared to both unreacted albite and the fresher appearing regions of reacted albite. This result demonstrates that (1) initial dissolution produces spatially discontinuous altered layers, and (2) the depletion of both alkali cations and aluminum from the near surface region is associated with locally enhanced alkali-feldspar dissolution. This latter observation is strong evidence that the inverse relationship observed between far from equilibrium constant temperature/pH alkali feldspar dissolution rates and aqueous alkali cation and/or aluminum concentration stems from a reaction mechanism involving the selective removal of these elements from the mineral structure. The Raman spectra of increasingly altered surface regions confirm that alkali feldspar dissolution is the result the sequential breaking of bonds in the mineral structure. A close correspondence is apparent between the Raman spectra of amorphous silica and the reacted albites most altered regions. As altered layers on albite surfaces are discontinuous, the accurate determination of their thickness requires techniques which can distinguish chemical differences on a micron scale.

Gibbs free energy of formation of kaolinite from solubility measurement in basic solution between 60 and 170 °C

Abstract The solubility of a natural hydrothermal kaolinite was measured in buffered alkaline solutions at temperatures ranging from 60 to 170 °C. At temperatures below 110 °C equilibrium was obtained from undersaturation, while above 110 °C two different steady states were obtained, one from undersaturation and another from supersaturation. Raman spectroscopy studies of the solid phases recovered at the end of each run, demonstrated that the “undersaturated” experiments attained equilibrium with respect to kaolinite, while the “supersaturated” experiments attained equilibrium with an illite-like mineral. Regression of kaolinite solubility products yields a Gibbs free energy of formation of kaolinite (Al 2 Si 2 O 5 (OH) 4 ) at 25 °C of −907.68 ± 0.2 kcal/mol. This value is more negative than most literature values, but similar to that generated from calorimetric data by Barany and Kelley (1961) . Furthermore, calculated thermodynamic parameters generated from kaolinite solubility obtained in this study are in close agreement with those deduced from boehmite-kaolinite equilibrium data at temperatures higher than 200 °C.

Raman Spectroscopic Study of Aluminum Silicate Complexes at 20°C in Basic Solutions

Raman spectroscopic measurements were performed at ambient temperature onaqueous silica-bearing solutions (0.005 < mSi < 0.02; 0 < pH < 14). The spectraare consistent with the formation of monomeric Si(OH)o4, SiO(OH)−3 andSiO2(OH)2−2 species at acid to neutral, basic, and strongly basic pH, respectively.Raman spectra of aqueous Al-bearing solutions at basic pH confirm thepredominance of the Al(OH)−4 species in a wide concentration range (0.01 < mAl < 0.1).Raman spectra of basic solutions (12.4 < pH < 14.3), containing both Al andSi, exhibit a strong decrease in intensities of SiO(OH)−3, SiO2(OH)2−2, andAl(OH)−4 bands in comparison with Al-free Si-bearing and Si-free Al-bearingsolutions of the same metal concentration and pH, suggesting the formation ofsoluble Al—Si complexes. The amounts of complexed Al and Si derived fromthe measurements of the Al and Si band intensities in strongly basic solutions(pH ∼ 14) are consistent with the formation, between Al(OH)−4 andSiO2(OH)2−2, of the single Al—Si dimer SiAlO3(OH)3−4 according to the reactionSiO2(OH)2−2 + Al(OH)−4 ⇔ SiAlO3(OH)3−4 + H2OAt lower pH (∼ 12.5) the changes in band intensities are consistent with theformation of several, likely more polymerized, Al—Si complexes.

Experimental determination of aqueous sodium-acetate dissociation constants at temperatures from 20 to 240°C

Abstract Dissociation constants of sodium acetate ion pair (NaCH 3 COO 0 ) were determined at the liquid–vapor saturation pressure by Raman spectroscopy at temperatures from 20 to 240°C and by potentiometry at temperatures from 25 to 172°C. Large differences were found between these two experimentally determined data sets; dissociation constants generated from Raman spectroscopic data were found to be ∼1.5 orders of magnitude higher than corresponding constants obtained using potentiometric measurements. Values generated from Raman spectroscopic data are consistent with the high temperature NaCH 3 COO 0 dissociation constants reported by [Oscarson, J.L., Gillespie, S.E., Christensen, J.J., Izatt, R.M., Brown, P.R., 1988. Thermodynamic quantities for the interaction of H + and Na + with C 2 H 3 O 2 − and Cl − in aqueous solution from 275 to 320°C. J. Soln. Chem., Vol. 17, pp. 865-885], whereas those obtained by potentiometry are consistent with the low temperature NaCH 3 COO 0 dissociation constants measured by [Archer, D.W., Monk, C.B., 1964. Ion-association constants of some acetates by pH (glass electrode) measurements. J. Chem. Soc., pp. 3117–3122.] and [Daniele, P.G., De Robertis, A., De Stefano, C., Sammartano, S., Rigano, C., 1985. On the possibility of determining the thermodynamic parameters for the formation of weak complexes using a simple model for the dependence on the ionic strength of activity coefficients: Na + , K + , and Ca 2+ complexes of low molecular weight ligands in aqueous solution. J. Chem. Soc. Dalton, pp. 2353–2361.], as well as the predictions of [Shock, E.L., Koretsky C.M., 1993. Metal–organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures. Geochim. Cosmochim. Acta, Vol. 57, pp. 4899–4922.] and [Benezeth, P., Castet, S., Dandurand, J.L., Gout, R., Schott, J., 1994. Experimental study of aluminum acetate complexing between 60 and 200°C. Geochim. Cosmochim. Acta, Vol. 58, pp. 4561–4571]. The origin of these differences likely stems in the species being sampled by different experimental techniques; Raman spectroscopy only identifies inner sphere sodium-acetate complexes as associated species whereas potentiometric measurements identify both inner and outer sphere sodium-acetate complexes as associated species. Regression of sodium acetate ion pair dissociation constants obtained from Raman spectroscopy and potentiometry enable characterization of true dissociation constants for both inner and outer sphere complexes. These latter parameters are used to assess the formation of these complexes at temperatures and solution compositions typical of natural fluids.

Experiments on phase transformations and chemical reactions of mechanically activated minerals by grinding: Petrogenetic implications

Abstract Prolonged grinding increases the energy of solids by the production of stored energy in the form of new surfaces and internal defects. Moreover, grinding also generates quasi-hydrostatic pressures which can result in polymorphic transformations and mineral decomposition. Here we demonstrate the solid-state transformation of metastable to stable polymorphs (aragonite → calcite, anatase → rutile); the transformation of low-pressure to high-pressure phases (calcite → aragonite); and the lowering of the dehydration and decarbonation temperatures of minerals (siderite → magnetite or hematite, diaspore → corundum). In the presence of a fluid phase, stored energy from grinding can be released, resulting in accelerated reaction rates and, more importantly, phase transformations. In this paper we demonstrate the following transformations: ground calcite → magnesian calcite (at low Mg2+ concentration in solution), ground calcite → aragonite (at high Mg2+ concentration), ground magnesite → hydromagnesite, and ground dolomite → aragonite + Mg2+. Assuming an analogy between laboratory and natural grinding, tectonic activity may have important consequences on the release of hydrothermal fluids, the solubilization of minerals and on solid-state transformations. As examples the possible role of deformation on the formation of metamorphic aragonite and diaspore-bauxites is discussed.

Raman spectroscopic study of arsenic speciation in aqueous solutions up to 275°C

Raman spectroscopic measurements were performed on As2O3–H2O solutions (0.02⩽[As]⩽5.2 mol kg-1) at temperatures from 20 to 275°C. At 20°C the spectrum of arsenic solutions of low concentration (0.02–0.5 mol kg-1) is the same over a wide range of pH (0–8). It only exhibits a polarized band at 700 cm-1 with a depolarized shoulder at 655 cm-1. These bands can be attributed to the pyramidal molecule As(OH)30. The spectrum of these solutions does not change significantly with increasing temperature. The only change consists in a small shift of the two bands towards low wavenumbers. In solutions of medium As concentration (1 mol kg-1), a broadening of the 700 cm-1 band towards low wavenumbers is observed. This can be explained by the formation of a non-dehydrated dimeric species which forms via hydrogen bridging bonds. At higher As concentrations (2–5.2 mol kg-1), a new band appears at 525 cm-1, the intensity of which increases with increasing As concentration and temperature. By comparison with the spectrum of fused arsenic oxide, this band can be attributed to As—O—As bonds. This suggests that polymeric species are present in concentrated As solutions. In the most concentrated solutions (4.1 and 5.2 mol kg-1), a new band occurs at 380 cm-1, which can be attributed to the symmetric stretching of the As4O6 tetrahedron.