Solubility Of Corundum Research Articles

Thermodynamic Modeling of Solubility of Corundum in Water at Supercritical Conditions

Thermodynamic Modeling of Solubility of Corundum in Water at Supercritical Conditions

Correlation and prediction of thermodynamic properties of nonelectrolytes at infinite dilution in water over very wide temperature and pressure ranges (2000 K and 10 GPa)

Thermodynamic modeling of natural processes involving deep aqueous fluids requires the knowledge of the values of chemical potentials (the Gibbs energy) of aqueous species. An accurate prediction of thermodynamic properties at high T and P is a strong challenge. It is shown that geochemical models, including the well-known HKF-model, cannot be recommended for an indiscriminate use at supercritical temperatures to predict chemical potentials of nonelectrolytes at infinite dilution in water. Nevertheless, sufficiently accurate predictions of ϕ2∞ (the fugacity coefficients at infinite dilution in water) of aqueous nonelectrolytes up to 2000K and water densities up to 1500kgm−3, i.e. pressure up to 10–12GPa, can be made relying on known theoretical relations valid at various parts of the phase diagram of water. In essence, the method, proposed in this work, consists in the interpolation of properties between two known limits: the first one, at low water densities, is defined by the values of the second virial coefficients for water-solute interactions, and the second, at high water densities – by predictions of the theory of a mixture of hard spheres. The interpolation at moderate temperatures (700–1300K) and water densities (500–900kgm−3) is simplified by sufficiently accurate predictions of properties using a semiempirical variant of a corresponding-states principle. Presented examples of the prediction of fugacity coefficients of “gases” at infinite dilution in water and of an aqueous solubility of corundum over very wide ranges of water densities/pressures demonstrate the potential and generality of the proposed methods of evaluating the thermodynamic properties of aqueous neutral compounds.

Water in the deep Earth: The dielectric constant and the solubilities of quartz and corundum to 60 kb and 1200 °C

Comprehensive quantitative theoretical evaluation of water–rock interactions in the Earth has long been restricted to a pressure of 5.0kb – too low to address processes involving deep aqueous fluids. Yet such fluids are thought to play an important role in the long-term geologic cycling of many chemical elements. A reason for this restriction is the lack of information on the dielectric constant of water (εH2O) needed for the revised Helgeson–Kirkham–Flowers (HKF) equations of state for aqueous species. Equation of state coefficients are available for hundreds of aqueous species in SUPCRT92, but calculations using these species can only be made to 5.0kb (Shock et al., 1992).In the present study, the applicability of the revised HKF equations of state for aqueous species was extended to 60kb by developing estimates of (εH2O). We used a statistical mechanically-based equation for the dielectric constant of a hard-sphere fluid applicable to water (Franck et al., 1990). The equation was calibrated with experimental data, and data from a comprehensive analysis of the literature (Fernández et al., 1997), and then used to calculate (εH2O) to a density of 1.1gcm−3. The values of ln(εH2O) were found to be linear with ln(ρH2O) which enabled empirical extrapolation of (εH2O) to 60kb. Values of ρH2O were computed with a recent comprehensive evaluation consistent with experimental data and a molecular dynamics model for water (Zhang and Duan, 2005).The resulting dielectric constants were tested at 727°C and 58kb by comparison with the results of ab initio molecular dynamics calculations (Pan et al., 2013). Additional testing was carried out by computing standard Gibbs free energies of aqueous species using the new values of (εH2O) and ρH2O in the revised HKF equations to predict equilibrium constants which in turn enabled calculation of the solubilities of quartz and corundum for comparison with experimental measurements to 20kb and 1100°C. Our results strongly suggest that geochemically useful predictions can now be made that will facilitate analysis of water–rock interactions in the Earth at depths much greater than previously possible.

Thermodynamic model for mineral solubility in aqueous fluids: theory, calibration and application to model fluid‐flow systems

Geofluids(2010)10, 20–40AbstractWe present a thermodynamic model for mineral dissolution in aqueous fluids at elevated temperatures and pressures, based on intrinsic thermal properties and variations of volumetric properties of the aqueous solvent. The standard thermodynamic properties of mineral dissolution into aqueous fluid consist of two contributions: one from the energy of transformation from the solid to the hydrated‐species state and the other from the compression of solvent molecules during the formation of a hydration shell. The latter contribution has the dimension of the generalized Krichevskii parameter. This approach describes the energetics of solvation more accurately than does the Born electrostatic theory and can be extended beyond the limits of experimental measurements of the dielectric constant of H2O. The new model has been calibrated by experimental solubilities of quartz, corundum, rutile, calcite, apatite, fluorite and portlandite in pure H2O at temperatures up to 1100°C and pressures up to 20 kbar. All minerals show a steady increase in solubility along constant geothermal gradients or water isochores. By contrast, isobaric solubilities initially increase with rising temperature but then decline above 200–400°C. This retrograde behavior is caused by variations in the isobaric expansivity of the aqueous solvent, which approaches infinity at its critical point. Oxide minerals predominantly dissolve to neutral species; so, their dissolution energetics involve a relatively small contribution from the solvent volumetric properties and their retrograde solubilities are restricted to a relatively narrow window of temperature and pressure near the critical point of water. By contrast, Ca‐bearing minerals dissolve to a variety of charged species; so, the energetics of their dissolution reactions involve a comparatively large contribution from volume changes of the aqueous solvent and their isobaric retrograde solubility spans nearly all metamorphic and magmatic conditions. These features correlate with and can be predicted from the standard partial molar volumes of aqueous species.The thermodynamic model can be used over much wider range of settings for terrestrial fluid–rock interaction than has previously been possible. To illustrate, it is integrated with transport theory to show quantitatively that integrated fluid fluxes characteristic of crustal shear zones are capable of precipitating quartz or calcite veins from low‐ and medium‐grade metamorphic conditions, at a geothermal gradient of 20°C km−1. For subduction zones, modeled by a geotherm of 7°C km−1, the required fluid fluxes are one to two orders of magnitude lower and predict enhanced efficiency of mass transfer and metasomatic precipitation in comparison with orogenic settings. The new model thus can be applied to shallow hydrothermal, metamorphic, magmatic and subduction fluids, and for retrieval of dependent thermodynamic properties for mass transfer or geodynamic modeling.

Solubility of corundum in aqueous KOH solutions at 700 °C and 1 GPa

The solubility of corundum in aqueous KOH solutions was measured at 700 °C and 1 GPa, using a piston-cylinder apparatus and a weight-loss method. Total potassium molality ( m K) ranged from 0.0011 to 3.9. At the lowest m K, corundum solubility ( m Al) was 0.0016 ± 0.0004 molal, which is slightly higher than that in pure H 2O at the same conditions (0.0011 molal). Corundum solubility increased with added KOH to a maximum of m Al = 2.66 at the highest m K. At m K ≥ 0.03, m Al increased linearly with m K, with d m Al / d m K = 1. The results were combined with previous work to evaluate the stability of the neutral ion-pair KAlO 2,aq at 700 °C and 1 GPa. We obtained an optimal fit to the experimental data with an equilibrium constant ( K) for the reaction KOH aq = K + + OH − of 10 − 1.206 , which was extrapolated from the data of Ho and Palmer [Ho, P.C. and Palmer, D.A., 1997. Ion association of a dilute aqueous potassium chloride and potassium hydroxide solutions to 600 °C and 300 MPa determined by electrical conductance measurements. Geochimica et Cosmochimica Acta, 61, 15, 3027–3040.] using linear isothermal correlations between the logarithms of H 2O density and K. This gave an equilibrium constant for the reaction KAlO 2,aq = K + +AlO 2 − of 10 − 0.299 . The results permit assessment of the dominant aqueous species in K–Al–O–H fluids at high pressure and temperature. We find that the dominant Al-bearing species in such fluids is predicted to be the neutral hydrate (HAlO 2,aq) at m K < 0.01 (pH < 4.8), whereas AlO 2 − predominates to higher m K and pH, over most geologically realistic conditions. The KAlO 2,aq ion pair will only be the most abundant Al-bearing species at very high pH (> 8), which corresponds to KOH molality of > 10 at 700 °C and 1 GPa. Thus, KAlO 2,aq is not a major reservoir for dissolved aluminum in low-chloride crustal and mantle aqueous fluids.

Lattice diffusion and solubility of argon in forsterite, enstatite, quartz and corundum

Argon diffusivities and solubilities in single, gem-quality crystals of forsterite, enstatite, quartz and corundum were determined from experiments conducted between 0.5 and ~ 6000 bars Ar pressure and temperatures from 425 °C to 1200 °C. Polished single-crystal slabs and specimens with natural facets were placed in open containers and exposed to an argon atmosphere either in a pressure vessel or in a gas-flow tube furnace at near-atmospheric pressure. Argon atoms from the pressure medium diffused into the crystals to produce near-surface concentration gradients, which were directly profiled using Rutherford backscattering spectrometry. The following Arrhenius relations were obtained for Ar diffusion: F o r s t e r i t e : D A r = 7.2 × 10 - 20 exp ( - 42 k J m o l - 1 / R T ) m 2 s - 1 E n s t a t i t e : D A r = 1.3 × 10 - 20 exp ( - 32 k J m o l - 1 / R T ) m 2 s - 1 Q u a r t z : D A r = 3.1 × 10 - 19 exp ( - 43 k J m o l - 1 / R T ) m 2 s - 1 C o r u n d u m : D A r = 1.2 × 10 - 20 exp ( - 33 k J m o l - 1 / R T ) m 2 s - 1 The reported activation energies probably represent apparent activation energies that are a combination of the effects of lattice diffusion of Ar atoms with trapping in point-defect vacancies. There are no discernible differences in Ar diffusion in different crystallographic directions, and diffusivities do not vary as a function of the intrinsic oxygen fugacity of the experimental vessels. In addition to diffusivities, the uptake gradients also yielded lattice solubilities of argon, as represented by the concentrations at the mineral surfaces. There are small differences between the solubilities of forsterite, quartz and corundum, but all solubilities are high at ~ 2000 ppm (by mass). The Ar solubilities for the iron-bearing minerals enstatite and San Carlos olivine are much higher than for the other minerals. The mean value for enstatite is 30–60% higher than the iron-free minerals, owing, perhaps, to incorporation of Ar in point defects that are more abundant in enstatite due to oxidation of minor FeO. Argon solubilities are independent of Ar pressure at P Ar > ~ 1 bar, but slightly lower at P Ar ≈ 0.5–1 bar. This behavior suggests that all sites capable of accommodating Ar atoms are filled at Ar pressures of ~ 1 bar. In all cases, Ar appears to be compatible in the minerals investigated, although the degree of compatibility will depend upon assumptions concerning the minimum Ar fugacity required to fully populate the available vacancies. A series of validation experiments and surface-sensitive analyses were conducted to confirm that the near-surface regions of crystals used in the experiments were crystalline, unreacted and free of structural damage that could potentially influence diffusion and solubility results.

Solubility of corundum in the system Al 2O 3–SiO 2–H 2O–NaCl at 800 °C and 10 kbar

The solubility of corundum was measured at 800 °C and 10 kbar in NaCl–H 2O–SiO 2 fluids over NaCl mole fractions ( X NaCl) of 0 to 0.5 and SiO 2 concentrations of zero to quartz or albite + melt saturation, depending on X NaCl. Experiments were performed in a piston-cylinder apparatus. Dissolved Al 2O 3 and SiO 2 were determined by weight losses of corundum and quartz crystals, or by bulk compositions of bracketing experiments. Although kyanite and sillimanite are slightly more stable than corundum + quartz at these pressures ( P) and temperatures ( T), neither appeared in any of the experiments. Results indicate that the enhancement of corundum solubility by NaCl at this P and T is further promoted by the addition of SiO 2. At quartz saturation, significant enhancement occurs in initially pure H 2O and addition of NaCl yields yet higher Al 2O 3 concentrations. At 0.03 ≤ X NaCl ≤ 0.1, quartz saturation is replaced by albite + silicate melt. The anhydrous melt composition is nearly on the join NaAlSi 3O 8–SiO 2. Al 2O 3 molality rises rapidly with NaCl concentration to 0.038 at X NaCl = 0.1 and then increases more slowly to 0.052 at halite saturation. There is, in addition, a measurable reciprocal enhancement of Si solubility by virtue of dissolved Al at a fixed X NaCl. Quench pH in NaCl-bearing fluids is strongly acidic and Na/Cl < 1 in quench solutes, suggesting low pH at high P and T. These observations, combined with previous work indicating Si–Al and Na–Al complexing, lead to the hypothesis that the reciprocal solubility enhancement of Al and Si is due to formation of Na-aluminosilicate complexes. Mass balance consideration suggests that the bulk Si/Al ratio of the group of complexes ranges from 1 to 2 in the NaCl-free system, to > 3 at high X NaCl. Our data show that Al 2O 3 is a moderately soluble component in quartz + aluminosilicate-saturated rocks in the presence of intergranular salt solutions at deep-crustal metamorphic P– T conditions and commonly realized salinities. Al 2O 3 should not be regarded as a fixed reference component, because it may exhibit substantial mobility.

Solubility of grossular, Ca 3Al 2Si 3O 12, in H 2O–NaCl solutions at 800 °C and 10 kbar, and the stability of garnet in the system CaSiO 3–Al 2O 3–H 2O–NaCl

The solubility and stability of synthetic grossular were determined at 800 °C and 10 kbar in NaCl–H 2O solutions over a large range of salinity. The measurements were made by evaluating the weight losses of grossular, corundum, and wollastonite crystals equilibrated with fluid for up to one week in Pt capsules and a piston-cylinder apparatus. Grossular dissolves congruently over the entire salinity range and displays a large solubility increase of 0.0053 to 0.132 molal Ca 3Al 2Si 3O 12 with increasing NaCl mole fraction ( X NaCl) from 0 to 0.4. There is thus a solubility enhancement 25 times the pure H 2O value over the investigated range, indicating strong solute interaction with NaCl. The Ca 3Al 2Si 3O 12 mole fraction versus NaCl mole fraction curve has a broad plateau between X NaCl = 0.2 and 0.4, indicating that the solute products are hydrous; the enhancement effect of NaCl interaction is eventually overtaken by the destabilizing effect of lowering H 2O activity. In this respect, the solubility behavior of grossular in NaCl solutions is similar to that of corundum and wollastonite. There is a substantial field of stability of grossular at 800 °C and 10 kbar in the system CaSiO 3–Al 2O 3–H 2O–NaCl. At high Al 2O 3/CaSiO 3 bulk compositions the grossular + fluid field is limited by the appearance of corundum. Zoisite appears metastably with corundum in initially pure H 2O, but disappears once grossular is nucleated. At X NaCl = 0.3, however, zoisite is stable with corundum and fluid; this is the only departure from the quaternary system encountered in this study. Corundum solubility is very high in solutions containing both NaCl and CaSiO 3: Al 2O 3 molality increases from 0.0013 in initially pure H 2O to near 0.15 at X NaCl = 0.4 in CaSiO 3-saturated solutions, a >100-fold enhancement. In contrast, addition of Al 2O 3 to wollastonite-saturated NaCl solutions increases CaSiO 3 molality by only 12%. This suggests that at high pH (quench pH is 11–12), the stability of solute Ca chloride and Na–Al ± Si complexes account for high Al 2O 3 solubility, and that Ca–Al ± Si complexes are minor. The high solubility and basic dissolution reaction of grossular suggest that Al may be a very mobile component in calcareous rocks in the deep crust and upper mantle when migrating saline solutions are present.

Micrometer-scale cavities in fibrous and cloudy diamonds — A glance into diamond dissolution events

Micrometer sized internal cavities in diamonds preserve evidence of diamond dissolution events. Combining the methods of focused ion beam (FIB) sample preparation and transmission electron microscopy (TEM) enables these features to be studied in detail. Micrometer-scale cavities are found in the inner parts of fibrous and cloudy kimberlitic diamonds. Their filling consists of amorphous matrix, secondary nano-crystals, volatiles and in some cases larger resorbed crystals. Trapped minerals include corundum, Kappa-alumina, quartz, olivine, moissanite-6H and Ca–Mg carbonates. This is the first observation of Kappa-alumina in nature. Secondary nano-minerals are observed within the amorphous matrix and include carbonates, Al-oxide, fluorite, ilmenite and secondary diamond crystals. The amorphous matrix is spongy and its composition is dominated by amorphous carbon, nitrogen, chlorine and also contains water. When no crystalline phases are observed, the matrix is also enriched in alumina, silica and in some cases calcium. We propose that micrometer scale cavities in diamonds form during dissolution events induced by the introduction of oxidizing hydrous fluids into the diamond growth area. Hydrous fluids are the main dissolving agents for most kimberlitic diamonds [Fedortchouk, Y., Canil, D., Semenets, E., 2007. Mechanisms of diamond oxidation and their bearing on the fluid composition in kimberlite magmas. Am. Mineral. 92, 1200–1212]. At diamond forming conditions silica and alumina are enriched in hydrous fluids that are in equilibrium with eclogites [Kessel, R., Ulmer, P., Pettke, T., Schmidt, M.W., Thompson, A.B., 2005. The water-basalt system at 4 to 6 GPa: Phase relations and second critical endpoint in a K-free eclogite at 700 to 1400 °C. Earth Planet. Sci. Lett. 237, 873–892]; this is consistent with the increased solubility of alumina with increased pressure and temperature in the Na–Cl bearing fluids [Manning, C.E., 2006. Mobilizing aluminum in crustal and mantle fluids. J. Geochem. Explor. 89, 251–253; Manning, C.E., 2007. Solubility of corundum + kyanite in H2O at 700C and 10 kbar: evidence for Al–Si complexing at high pressure and temperature. Geofluids. 7, 258–269]. Additionally, hydrous fluids may leach grain boundaries that are enriched in alumina in peridotitic environments [Hiraga, T., Anderson, I.M., Kohlstedt, D.L., 2004. Grain boundaries as reservoirs of incompatible elements in the Earth's mantle. Nature. 427, 699–703; Wirth, R., 1996. Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos, Arizona. Contrib. Mineral. Petrol. 124, 44–54]. Diamond dissolution will form oxidized carbon species and may decrease the solubility of silica and alumina in the dissolving agent leading to their precipitation. Diamond forming fluids that are trapped in sub-micrometer inclusions in the same fibrous diamonds are the dominant fluid component in the diamond growth area. Corrosive hydrous fluids are less common and appear as short, discrete events, followed by the return of the common diamond forming fluids and continued diamond precipitation.

Solubility of corundum + kyanite in H2O at 700°C and 10 kbar: evidence for Al‐Si complexing at high pressure and temperature

AbstractThe solubility of the assemblage corundum + kyanite in H2O was determined at 700°C and 10 kbar, using a piston‐cylinder apparatus and rapid‐quench/fluid‐extraction techniques. Weighted mean concentrations of total Al and Si were 5.80 ± 0.03 mmol kg−1 H2O and 0.308 ± 0.003 mol kg−1 H2O, respectively (1σ errors). The Al concentration is nearly five times higher than that of corundum solubility in pure H2O. This difference is interpreted to arise from complexing between Si and Al to form HAlSiO4,aq species. Charged or more polymerized species are also possible, but their abundance cannot be constrained based on these experiments. Assumption of a single aqueous aluminosilicate complex permits calculation of the thermodynamic consequences of Al‐Si interaction in high‐pressure fluids, as well as phase diagrams for the system Al‐Si‐O‐H. Formation of Al‐Si complexes leads to a large increase in dissolved Al with increasing Si in solution, such that Al concentration in equilibrium with kyanite + quartz is predicted to be 7.1 mmolal, higher than with kyanite + corundum. The elevated concentration of Si in deep‐crustal and mantle aqueous fluids suggests that Al must readily be dissolved and transported by Al‐Si complexing in high‐pressure metamorphic and metasomatic environments. The results provide a simple explanation for the common observation of kyanite + quartz segregations in eclogites and Barrovian metamorphic rocks.

The solubility of corundum in H 2O at high pressure and temperature and its implications for Al mobility in the deep crust and upper mantle

The solubility of corundum in H 2O was measured at 700–1100 °C and 0.5–2.0 GPa using a piston–cylinder apparatus and weight-loss methods. At 700 °C, 0.5 GPa, Al concentration is 0.41 ± 0.15 millimolal (1 σ analytical error). Corundum solubility increases strongly with pressure ( P) and temperature ( T), reaching 2.57 ± 0.15 millimolal at 700 °C, 2.0 GPa, and 27.3 ± 1.5 millimolal at 1100 °C, 2.0 GPa. The results were fitted with the equation log m Al = 1.131 + 3827 / T − (2.277 − 7336 / T)log ρ H 2O , where m Al is molality of Al, ρ H 2O is the density of H 2O in g/cm 3, and T is in Kelvin. The data indicate that Al can have significant solubility in H 2O during metamorphic and metasomatic processes in the deep crustal and upper mantle.

Solubilities of corundum, wollastonite and quartz in H 2O–NaCl solutions at 800 °C and 10 kbar: Interaction of simple minerals with brines at high pressure and temperature

Solubilities of corundum (Al 2O 3) and wollastonite (CaSiO 3) were measured in H 2O–NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H 2O and NaCl at pressure ( P) and temperature ( T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al 2O 3 molality ( m Al 2 O 3 ) of 0.0013(1) (1 σ error) in pure H 2O and then leveling off to a maximum of ∼0.015 at halite saturation ( X NaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H 2O, X Al 2 O 3 / X Al 2 O 3 ° , passes through a maximum at X NaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H 2O at this P and T ( m CaSiO 3 = 0.0167 ( 6 ) ) . It undergoes great enhancement, with a maximum solubility relative to that in H 2O at X NaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H 2O–NaCl–SiO 2, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from m SiO 2 = 1.248 in pure H 2O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H 2O reproduces the data and indicates that hydrogen bonding of molecular H 2O to dissolved silica species is thermodynamically negligible. The maxima in X Al 2 O 3 / X Al 2 O 3 ° for corundum and wollastonite indicate that the solute products include hydrates and Na + and/or Cl − species produced by molar ratios of reactant H 2O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism.

Mobilizing aluminum in crustal and mantle fluids

Aluminum is a major rock-forming element, but its low solubility in H 2O at upper crustal conditions has led to the assumption that it is effectively immobile during fluid–rock interaction in the deep crust and upper mantle. However, new experimental results on Al solubility in H 2O–SiO 2–NaCl fluids at high pressure and temperature challenge this view. The combined effects of temperature, pressure, dissolved SiO 2 and NaCl yield increases in corundum (Al 2O 3) solubility at 800 °C, 1 GPa of > 10 3 or 10 4 relative to that at < 0.3 GPa in pure H 2O. Enhancement of Al solubility is best explained by formation of polynuclear Na–Al–Si–O clusters and/or polymers. Aluminum should not be assumed to be immobile during fluid–rock interaction in deep environments.

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.

Experimental determination and analysis of the solubility of corundum in 0.1-molal CaCl 2 solutions between 400 and 600°C at 0.6 to 2.0 kbar

Corundum (α-Al 2O 3) solubility was measured in 0.1-molal CaCl 2 solutions from 400 to 600°C between 0.6 and 2.0 kbar. The Al molality at 2 kbar increases from 3.1 × 10 −4 at 400°C to 12.7 × 10 −4 at 600°C. At 1 kbar, the solubility increases from 1.5 × 10 −4 m at 400°C to 3.4 × 10 −4 m at 600°C. These molalities are somewhat less than corundum solubility in pure H 2O (Walther, 1997) at 400°C but somewhat greater at 600°C. The distribution of species was computed considering the Al species Al(OH) 3 0 and Al(OH) 4 −, consistent with the solubility of corundum in pure H 2O of Walther (1997) and association constants reported in the literature. The calculated solubility was greater than that measured except at 600°C and 2.0 kbar, indicating that neutral-charged species interactions are probably important. A Setchénow model for neutral species resulted in poor fitting of the measured values at 1.0 kbar. This suggests that Al(OH) 3 0 has a greater stability relative to Al(OH) 4 − than given by the models of Pokrovskii and Helgeson (1995) or Diakonov et al. (1996). The significantly lower Al molalities in CaCl 2 relative to those in NaCl solutions at the same concentration confirm the suggestions of Walther (2001) and others that NaAl(OH) 4 0 rather than an Al-Cl complex must be significant in supercritical NaCl solutions to give the observed increase in corundum solubility with increasing NaCl concentrations.

Experimental determination and analysis of the solubility of corundum in 0.1 and 0.5 m NaCl solutions between 400 and 600°C from 0.5 to 2.0 kbar

Corundum, α-Al2O3, solubility was measured in 0.1 and 0.5 m NaCl solutions from 400 to 600°C between 0.5 and 2.0 kbar. The base-10 logarithm of Al molality at 2 kbar in 0.5 m NaCl increases from −2.45 at 400°C to −1.52 at 600°C. At 1 kbar, the solubility increases from −2.33 at 400°C to −1.03 at 600°C. These molalities are between 1 and 3 orders of magnitude greater than corundum solubility in pure H2O (Walther, 1997). In 0.1 m NaCl, corundum solubility begins to decrease below that in pure water at 400°C as pressures are decreased below ∼1.2 kbar. A distribution of species calculation considering the Al species NaAl(OH)40, Al(OH)30 and Al(OH)4− and using association constants reported in the literature demonstrate that Al(OH)4− is a significant species in equilibrium with corundum in pure H2O. The calculations also indicate that another Al aqueous species containing Na or Cl besides NaAl(OH)40 must be present in NaCl solutions to give the observed increase in Al with increasing NaCl concentrations to 0.5 m.

Experimental determination and interpretation of the solubility of corundum in H 2O between 350 and 600°C from 0.5 to 2.2 kbar

Corundum solubility was measured in H2O between 350° and 600°C from 0.5 to 2.2 kbar. The base-10 logarithm of molality of Al in equilibrium with corundum along the 0.5 kbar isobar decreases with increasing temperature from −3.44 at 400°C to −5.52 at 600°C, but at 2.0 kbar the logarithm of molality of Al increases from −3.25 at 400°C to −3.09 at 600°C. Electrostatic energetics appear important in stabilizing the dominate Al species as isotherms of solubility are a linear function of the reciprocal of the dielectric constant of H 2O. These results together with values reported for corundum solubility in KOH solutions (Pascal and Anderson, 1989; Azaroual et al., 1996) are used to constrain the dominant Al species in pure H 2O solutions to Al(OH) 3 0. The base-10 logarithm of the molal equilibrium constants for the reaction: Al(OH) 4 − + H + = Al(OH) 3 0 + H 2O at 2.0 kbar and 400°, 500°, 600°, and 700°C are computed to be 6.33, 5.74, 6.39, and 6.75, respectively. It is argued that KAl(OH) 4 0 is not an important species in KOH aqueous solutions as increasing Al(OH) 4 − plus Al(OH) 3 0 with increasing pH and ionic strength can account for the increased corundum solubility within experimental uncertainty.

Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: the system Al 2O 3H 2OKOH

Experimental solubilities of gibbsite and corundum in aqueous KOH solutions at elevated temperatures and pressures reported by Fricke and Jucaitis (1930), Wesolowski (1992), and Pascal and Anderson (1989) were used together with the Hückel (1925) and Setchénow (1892) equations for activity coefficients of aqueous species and the thermodynamic properties of the aluminate ion AlO 2 − to retrieve dissociation constants for the potassium aluminate ion pair KAlO 2 0. The calculations were carried out using values of the Hückel extended-term parameter for aqueous KOH and dissociation constants for KOH 0 generated in the present study. Supercritical solubilities of corundum reported in the literature were regressed with the aid of a speciation model which explicitly provides for the formation in solution of triple ion clusters (Oelkers and Helgeson, 1990). Values of the dissociation constant for KAlO 2 0 generated from these data were subsequently regressed with the revised Helgeson-Kirkham-Flowers (HKF) equations of state (Tanger and Helgeson, 1988; Shock et al., 1989) to obtain HKF equations of state coefficients and the standard partial molal thermodynamic properties at 25°C and 1 bar of KAlO 2 0, which can be used to calculate the standard partial molal thermodynamic properties of the species at temperatures to 1000°C and pressures to 5 kbar. Combining these parameters and properties with those for AlO 2 − (Pokrovskii and Helgeson, 1995) permits calculation to within 0.05 log units of the experimental solubilities of corundum in KOH solutions reported by Barns et al. (1963) and Anderson and Burnham (1967) at temperatures to 800°C and pressures to 5 kbar. Similar calculations carried out for temperatures from 80 to 300°C at P SAT ( P SAT refers to 1 bar at temperatures < 100°C and to the equilibrium pressure for coexisting liquid and vapor H 2O at higher temperatures) indicate that addition of KCl to dilute KOH solutions increases the solubilities of gibbsite, boehmite, and diaspore in response to increasing formation of KAlO 2 0 and the decrease in the activity coefficient of AlO 2 −. At temperatures ∼ ≤ 80°C, the two factors contribute comparably to the solubility of gibbsite in solutions with KCl concentrations up to 5 molal.

Corundum solubility and aluminum speciation in KOH aqueous solutions at 400°C from 0.5 to 2.0 kbar

Corundum solubility has been measured in KOH aqueous solutions under supercritical conditions (400°C, from 0.5 to 2 kbar). The investigated concentration range (0.001 < MKOH < 0.1 mol/kg H2O) allowed to vary the species balance between diluted solutions dominated by aluminate ion, Al(OH)4− , and more concentrated ones in which association of aluminate ion with K+ ion is appreciable. The dissociation constant of the complex KAl(OH)40 and the equilibrium constant for hydrolysis of corundum have been simultaneously fitted to the measured solubilities through a speciation calculation.The corundum hydrolysis (1/2 Al2O3 + 5/2 H2O ⇔ Al(OH)4− + H+) equilibrium constant is in close agreement with the values calculated from thermodynamical data for corundum (Berman, 1988) and aqueous species (Johnson et al., 1992). Using the Density Model (Anderson et al., 1991) for extrapolating thermodynamical properties of aqueous species results in appreciably higher values for this equilibrium constant, not consistent with experimental data, due to the inadequacy of the Density Model for extrapolating water properties in the supercritical P-T range.For the KA1 (OH)40 dissociation reaction, the following values of log K: −1.94 ± 0.17, −1.73 ± 0.14, and −1.56 ± 0.11, are obtained at 0.5, 1.0, and 2.0 kbar, respectively. These constants correspond to equal contributions of the aluminate ion and of the KAl(OH)40 complex to the total dissolved A1 in 0.1 m KOH solutions. At 600 and 700°C, 2 kbar, the same equilibrium constants calculated from literature experimental corundum solubilities are −2.43 and −3.16, respectively.From the calculated aluminate ion concentration in the system Al2O3H2O and the literature values of corundum solubility in water, the neutral hydroxyde Al(OH)3 is inferred to be probably the dominant A1 aqueous species in the neutral pH range in this system.

An experimental and computational study of sodium-aluminum complexing in crustal fluids

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.