Eric H. Oelkers

SUPCRT92: a software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000 ° C

Recent advances in theoretical geochemistry permit calculation of the standard molal thermodynamic properties of a wide variety of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000°C. The SUPCRT92 software package facilitates practical application of these recent theories, equations, and data to define equilibrium constraints on geochemical processes in a wide variety of geologic systems. The SUPCRT92 package is composed of three interactive FORTRAN 77 programs, SUPCRT92, MPRONS92, and CPRONS92, and a sequential-access thermodynamic database, SPRONS92.DAT. The SUPCRT92 program reads or permits user-generation of its two input files, CON and RXN, retrieves data from the direct-access equivalent of SPRONS92.DAT, calculates the standard molal Gibbs free energy, enthalpy, entropy, heat capacity, and volume of each reaction specified on the RXN file through a range of conditions specified on the CON file, and writes the calculated reaction properties to the output TAB file and, optionally, to PLT files that facilitate their graphical depiction. Calculations can be performed along the liquid side of the H2O vaporization boundary by specifying either temperature (T) or pressure (P), and in the single-phase regions of fluid H2O by specifying either T and P, T and H2O density, T and log K, or P and log K. SPRONS92.DAT, which contains standard molal thermodynamic properties at 25°C and 1 bar, equation-of-state parameters, heat capacity coefficients, and phase transition data for approximately 500 minerals, gases, and aqueous species, can be augmented or otherwise modified using MPRONS92, and converted to its direct-access equivalent using CPRONS92.

The rainbow vent fluids (36°14′N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids

Fluids were collected from the Rainbow vent field (36°14′N) on the Mid-Atlantic Ridge (MAR) during the 1997 diving FLORES cruise. This vent field, in ultramafic rocks at a depth of 2300 m, is composed of ∼10 black smokers emitting acidic (pH∼2.8) fluids at 365 °C. The low pH of the hot-temperature Rainbow fluids likely results from seawater–ultramafic rock interaction that releases H+ ions into reducing hydrothermal fluids. Fluid chemistry is strongly influenced by phase separation generating Cl-rich brines (ClEM=750 mM) strongly enriched with Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Cs, Pb, Y, and rare earth elements (REE). REE and transition metal abundance (particularly Fe and Mn) in the Rainbow fluids is dramatically higher than in other MAR fluids. The abundance of trace element and REE enrichment is due to the greater solubility of these elements that is strongly favored by Cl-complexation at low-pH and high-temperature conditions. Chondrite-normalized REE patterns show strong LREE enrichment with evidence of the typical Eu anomaly. This REE partitioning suggests that either (1) ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and/or (2) that the unique Rainbow fluid temperature, pH, and redox state issued from the ultramafic character of leached substratum can produce unique REE partitioning.

The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions

Abstract Analysis of aluminosilicate steady-state dissolution/precipitation rates indicate that in contrast to what is commonly assumed, the constant pH rates are not independent of chemical affinity at far from equilibrium conditions. Rather, the logarithm of these rates for albite and kaolinite are linear functions of the logarithm of aqueous Al concentration over wide ranges of saturation states. Consideration of both the steady-state rates and the surface chemistry of these minerals following dissolution indicates that these rates are consistent with their control by the decomposition of an Al-deficient, silica-rich surface precursor complex. Taking account of reactions written to form this complex leads to a rate equation for the dissolution/precipitation of these minerals that accurately describes their variation on pH, aqueous Al concentration, and chemical affinity. By analogy, it appears likely that the rates of numerous other aluminosilicate dissolution/crystallization reactions are also consistent with their control by the decomposition of similar precursor complexes. It follows from these observations that 1. (1) the generation of steady state dissolution rate constants from experiments performed in batch type reactors, 2. (2) the interpretation of the pH dependence of aluminosilicate dissolution reactions requires explicit account of the effects of aqueous Al concentration on these rates.

General kinetic description of multioxide silicate mineral and glass dissolution

Abstract The dissolution mechanisms of multioxide silicate minerals and glasses differ from those of single (hydr)oxides because their dissolution may require the breaking of more than one metal-oxygen bond type. A general kinetic description of major rock forming multioxide silicate dissolution is developed in the present study by assuming the following: (1) the relative rates at which various metal-oxygen bonds are broken within a multioxide structure are consistent with the relative dissolution rates of the single (hydr)oxides; (2) the difference in the rates of breaking each metal-oxygen bond type is sufficiently large such that the reaction breaking one bond type can attain equilibrium before breaking substantial quantities of slower breaking metal-oxygen bonds; and (3) those metal oxygen bonds that break before the final destruction of the structure liberate metal atoms via metal-proton exchange reactions. Multioxide dissolution proceeds via a series of metal-proton exchange reactions until the mineral or glass structure is destroyed. This metal-proton exchange reaction sequence is shown to be consistent with leached layer compositions at acidic conditions. The last metal-proton exchange reaction in the series is slowest and thus rate controlling. Of these slowest exchanging metals, those partially freed from the structure by being adjacent to previously exchanged metals are liberated faster than those completely attached to the mineral or glass and thus constitute the rate-controlling precursor complex. The identity and reactions forming this precursor complex are used within the context of transition-state theory to derive equations that describe accurately the dissolution rates of the major rock-forming multioxide silicate minerals and glasses as a function of solution composition over the full range of chemical affinity.

The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25°C and pH = 3 and 11

Steady state basaltic glass dissolution rates were measured as a function of aqueous aluminum, silica, and oxalic acid concentration at 25° C and pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basaltic glass, and exhibited stoichiometric Si versus Al release. Rates are independent of aqueous silica activity, but decrease with increasing aqueous aluminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experimental uncertainty using where r signifies the surface area normalized basaltic glass steady state dissolution rate, k refers to a rate constant equal to 10-11.65 (mol of Si)/cm2/s, and ai represents the activity of the subscripted aqueous species. The observation that all rates obtained in the present study can be described by a single regression equation supports strongly the likelihood that basaltic glass dissolution is controlled by a single mechanism at both acidic and basic pH and in both the presence and absence of organic acids. Taking account of the dissolution mechanisms of similarly structured and compositioned minerals, and previously published studies of basaltic glass dissolution behavior, basaltic glass dissolution likely proceeds via 1) the relatively rapid and essentially complete removal of univalent and divalent cations from the near surface; 2) aluminum releasing exchange reactions between three aqueous H+ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. The breaking of Al-O bonds does not destroy the glass framework; it only partially liberates the silica tetrahedral chains by removing adjoining Al atoms. Basaltic glass dissolution rates are proportional to the concentration of partially detached framework Si tetrahedra near the surface, which is linked through the law of mass action for the Al/proton exchange reaction to aqueous aluminum activity. Copyright

Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 °C and 5 kbar

Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

Mechanism, rates, and consequences of basaltic glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a function of pH and temperature

This study is aimed at quantifying surface reaction controlled basaltic glass dissolution rates at far-from-equilibrium conditions. Towards this aim, steady-state basaltic glass dissolution rates were measured as a function of pH from 2 to 11 at temperatures from 6° to 50°C, and at near neutral conditions to 150°C. All rates were measured in open system titanium mixed flow reactors. Measured dissolution rates display a common pH variation; dissolution rates decrease dramatically with increasing pH at acid conditions, minimize at near neutral pH, and increase more slowly with increasing pH at basic conditions. The pH at which basaltic glass dissolution minimizes decreases with increasing temperature. Dissolution rates were interpreted within the context of a multioxide dissolution model. Constant temper- ature rates are shown to be consistent with their control by partially detached Si tetrehedra at the basaltic glass surface. Regression of far-from-equilibrium dissolution rates obtained in the present study and reported in the literature indicate that all data over the temperature and pH range 6° T 300°C and 1 pH 11 can be described within uncertainty using

Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis

Steady-state dissolution rates of anorthite (An96) were measured as a function of aqueous Si, Al, and Ca concentration at temperatures from 45 to 95°C and over the pH range 2.4 to 3.2 using a Ti mixed-flow reactor. All dissolution experiments exhibited stoichiometric dissolution. The concentration of aqueous Si, Al, and Ca ranged from ∼7 X× 10−5 to ∼1 × 10−3 molal, ∼6 × 10−5 to ∼3.4 × 10−3 molal, and ∼5 × 10−5 to ∼0.1 molal, respectively, corresponding to calculated anorthite chemical affinities ranging from ∼ 115 to ∼65 kJ/mol. Measured anorthite dissolution rates at constant temperature are proportional to αH+1.5, where αH+ designates the activity of the hydrogen ion, and consistent with an apparent activation energy of 18.4 kJ/mol. Anorthite dissolution rates are independent of aqueous Al concentration, which is in contrast with the alkali feldspars, whose constant pH, far from equilibrium rates are proportional to αAl+3−0.33 (Oelkers et al., 1994; Gautier et al., 1994; E. H. Oelkers and J. Schott, unpubl. data). This difference suggests a distinctly different dissolution mechanism. For the case of both types of feldspars it appears that Al is more readily removed than Si from the aluminosilicate framework. Because it has a SiAl ratio of 3, the removal of Al from the alkali feldspar framework leaves partially linked Si tetrehedra. Removal of Si still requires the breaking of SiO bonds, and thus the overall alkali feldspar dissolution rate is controlled by the decomposition of a silica-rich surface precursor. The variation of alkali feldspar dissolution rates with aqueous Al activity stems from the fact that the formation of this precursor requires the removal of Al. In contrast, because it has a SiAl ratio of 1, the removal of Al from the anorthite framework leaves completely detached Si tetrehedra. As a result, the removal of Si does not require the breaking of SiO bonds, the rate controlling precursor complex is not formed by the removal of Al, and the overall dissolution rate is independent of aqueous Al concentration at far from equilibrium conditions. It can be inferred from these results that the variation of far from equilibrium aluminosilicate dissolution rates on aqueous Al depends on the number and relative strength of different bond types that need to be broken for mineral hydrolysis.

Are quartz dissolution rates proportional to B.E.T. surface areas

Abstract A single quartz powder was dissolved at 200°C and 250°C under far from equilibrium conditions in atmosphere-equilibrated deionized water during a sequential series of experiments performed over one year in a titanium open system mixed flow reactor. Scanning electron microscope (SEM) photomicrographs show that morphological changes of this powder were dominated by grain edge rounding and etch pit formation. Etch pit walls rapidly evolve into unreactive negative crystal faceted forms; etch pit density and diameter are essentially constant during the experiments, indicating that dissolution predominantly deepened rather than widened etch pits. Measured B.E.T. surface areas increase linearly with mass of quartz dissolved to a value 5.6 times greater than that of the initial powder during the course of the experiments. Nevertheless, measured 200°C far from equilibrium mass normalized dissolution rates remained constant within experimental uncertainty. It is concluded that the bulk of the observed increase in B.E.T. surface areas during dissolution consisted of essentially unreactive etch pit walls which contribute negligibly to mineral dissolution. As similar etch pit development is common in natural systems, geometric rather than B.E.T. surface areas may provide a more accurate parameter for estimating dissolution rates.

Experimental study of K-feldspar dissolution rates as a function of chemical affinity at 150°C and pH 9

Steady state dissolution rates of a K-rich feldspar (K0.81Na0.15Ba0.03Al1.05Si2.96O8) were measured as a function of chemical affinity and aqueous Si and Al concentration in solutions containing 5 × 10−3 m total K using a titanium mixed flow reactor at a temperature of 150°C and pH of 9.0. All dissolution experiments exhibited stoichiometric dissolution with respect to Al and Si. The concentration of aqueous silica and Al ranged from 1 × 10−6to 5 × 10−4mol/kg and 4 × 10−7to 5 × 10−4mol/kg, respectively, corresponding to K-feldspar chemical affinities ranging from ~90 to ~5 kJ/mol. Logarithms of measured dissolution rates are an inverse linear function of aqueous aluminum concentration, but independent of aqueous silica concentration at all chemical affinities greater than ~20 kJ/mol. These rates become increasingly controlled by chemical affinity as equilibrium is approached. This variation of steady state dissolution rates is consistent with their control by the decomposition of silica rich/aluminum deficient surface precursor complex. Taking account of transition state theory and the identity of reactions to form this precursor complex, an equation was derived to describe the steady state dissolution rates over the full range of chemical affinity. A simplified but less general version of this equation, which can be used to describe the steady state rates (r) obtained in the present study can be expressed as r=k+1aAl(OH)+−aH+13 (1 − exp (− A3RT)) where k+ stands for a rate constant equal to 1.7 × 10−17 mol/cm2/s, aH+ and aAl(OH)4− designate the activities of H+ and Al(OH)4− , respectively, A refers to the chemical affinity of the overall reaction, R signifies the gas constant and T denotes the temperature in K. Corresponding experiments performed in a batch-type reactor illustrate the consistency between dissolution rates generated in open and closed systems.