Geology topics

Niel Plummer

Publications and source records attributed to Niel Plummer.

At least 109 records · Page 6Linked to original sources

USGS1992

Dissolution of aragonite-strontianite solid solutions in nonstoichiometric Sr (HCO3)2-Ca (HCO3)2-CO2-H2O solutions

Synthetic strontianite-aragonite solid-solution minerals were dissolved in CO 2 -saturated non-stoichiometric solutions of Sr(HCO 3 ) 2 and Ca(HCO 3 ) 2 at 25°C. The results show that none of the dissolution reactions reach thermodynamic equilibrium. Congruent dissolution in Ca(HCO 3 ) 2 solutions either attains or closely approaches stoichiometric saturation with respect to the dissolving solid. In Sr(HCO 3 ) 2 solutions the reactions usually become incongruent, precipitating a Sr-rich phase before reaching stoichiometric saturation. Dissolution of mechanical mixtures of solids approaches stoichiometric saturation with respect to the least stable solid in the mixture. Surface uptake from subsaturated bulk solutions was observed in the initial minutes of dissolution. This surficial phase is 0–10 atomic layers thick in Sr(HCO 3 ) 2 solutions and 0–4 layers thick in Ca(HCO 3 ) 2 solutions, and subsequently dissolves and/or recrystallizes, usually within 6 min of reaction. The initial transient surface precipitation (recrystallization) process is followed by congruent dissolution of the original solid which proceeds to stoichiometric saturation, or until the precipitation of a more stable Sr-rich solid. The compositions of secondary precipitates do not correspond to thermodynamic equilibrium or stoichiometric saturation states. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of solid solutions on surfaces of aragonite and strontianite single crystals immersed in Sr(HCO 3 ) 2 and Ca(HCO 3 ) 2 solutions, respectively. In Sr(HCO 3 ) 2 solutions, the XPS signal from the outer ~ 60 Å on aragonite indicates a composition of 16 mol% SrCO 3 after only 2 min of contact, and 14–18 mol% SrCO 3 after 3 weeks of contact. The strontianite surface averages approximately 22 mol% CaCO 3 after 2 min of contact with Ca(HCO 3 ) 2 solution, and is 34–39 mol% CaCO 3 after 3 weeks of contact. XPS analysis suggests the surface composition is zoned with somewhat greater enrichment in the outer ~25 Å (as much as 26 mol% SrCO 3 on aragonite and 44 mol% CaCO 3 on strontianite). The results indicate rapid formation of a solid-solution surface phase from subsaturated aqueous solutions. The surface phase continually adjusts in composition in response to changes in composition of the bulk fluid as net dissolution proceeds. Dissolution rates of the endmembers are greatly reduced in nonstoichiometric solutions relative to dissolution rates observed in stoichiometric solutions. All solids dissolve more slowly in solutions spiked with the least soluble component ((Sr(HCO 3 ) 2 )) than in solutions spiked with the more soluble component (Ca(HCO 3 ) 2 ), an effect that becomes increasingly significant as stoichiometric saturation is approached. It is proposed that the formation of a non-stoichiometric surface reactive zone significantly decreases dissolution rates.

Geochimica et Cosmochimica Acta↗

USGS1991

Geochemical evolution of water in the Madison Aquifer in parts of Montana, South Dakota, and Wyoming

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Professional Paper↗

USGS1991

An interactive code (NETPATH) for modeling NET geochemical reactions along a flow PATH

No abstract available.

Water-Resources Investigations Report↗

USGS1990

Reaction paths and equilibrium end-points in solid-solution aqueous-solution systems

Equations are presented describing equilibrium in binary solid-solution aqueous-solution (SSAS) systems after a dissolution, precipitation, or recrystallization process, as a function of the composition and relative proportion of the initial phases. Equilibrium phase diagrams incorporating the concept of stoichiometric saturation are used to interpret possible reaction paths and to demonstrate relations between stoichiometric saturation, primary saturation, and thermodynamic equilibrium states. The concept of stoichiometric saturation is found useful in interpreting and putting limits on dissolution pathways, but there currently is no basis for possible application of this concept to the prediction and/ or understanding of precipitation processes. Previously published dissolution experiments for (Ba, Sr)SO 4 and (Sr, Ca)C̈O 3orth. solids are interpreted using equilibrium phase diagrams. These studies show that stoichiometric saturation can control, or at least influence, initial congruent dissolution pathways. The results for (Sr, Ca)CO 3orth. solids reveal that stoichiometric saturation can also control the initial stages of incongruent dissolution, despite the intrinsic instability of some of the initial solids. In contrast, recrystallisation experiments in the highly soluble KCl-KBr-H 2 O system demonstrate equilibrium. The excess free energy of mixing calculated for K(Cl, Br) solids is closely modeled by the relation G E = χ KBr χ KCl RT [ a 0 + a 1 (2 χ KBr −1)], where a 0 is 1.40 ± 0.02, a 1 , is −0.08 ± 0.03 at 25°C, and χ KBr and χ KCl are the mole fractions of KBr and KCl in the solids. The phase diagram constructed using this fit reveals an alyotropic maximum located at χ KBr = 0.676 and at a total solubility product, ΣΠ = [ K + ]([ Cl − ] + [ Br − ]) = 15.35.

Geochimica et Cosmochimica Acta↗

USGS1989

Thermodynamics of magnesian calcite solid-solutions at 25°C and 1 atm total pressure

The stability of magnesian calcites was reexamined, and new results are presented for 28 natural inorganic, 12 biogenic, and 32 synthetic magnesian calcites. The magnesian calcite solid-solutions were separated into two groups on the basis of differences in stoichiometric solubility and other physical and chemical properties. Group I consists of solids of mainly metamorphic and hydrothermal origin, synthetic calcites prepared at high temperatures and pressures, and synthetic solids prepared at low temperature and very low calcite supersaturations ( ) from artificial sea water or NaClMgCl 2 CaCl 2 solutions. Group I solids are essentially binary s of CaCO 2 and MgCO 2 , and are thought to be relatively free of structural defects. Group II solid-solutions are of either biogenic origin or are synthetic magnesian calcites and protodolomites (0–20 and &sim; 45 mole percent MgCO 3 ) prepared at high calcite supersaturations ( ) from NaClNa 2 SO 4 MgCl 2 CaCl 2 or NaClMgCl 2 CaCl 2 solutions. Group II solid-solutions are treated as massively defective solids. The defects include substitution foreign ions (Na + and SO 4 2− ) in the magnesian calcite lattice (point defects) and dislocations (~2 · 10 9 cm −2 ). Within each group, the excess free energy of mixing, G E , is described by the mixing model , where x is the mole fraction of the end-member Ca 0.5 Mg 0.5 CO 3 in the solid-solution. The values of A 0 and A 1 for Group I and II solids were evaluated at 25°C. The equilibrium constants of all the solids are closely described by the equation ln , where K C and K D are the equilibrium constants of calcite and Ca 0.5 Mg 0.5 CO 3 . Group I magnesian calcites were modeled as sub-regular solid-solutions between calcite and dolomite, and between calcite and “disordered dolomite”. Both models yield almost identical equilibrium constants for these magnesian calcites. The Group II magnesian calcites were modeled as sub-regular solid-solutions between defective calcite and protodolomite. Group I and II solid-solutions differ significantly in stability. The rate of crystal growth and the chemical composition of the aqueous solutions from which the solids were formed are the main factors controlling stoichiometric solubility of the magnesian calcites and the density of crystal defects. The literature on the occurrence and behavior of magnesian calcites in sea water and other aqueous solutions is also examined.

Geochimica et Cosmochimica Acta↗

USGS1988

A computer program incorporating Pitzer's equations for calculation of geochemical reactions in brines

The program named PHRQPITZ is a computer code capable of making geochemical calculations in brines and other electrolyte solutions to high concentrations using the Pitzer virial-coefficient approach for activity-coefficient corrections. Reaction-modeling capabilities include calculation of (1) aqueous speciation and mineral-saturation index, (2) mineral solubility, (3) mixing and titration of aqueous solutions, (4) irreversible reactions and mineral water mass transfer, and (5) reaction path. The computed results for each aqueous solution include the osmotic coefficient, water activity , mineral saturation indices, mean activity coefficients, total activity coefficients, and scale-dependent values of pH, individual-ion activities and individual-ion activity coeffients , and scale-dependent values of pH, individual-ion activities and individual-ion activity coefficients. A data base of Pitzer interaction parameters is provided at 25 C for the system: Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O, and extended to include largely untested literature data for Fe(II), Mn(II), Sr, Ba, Li, and Br with provision for calculations at temperatures other than 25C. An extensive literature review of published Pitzer interaction parameters for many inorganic salts is given. Also described is an interactive input code for PHRQPITZ called PITZINPT. (USGS)

Water-Resources Investigations Report↗

USGS1987

Thermodynamics of aragonite-strontianite solid solutions: Results from stoichiometric solubility at 25 and 76°C

Dissolution of synthetic strontianite-aragonite solid solutions was followed analytically to stoichiometric saturation using large solid to solution ratios in CO 2 -H 2 O solution at 25 and 76°C. The compositional dependence of the equilibrium constant was calculated from the composition of saturated (stoichiometric) solutions and used to calculate the activities and activity coefficients of CaCO 3 and SrCO 3 in the solid Ca (1− x ) Sr x CO 3 at 25 and 76°C. The results show that the solid-solution is not regular but unsymmetrical. The excess free energy of mixing is closely modeled for all compositions by the relation where A 0 is 8.49 ± 0.30 and 7.71 ± 0.20 KJ/mole and A 1 is −4.51 ± 0.20 and −3.36 ± 0.40 KJ/mole at 25 and 76°C, respectively. The equilibrium constant is denned as a function of the SrCO 3 mole fraction, x , by the relation where R is the gas constant, T is in Kelvins and K A and K S are the aragonite and strontianite equilibrium constants. The experimental results indicate the Henry's law coefficients of SrCO 3 in aragonites containing 0 to 6 mole percent SrCO 3 are approximately 91± 8 and 23 ± 1 at 25 and 76°C, respectively and for strontianites the Henry's law coefficients and applicable compositional ranges are approximately 7.3 ± 0.3 (0.84 ≤ x ≤ 1.00) and 3.3 ± 0.5 (0.50 ≤ x ≤ 1.00) at 25 and 76°C, respectively. Substitution of small amounts of Sr in aragonite and Ca in strontianite initially increases the stability of the solid. The most stable aragonites and strontianites contain 0.58 ± 0.03 and 12.5 ± 1.1 mole percent SrCO 3 and CaCO 3 at 25°C and 3.1 ± 0.3 and 17.2 ± 1.1 mole percent SrCO 3 and CaCO 3 at 76°C, respectively. The spinode occurs over the regions 0.065 ± 0.001 ≤ x ≤ 0.620 ± 0.014 at 25°C and 0.103 ± 0.007 ≤ x ≤ 0.585 ± 0.019 at 76°C where all compositions are unstable. A miscibility gap occurs over the compositional ranges 0.0058 ± 0.0003 ≤ x ≤ 0.875 ± 0.011 at 25°C and 0.031 ± 0.003 ≤ x ≤ 0.828 ± 0.011 at 76°C and is in reasonable agreement with reported compositions of natural aragonites and strontianites. Marine aragonites are neither at equilibrium nor stoichiometric saturation with surface seawater. The experimentally observed distribution coefficient of Sr in aragonite is 12 times larger than the calculated equilibrium value (0.095) at 25°C. Naturally occurring strontianites contain large amounts of calcium primarily because Ca/Sr ratios in natural waters are typically large. Neither equilibrium nor stoichiometric saturation is observed at 76°C during laboratory recrystallization of strontianite-aragonite solid solutions even after apparent 100 percent conversion to a narrow secondary composition and demonstration of a nearly constant composition system for periods of 300 hours.

Geochimica et Cosmochimica Acta↗

USGS1987

pH measurement of low-conductivity waters

pH is an important and commonly measured parameter of precipitation and other natural waters. The various sources of errors in pH measurement were analyzed and procedures for improving the accuracy and precision of pH measurements in natural waters with conductivities of < 100 uS/cm at 25 C are suggested. Detailed procedures are given for the preparation of dilute sulfuric acid standards to evaluate the performance of pH electrodes in low conductivity waters. A daily check of the pH of dilute sulfuric acid standards and deionized water saturated with a gas mixture of low carbon dioxide at partial pressure (air) prior to the measurement of the pH of low conductivity waters is suggested. (Author 's abstract)

Water-Resources Investigations Report↗

USGS1986

The solubility of BaCO 3 (cr) (witherite) in CO 2 -H 2 O solutions between 0 and 90°C, evaluation of the association constants of BaHCO 3 + (aq) and BaCO 3 0 (aq) between 5 and 80°C, and a preliminary evaluation of the thermodynamic properties of Ba 2+ (aq)

One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO 3 ( cr ) = Ba 2+ ( aq ) + CO 3 2− ( aq ) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by log K = 607.642 + 0.121098 T − 20011.25/ T − 236.4948 log T where T is in degrees Kelvin. The log K of BaCO 3 (cr), the Gibbs energy, the enthalpy and entropy of the reaction at 298.15 K are −8.562, 48.87 kJ · mol −1 , 2.94 kJ · mol −1 and −154.0 J · mol −1 · K −1 , respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO 3 + (aq) and BaCO 3 0 (aq) Three different methods were used to evaluate the association constant of BaHCO 3 + (aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba 2+ ( aq ) + HCO 3 − ( aq ) = BaHCO 3 + ( aq ) is given by log K BaHCO 3 + = −3.0938 + 0.013669 T . The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15°K are 0.982, −5.606 kJ · mol −1 , 23.26 kJ · mol −1 and 96.8 J · mol −1 · K −1 , respectively. The temperature dependence of the equilibrium constant for the reaction Ba 2+ ( aq ) + CO 2− 3 ( aq ) = BaCO 0 3 ( aq ) is given by log K BaCO 3 0 = 0.113 + 0.008721 T . The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15° K are 2.71, −15.49 kJ · mol −1 , 14.84 kJ · mol −1 and 101.7 J· mol −1 · K −1 . The above model leads to reliable calculations of the aqueous speciation and solubility of witherite in the system BaCO 3 -CO 2 -H 2 O from 0 to more than 90°C. Literature data on witherite solubility were re-evaluated and compared with the results of this study. Problems in the thennodynamic selections of Ba compounds are considered. Newer data require the revision of Δ f H° and Δ f G° of Ba 2+ (aq) to −532.5 and −555.36 kJ · mol −1 , respectively, for agreement with solubility data.

Geochimica et Cosmochimica Acta↗

USGS1985

Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites

Significant amounts of SO 4 2− , Na + , and OH − are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO 4 2− . Aragonites and most biogenic low Mg-calcites contain significant amounts of Na + , but very low concentrations of SO 4 2− . The SO 4 2− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na + and SO 4 2− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO 4 2− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO 4 2− is the same as that of aragonite. Na + appears to have very little effect on the solubility product of calcites. The amounts of Na + and SO 4 2− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ( D "> D ) of SO 4 2− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k 0 + k 1 R "> D = k 0 + k 1 R where k 0 "> k 0 and k 1 "> k 1 are constants equal to 6.16 × 10 −6 "> 6.16 × 10 −6 and 3.941 × 10 −6 "> 3.941 × 10 −6 , respectively, and R "> R is the rate of crystal growth of calcite in mg·min −1 ·g −1 of seed. The data on Na + are consistent with the hypothesis that a significant amount of Na + occupies interstitial positions in the calcite structure. The distribution of Na + follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na + distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na + contents of calcites are not very accurate indicators of environmental salinities.

Geochimica et Cosmochimica Acta↗

USGS1984

The solubility of strontianite (SrCO 3 ) in CO 2 -H 2 O solutions between 2 and 91°C, the association constants of SrHCO + 3 (aq) and SrCO 0 3 (aq) between 5 and 80°C, and an evaluation of the thermodynamic properties of Sr 2+ (aq) and SrCO 3 (cr) at 25°C and 1 atm total pressure

Seventy new measurements (Sr T -P co2 of the solubility of strontianite were used to evaluate the equilibrium constant for the reaction SrCO 3 ( cr ) = Sr 2+ ( aq ) + CO 2− 3 ( aq ) between 2 and 91 °C. The temperature dependence of the equilibrium constant is given by the expression Log K = 155.0305 − 7239.594/ T − 56.58638 log T where T is in degrees Kelvin. The log K of strontianite, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are −9.271 ± 0.020, 52.919 ± 0.08 kJ · mol −1 , −1.67 ± 1.30 kJ · mol −1 , and −183.1 ± 4.0 J · mol −1 · K −1 , respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs SrHCO + 3 (aq) and SrCO 0 3 (aq) which were evaluated by potentiometric methods between 5 and 80°C. The equilibrium constant for the association reaction Sr 2+ ( aq ) + HCO − 3 ( aq ) = SrHCO + 3 aq ) is given by the expression Log K SrHCO + 3 = −3.248 + 0.014867 T . The log of the association constant, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are 1.18, −6.76 kJ · mol −1 , 25.30 kJ · mol −1 , and 107.5 J · mol −1 · K −1 , respectively. The equilibrium constant for the association reaction Sr 2+ ( aq ) + CO 2− 3 ( aq ) = SrCO 0 3 aq ) is given by the expression Log K SrCO 0 3 = −1.019 + 0.012826 T . The log of the association constant, the Gibbs energy, enthalpy, and entropy of the reaction at 25°C are 2.81, −16.01 kJ · mol −1 ,21.83 kJ · mol −1 , and 126.9 J · mol −1 · K −1 , respectively. These results lead to reliable calculation of the aqueous speciation and solubility of strontianite in the system SrCO 3 -CO 2 -H 2 O from 0 to more than 90°C. Literature data on the solubility of strontianite have been evaluated and compared with these results. Our new data for strontianite have been used in an evaluation of the thermodynamic properties of Sr 2+ (aq), SrCO 3 (cr) and related compounds. The following values are recommended for the standard enthalpy (kJ · mol −1 ), Gibbs energy (kJ · mol −1 ), and entropy (J · mol −1 · K −1 ), respectively, of Sr 2+ aq): −550.90 ± 0.50, −563.83 ± 0.8 and −31.50 ± 2.0, and for SrCO 3 (cr): −1225.77 ± 1.1, −1144.73 ± 1.0 and 97.2.

Geochimica et Cosmochimica Acta↗

USGS1984

Chemical and X-ray analyses of 175 calcite and marble samples from the collection of the National Museum of Natural History, Smithsonian Institution

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Open-File Report↗

USGS1984

Partial compilation and revision of basic data in the WATEQ programs

Several portions of the basic data in the WATEQ series of computer programs (WATEQ, WATEQF, WATEQ2, WATEQ3, and PHREEQE) are compiled. The density and dielectric constant of water and their temperature dependence are evaluated for the purpose of updating the Debye-Huckel solvent parameters in the activity coefficient equations. The standard state thermodynamic properties of the Fe2+ and Fe3+ aqueous ions are refined. The main portion of this report is a comprehensive listing of aluminum hydrolysis constants, aluminum fluoride, aluminum sulfate, calcium chloride, magnesium chloride, potassium sulfate and sodium sulfate stability constants, solubility product constants for gibbsite and amorphous aluminum hydroxide, and the standard electrode potentials for Fe (s)/Fe2+(aq) and Fe2 +(aq)/Fe3+(aq). (USGS)

Water-Resources Investigations Report↗

USGS1983

Development of reaction models for ground-water systems

Methods are described for developing geochemical reaction models from the observed chemical compositions of ground water along a hydrologic flow path. The roles of thermodynamic speciation programs, mass balance calculations, and reaction-path simulations in developing and testing reaction models are contrasted. Electron transfer is included in the mass balance equations to properly account for redox reactions in ground water. The mass balance calculations determine net mass transfer models which must be checked against the thermodynamic calculations of speciation and reaction-path programs. Although reaction-path simulations of ground-water chemistry are thermodynamically valid, they must be checked against the net mass transfer defined by the mass balance calculations. An example is given testing multiple reaction hypotheses along a flow path in the Floridan aquifer where several reaction models are eliminated. Use of carbon and sulfur isotopic data with mass balance calculations indicates a net reaction of incongruent dissolution of dolomite (dolomite dissolution with calcite precipitation) driven irreversibly by gypsum dissolution, accompanied by minor sulfate reduction, ferric hydroxide dissolution, and pyrite precipitation in central Florida. Along the flow path, the aquifer appears to be open to CO 2 initially, and open to organic carbon at more distant points down gradient.

Geochimica et Cosmochimica Acta↗

USGS1983

Process and rate of dedolomitization: Mass transfer and C14 dating in a regional carbonate aquifer

Regional dedolomitization is the major process that controls the chemical character of water in the Mississippian Pahasapa Limestone (Madison equivalent) surrounding the Black Hills, South Dakota and Wyoming. The process of dedolomitization consists of dolomite dissolution and concurrent precipitation of calcite; it is driven by dissolution of gypsum. Deuterium and oxygen isotopic data from the ground water, coupled with regional potentiometric maps, show that recharge occurs on the western slope of the Black Hills and that the water flows northward and westward toward the Powder River Basin. A significant part flows around the southern end of the Black Hills to replenish the aquifer to the east of the Hills. Depth of flow was inferred from interpretation of the silica geothermometer based on the temperature-dependent solubilities of quartz and chalcedony in water. Chemical effects of warm water in the Pahasapa Limestone include changes in the solubility products of minerals, conversion of gypsum to anhydrite, solution and precipitation of minerals, and increases in the tendency for outgassing of carbon dioxide. Where sulfate reduction is not important, sulfur isotope data show that (1) in the Mississippian aquifer, most of the sulfate is from dissolution of gypsum and (2) some wells and springs have a hydrologic connection with overlying Permian and Pennsylvanian evaporites. Sulfate ion concentration, a progress variable, shows a strong correlation with pH as a result of the combined effects of the dedolomitization reactions. Mass-balance and mass-transfer calculations were used to adjust 14 C values to determine a range of ground-water flow velocities between 2 and 20 m/yr. These velocities are characteristic of carbonate aquifers. The average rates of dolomite and gypsum dissolution are 1.7 × 10 −4 and 3.4 × 10 −4 mmol/kg of H 2 O/yr, respectively. The precipitation of calcite is occurring at the rate of 3.4 × 10 −4 mmol/kg of H 2 O/yr. The close agreement among the model results demonstrates that dedolomitization is controlling water-rock interactions in this regional carbonate aquifer system.

Geological Society of America Bulletin↗

USGS1983

PHRQINPT, an interactive computer program for constructing input data sets to the geochemical simulation program PHREEQE

PHRQINPT is a FORTRAN 77 program which facilitates formulation of the input data file to PHREEQE (Parkhurst, Thorstenson, and Plummer, 1980). PHRQINPT runs interactively and contains many features to help the user construct the input data set. This report describes these features and gives instructions on the use and implementation of PHRQINPT. A listing of the program and the two accompanying data sets are given in the attachments, along with two examples. (USGS)

Water-Resources Investigations Report↗

USGS1982

The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O

Calculations based on approximately 350 new measurements (Ca T -PCO 2 ) of the solubilities of calcite, aragonite and vaterite in CO 2 -H 2 O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants K C , K A , and K V respectively, for the reaction CaCO 3 (s) = Ca 2+ + CO 2− 3 : where T is in o K. At 25°C the logarithms of the equilibrium constants are −8.480 ± 0.020, −8.336 ± 0.020 and −7.913 ± 0.020 for calcite, aragonite and vaterite, respectively. The equilibrium constants are internally consistent with an aqueous model that includes the CaHCO + 3 and CaCO 0 3 ion pairs, revised analytical expressions for CO 2 -H 2 O equilibria, and extended Debye-Hückel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO 2 -dependence if the CaHCO + 3 association constant is between 0 and 90°C, corresponding to the value log K Cahco + 3 = 1.11 ± 0.07 at 25°C. The CaCO 0 3 association constant was measured potentiometrically to be between 5 and 80°C, yielding log K CaCO 0 3 = 3.22 ± 0.14 at 25°C. The CO 2 -H 2 O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of K H , K 1 and K 2 are , and log K 2 = −107.8871 − 0.03252849 T + 5151.79/ T + 38.92561 log T − 563713.9/ T 2 which may be used to at least 250°C. These expressions hold for 1 atm. total pressure between 0 and 100°C and follow the vapor pressure curve of water at higher temperatures. Extensive measurements of the pH of Ca-HCO 3 solutions at 25°C and 0.956 atm PCO 2 using different compositions of the reference electrode filling solution show that measured differences in pH are closely approximated by differences in liquid-junction potential as calculated by the Henderson equation. Liquid-junction corrected pH measurements agree with the calculated pH within 0.003-0.011 pH. Earlier arguments suggesting that the CaHCO + 3 ion pair should not be included in the CaCO 3 -CO 2 -H 2 O aqueous model were based on less accurate calcite solubility data. The CaHCO + 3 ion pair must be included in the aqueous model to account for the observed PCO 2 -dependence of aragonite solubility between 317 ppm CO 2 and 100% CO 2 . Previous literature on the solubility of CaCO 3 polymorphs have been critically evaluated using the aqueous model and the results are compared.

Geochimica et Cosmochimica Acta↗

USGS1982

Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity, and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

We have calculated the total individual ion activity coefficients of carbonate and calcium, and , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of and are 0.038 ± 0.002 and 0.173 ± 0.010 , respectively. These values of and are independent of liquid junction errors and internally consistent with the value . By defining and on a common scale ( ), the product is independent of the assigned value of and may be determined directly from thermodynamic measurements in seawater. Using the value and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research 25 , 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.

Geochimica et Cosmochimica Acta↗