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Thermodynamic stability studies of the basic copper carbonate mineral,malachite

James L Symes Dana R Kester 《Geochimica et cosmochimica acta》1984,48(11):2219-2229

Natural malachite is a well defined solid demonstrating reproducible solubility behavior over a wide range of pH. The following equilibrium constants associated with the malachite dissolution equilibrium at 25°C, 1 atm were determined:

K_{sp} = a^{2}_{cu^{2+}} a_{CO_{3}^{2?}} K^{2}_{w} a^{2}_{H+} = 3.5 ± 0.6 × 10^{?34}

(infinite dilution)

K^{1}_{sp} = [Cu^{2+}]^{2} [CO^{2?}_{3}]K^{2}_{w} a^{2}_{H+} = 10. ± 0.2 × 10^{?32}

(0.72 ionic strength)

K′_{sp} = m^{2}_{Cu^{2+}} m_{COsu2?}_{3} K^{2}_{w} a^{2}_{H+} = 1.3 ± 0.1 × 10^{?28}

(36.9‰ salinity seawater). The temperature dependence of a “mixed” equilibrium constant, K_sp⁺, of the form:

K^{2}_{sp} = [Cu^{2+}]^{2} m_{CO^{2?}_{3}} K^{2}_{w} a^{2}^{H+}

has been measured at I = 0.72, yielding the relationship:

log K^{2}_{sp} = (? 9.8 ± 0.03) × 10^{4} (1 T°K) + (1.52 ± 0.09)

within a 5–25°C temperature range. The effect of pressure on the solubility of malachite in water and seawater was estimated from partial molar volume and compressibility data. For 25 °C at infinite dilution

K'_{sp} (1000 bar) K'_{sp} (0) = 240

and in seawater

K′_{sp} (1000) K'_{sp} (0) = 44

.Comparison of stoichiometric and apparent malachite equilibrium constants has been used to estimate the extent of copper(II) ion interaction at the ionic strength of seawater. In dilute carbonate medium (total alkalinity, TA = 2.4 meq/kg H₂O, pH 8.3), 2.9% of total dissolved copper exists as the free copper(II) ion and in seawater (S = 36.9%., TA = 2.3 meq/kg H₂O, pH = 8.1),

[Cu^{2+}] T(Cu)

is 3.1%.Total dissolved copper levels of approximately 450–750 nMol/Kg are necessary to attain malachite saturation conditions in the open ocean. Observations of malachite particles suspended in seawater must be explained by precipitation or solid phase substitution reactions from localized environments rather than by direct precipitation from bulk seawater. 相似文献

The effect of pressure on the solubility of minerals in water and seawater

Frank J Millero 《Geochimica et cosmochimica acta》1982,46(1):11-22

The effect of presure on the solubility of minerals in water and seawater can be estimated from In

(K^{P}_{sp} K^{0}_{sp}) + (?ΔVP + 0.5ΔKP^{2}) RT

where the volume (ΔV) and compressibility (ΔK) changes at atmospheric pressure (P = 0) are given by

ΔV = V ? (M^{+}, X^{?}) ? V ? [MX(s)]ΔK = K ? (M^{+}, X^{?}) ? K ? [MX(s)]

Values of the partial molal volume (

V ?

) and compressibilty (

K ?

) in water and seawater have been tabulated for some ions from 0 to 50°C. The compressibility change is quite large (~10 × 10^?3 cm³ bar^?1 mol^?1) for the solubility of most minerals. This large compressibility change accounts for the large differences observed between values of ΔV obtained from linear plots of In K_sp versus P and molal volume data (Macdonald and North, 1974; North, 1974). Calculated values of

K^{P}_{sp} K^{o}_{sp}

for the solubility of CaCO₃, SrSO₄ and CaF₂ in water were found to be in good agreement with direct measurements (Macdonald and North, 1974). Similar calculations for the solubility of minerals in seawater are also in good agreement with direct measurements (Ingle, 1975) providing that the surface of the solid phase is not appreciably altered. 相似文献

Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie

L. Dever R. Durand J.Ch. Fontes P. Vachier 《Geochimica et cosmochimica acta》1982,46(10):1947-1956

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions.During winter: the soil is only partly saturated in water and the CO₂ partial pressure is low (Ca 10^?3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and ¹³C content (

δ_{tidc}^{13} C = ?12,2%0

) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values:

δ_{G}^{13} C = ?21,5%0

;

A_{G}^{14} C = 118%

;

δ_{M}^{13} C = +2,9%0

;

A_{M}^{14} C = 28%

).During spring and summer: the soil moisture decreases, the input of biogenic CO₂ induces an increase of the soil CO₂ partial pressure (Ca from 3.10^?3 atm to 7.10^?3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO₂ with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO₂ dissolution and CO₂ effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δ_{L mes} = from ?9,4%0 to ?11,5%0, δ_{l calc} = from ?9,8%0 to ?13,9%0 A_{L mes} = 119%, A_{L calc} = from 119% to 125%).The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite. 相似文献

Density calculations for silicate liquids. I. Revised method for aluminosilicate compositions

Y Bottinga D Weill P Richet 《Geochimica et cosmochimica acta》1982,46(6):909-919

Equations are developed for calculating the density of aluminosilicate liquids as a function of composition and temperature. The mean molar volume at reference temperature T_r, is given by

V_{r} = ∑X_{i} V ?^{o}_{i} + X_{A} V ?^{o}_{A}

, where the summation is taken over all oxide components except A1₂O₃, X stands for mole fraction,

V ?^{o_{i}}

terms are constants derived independently from an analysis of volume-composition relations in alumina-free silicate liquids, and

V ?^{o}_{A}

is the composition-dependent apparent partial molar volume of Al₂O₃. The thermal expansion coefficient of aluminosilicate liquids is given by

α = ∑X_{i} \ ? ga_{i}^{o} + X_{A} \ ? ga_{A}^{o}

, where

\ ? ga_{i}^{o}

terms are constants independent of temperature and composition, and

\ ? ga^{o}_{A}

is a composition-dependent term representing the effect of Al₂O₃ on the thermal expansion. Parameters necessary to calculate the volume of silicate liquids at any temperature T according to V(T) = V_rexp[α(T-T_r)], where T_r = 1400°C have been evaluated by least-square analysis of selected density measurements in aluminosilicate melts. Mean molar volumes of aluminosilicate liquids calculated according to the model equation conform to experimentally measured volumes with a root mean square difference of 0.28

cc mole

and an average absolute difference of 0.90% for 248 experimental observations. The compositional dependence of

V ?^{o}_{A}

is discussed in terms of several possible interpretations of the structural role of Al³⁺ in aluminosilicate melts. 相似文献

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

L.Niel Plummer Eric T. Sundquist 《Geochimica et cosmochimica acta》1982,46(2):247-258

We have calculated the total individual ion activity coefficients of carbonate and calcium,

γ_{TCO_{3}^{2?}}

and

γ_{TCa^{2+}}

, 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

γ_{TCO^{2}_{3?}}

and

γ_{TCa_{2+}}

are

0.038 ± 0.002

and

0.173 ± 0.010

, respectively. These values of

γ_{TCO_{3}^{2?}}

and

γ_{TCa^{2+}}

are independent of liquid junction errors and internally consistent with the value

γ_{TCl^{?}} = 0.651

. By defining

γ_{TCa^{2+}}

and

γ_{TCO_{3}^{2?}}

on a common scale (

γ_{TCl^{?}}

), the product

γ_{TCa^{2+}}

γ_{TCO_{3}^{2?}}

is independent of the assigned value of

γ_{Cl^{?}}

and may be determined directly from thermodynamic measurements in seawater. Using the value

γ_{TCa^{2+}} γ_{TCO_{3}^{2?}} = 0.0067

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 Research25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data. 相似文献

The system pargasite-H₂O-CO₂: a model for melting of a hydrous mineral with a mixed-volatile fluid—I. Experimental results to 8 kbar

J.R Holloway 《Geochimica et cosmochimica acta》1973,37(3):651-666

The stability of the amphibole pargasite [NaCa₂Mg₄Al(Al₂Si₆))O₂₂(OH)₂] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H₂O was controlled independently of P by using mixtures of H₂O + CO₂ in the fluid phase. The mole fraction of H₂O in the fluid (

X_{H}_{2} O^{1fl}

) ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing

X_{H}_{2} O^{1fl}

at constant P. Above P ? 4 kbar stability T increases as

X_{H}_{2} O^{1fl}

is decreased below one, passes through a T maximum and then decreases with a further decrease in

X_{H}_{2} O^{1fl}

. This behavior is due to a decrease in the H₂O content of the silicate liquid as

X_{H}_{2} O^{1fl}

decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for

X_{H}_{2} O^{1fl} = 1

) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from

X_{H}_{2} O^{1fl} ? 0.6

at P = 5 kbar to

X_{H}_{2} O^{1fl} ? 0.4

at P = 8 kbar.The H₂O content of liquid coexisting with pargasite has been estimated as a function of

X_{H_{2}O}^{1fl}

at 5 and 8 kbar P, and can be used to estimate the H₂O content of magmas. Because pargasite is stable at low values of

X_{H}_{2} O^{1fl}

at high P and T, hornblende can be an important phase in igneous processes even at relatively low H₂O fugacities. 相似文献

Liquidus relationships in the system CaCO₃Ca(OH)₂CaS and the solubility of sulfur in carbonatite magmas

George R. Helz Peter J. Wyllie 《Geochimica et cosmochimica acta》1979,43(2):259-265

CaCO₃Ca(OH)₂CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO₃, Ca(OH)₂ and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO₃ = 46.1%, Ca(OH)₂ = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO₃ to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to

?_{S_{2}}^{12}

and inversely proportional to

?_{O_{2}}^{12}

; it also is likely to be directly proportional to melt basicity, defined here by

a_{CO_{3}^{2?}} ?_{CO_{2}}

. A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas. 相似文献

Stability and phase equilibria of chloride and carbonate bearing scapolites at 750°C and 4000 bar

David E. Ellis 《Geochimica et cosmochimica acta》1978,42(8):1271-1281

At 750°C and 4000 bar scapolite is stable relative to plagioclase + calcite over the range of plagioclase compositions An₅₃–An₈₃. The assemblage plagioclase + scapolite + calcite is stable relative to plagioclase + calcite over the ranges of plagioclase composition An₄₈-An₅₃ and An₈₃–An_91.5. When NaCl is present in the coexisting fluid the range of scapolite compositions stable relative to plagioclase increases. High mole fractions of NaCl in the fluid stabilize scapolite relative to plagioclases from An₂₅ to An₈₇ in the presence of excess calcite. Determination of the

Cl (Cl + CO_{3})

ratios of the synthetic scapolites shows that the range of stable scapolite compositions is significantly larger than heretofore proposed, and that even the chloride and carbonate bearing scapolites must be considered a four component solid solution. The K_D for the exchange of NaCl and CaCo₃ between coexisting scapolite, fluid and carbonate is given by the equation In

K_{D} = (?0.0028) [Al (Al + Si)]^{?5.5580}

. This equation implies that Cl-poor natural scapolites coexisted with fluids low in NaCl, and that regional occurrences of Cl-rich scapolites are likely to represent metamorphosed evaporite sequences. 相似文献

Carbonate equilibria in hydrothermal systems: First ionization of carbonic acid in NaCl media to 300°C

C.S Patterson G.H Slocum R.H Busey R.E Mesmer 《Geochimica et cosmochimica acta》1982,46(9):1653-1663

The ionization quotients of aqueous carbon dioxide (carbonic acid) have been precisely determined in NaCl media to 5 m and from 50° to 300°C using potentiometric apparatus previously developed at Oak Ridge National Laboratory. The pressure coefficient was also determined to 250°C in the same media. These results have been combined with selected information in the literature and modeled in two ways to arrive at the best fits and to derive the thermodynamic parameters for the ionization reaction, including the equilibrium constant, activity coefficient quotients, and pressure coefficients. The variation with temperature of the two fundamental quantities

Δ V ?^{o}

and

Δ C ?^{o}_{p}

were examined along the saturation vapor pressure curve and at constant density. The results demonstrated again that for reactions with minimal electrostriction changes the magnitudes and variations of

Δ C ?^{o}_{p}

and

Δ V ?^{o}

with temperature are small and, in addition,

Δ C ?_{p}

and

Δ V ?

are approximately independent of salt concentration.The results have also been applied to an examination of the solubility of calcite as a function of pH (in a given NaCl medium) for the neutral to acidic region both for systems with fixed CO₂ pressure and systems where the calcium ion concentration equals the concentration of carbon. The pH of saturated solutions of calcite with P_CO₂ of 12 bars increases from 5.1 to 5.5 between 100° and 300°C. 相似文献

10.

Diffusion of ions in sea water and in deep-sea sediments 总被引：3，自引：0，他引：3

Sandra Gregory 《Geochimica et cosmochimica acta》1974,38(5):703-714

The tracer-diffusion coefficient of ions in water, D_j⁰, and in sea water,

D_{j}^{1}

, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective

D_{j}^{1}

or D_j⁰ values. The tracer diffusion coefficients of ions in deep-sea sediments, D_j,sed., can be related to

D_{j}^{1}

D_{j,sed.} = D_{j}^{1} · α θ^{2}

, where θ is the tortuosity of the bulk sediment and a a constant close to one. 相似文献

11.

Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25°C

Howard M. May Philip A. Helmke Marion L. Jackson 《Geochimica et cosmochimica acta》1979,43(6):861-868

Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO₃ that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO₃)₃ solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9.The significant mononuclear ion species at equilibrium with gibbsite are Al³⁺, AlOH²⁺, Al(OH)⁺₂ and Al(OH)^?₄, and their ion activity products are

1K_{50} = 1.29 × 10^{8}

1K_{s1} = 1.33 × 10^{3}

1K_{s2} = 9.49 × 10^{?3}

and

1K_{s4} = 8.94 × 10^{?15}

. The calculated standard Gibbs free energies of formation (ΔG°_f) for the synthetic gibbsite and the A1OH²⁺, Al(OH)⁺₂ and Al(OH)^?₄ ions are ?276.0, ?166.9, ?216.5 and ?313.5 kcal mol^?1, respectively. These ΔG°_f values are based on the recently revised ΔG°_f value for Al³⁺ (?117.0 ± 0.3 kcal mol_?1) and carry the same uncertainty. The ΔG°_f of the natural gibbsite is ?275.1 ± 0.4 kcal mol^?, which suggests that a range of ΔG°_f values can exist even for relatively simple natural minerals. 相似文献

12.

Constantes de formation des complexes hydroxydés de l'aluminium en solution aqueuse de 20 a 70°C

Yves Couturier Gil Michard Gérard Sarazin 《Geochimica et cosmochimica acta》1984,48(4):649-659

Stability constants of hydroxocomplexes of Al(III):Al(OH)²⁺ and A1(OH)₄^? have been measured in the 20–70°C temperature range by reactions involving only dissolved species. The stability constant

^{1} K_{1}

of the first complex ion is studied by measuring pH of solutions of aluminium salts at several concentrations.

^{1} β_{4}

of aluminate ion is deduced from equilibrium constants of the reaction between the trioxalato aluminium (III) complex ion and Al³⁺ in acid medium, and between the same complex ion and A1(OH)₄^? in alkaline medium. The K values and the associated ΔH are

^{1} K_{1} = 10^{?5.00}

and ΔH₁ = 11.8 Kcal;

^{1} β_{4} = 10^{?22.20}

and ΔH₄ = 42.45 Kcal. These last results are not in agreement with the values of recent tables for

ΔG^{0} ?

and

ΔH^{0} ?

of Al³⁺ and Al(OH)₄^?. We suggest a consistent set of data for dissolved and solid Al species and for some aluminosilicates. 相似文献

13.

Settling velocities of particulate systems, 3. Power series expansion for the drag coefficient of a sphere and prediction of the settling velocity

F. Concha A. Barrientos 《International Journal of Mineral Processing》1982,9(2):167-172

The following equation has been previously developed for the drag coefficient of a sphere.

C_{D} = C_{0} [1 + (σ_{0} /Re^{12})]^{2}

In this work the authors propose a power series expansion for C₀ in terms of the Reynolds number:

C_{0} = 0 284153 Σ α=0 n B_{α} Re^{α}

A fifth-order polynomial permits obtaining the drag coefficient and the settling velocity of a sphere, up to a Reynolds number of 3 × 10⁵, with an average relative error of about 2%. 相似文献

14.

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes

Frank.J. Millero Robert H. Byrne 《Geochimica et cosmochimica acta》1984,48(5):1145-1150

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of

K_{1} = 30.0 ± 0.6

K_{2} = 106.7 ± 2.1

and

K_{3} = 73.0 ± 1.5

were obtained for the cumulative formation constants. [

Pb^{2+} + nCl^{?} → PbCl_{n}^{2?n})

]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution. 相似文献

15.

Light hydrocarbons in Red Sea brines and sediments

Roger A. Burke James M. Brooks William M. Sackett 《Geochimica et cosmochimica acta》1981,45(5):627-634

Light hydrocarbon (C₁-C₃) concentrations in the water from four Red Sea brine basins (Atlantis II, Suakin, Nereus and Valdivia Deeps) and in sediment pore waters from two of these areas (Atlantis II and Suakin Deeps) are reported. The hydrocarbon gases in the Suakin Deep brine (T = ~ 25°C,

Cl^{?} = ~ 85‰

CH_{4} = ~ 71 1

) are apparently of biogenic origin as evidenced by

C_{1} (C_{2} + C_{3}

) ratios of ~ 1000. Methane concentrations (6–8 μl/l) in Suakin Deep sediments are nearly equal to those in the brine, suggesting sedimentary interstitial waters may be the source of the brine and associated methane.The Atlantis II Deep has two brine layers with significantly different light hydrocarbon concentrations indicating separate sources. The upper brine (T = ~ 50°C,

Cl^{?} = ~ 73‰

, CH₄ = ~ 155 μl/l) gas seems to be of biogenic origin [

C_{1} (C_{2} + C_{3}) = ~1100

], whereas the lower brine (T = ~ 61°C,

Cl^{?} = ~ 155‰

, CH₄ = ~ 120μl/l) gas is apparently of thermogenic origin [

C_{1} (C_{2} + C_{3}) = ~ 50

]. The thermogenic gas resulting from thermal cracking of organic matter in the sedimentary column apparently migrates into the basin with the brine, whereas the biogenic gas is produced in situ or at the seawater-brine interface. Methane concentrations in Atlantis II interstitial waters underlying the lower brine are about one half brine concentrations; this difference possibly reflects the known temporal variations of hydrothermal activity in the basin. 相似文献

16.

Theoretical prediction of phase relations among aqueous solutions and minerals: Salton Sea geothermal system

Dennis K. Bird Denis L. Norton 《Geochimica et cosmochimica acta》1981,45(9):1479-1494

相似文献

17.

Correction of ground-water chemistry and carbon isotopic composition for effects of CO₂ outgassing

F.J. Pearson Donald W. Fisher L.N. Plummer 《Geochimica et cosmochimica acta》1978,42(12):1799-1807

Direct

P_{co_{2}}

measurements on water samples from several CO₂-charged warm springs are significantly higher than

P_{co_{2}}

values calculated from field pH and alkalinity (and other constituents). In addition, calcite saturation indices calculated from field pH and solution composition indicated supersaturation in samples which, on the basis of hydrogeologic concepts, should be near saturation or undersaturated. We attribute these discrepancies to uncertainties in field pH, resulting from CO₂ outgassing during pH measurement. Because samples for direct

P_{co_{2}}

measurement can be taken with minimal disturbance to the water chemistry, we have used the measured

P_{co_{2}}

to back calculate an estimate of the field pH and the carbon isotopic composition of the water before outgassing. By reconstructing water chemistry in this way, we find generally consistent grouping of

δ^{13} C

, pH, and degree of calcite saturation in samples taken from the same source at different times, an observation which we expect based on our understanding of the hydrogeology and geochemistry of the ground-water systems. This suggests that for very careful geochemical work, particularly on ground-waters much above ambient temperature,

P_{co_{2}}

measurements may provide more information on the system and a better estimate of its state of saturation with respect to carbonate minerals than can field measurements of pH. 相似文献

18.

Heat capacity measurements for cryolite (Na3AlF6) and reactions in the system NaFeAlSiOF

《Geochimica et cosmochimica acta》1987,51(12):3087-3103

相似文献

19.

Sedimentary iron monosulfides: Kinetics and mechanism of formation

Albert J Pyzik Sheldon E Sommer 《Geochimica et cosmochimica acta》1981,45(5):687-698

The reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide. Total, dissolved and acid extractable sulfide, thiosulfate, sulfate, and elemental sulfur were determined by spectrophotometric methods. Polysulfides, S₄^2? and S₅^2?, were determined from ultraviolet absorbance measurements and equilibrium calculations, while product hydroxyl ion was determined from pH measurements and solution buffer capacity.Elemental sulfur, as free and polysulfide sulfur, was 86% of the sulfide oxidation products; the remainder was thiosulfate. Rate expressions for the reduction and precipitation reactions were determined from analysis of electron balance and acid extractable iron monosulfide vs time, respectively, by the initial rate method. The rate of iron reduction in moles/liter/minute was given by

d(reduction Fe) dt = kS_{t}^{0.5} (J^{+})^{0.5} A_{FeOOH}^{1}

where S_t was the total dissolved sulfide concentration, (H⁺) the hydrogen ion activity, both in moles/ liter; and A_FeOOH the goethite specific surface area in square meters/liter. The rate constant, k, was 0.017 ± 0.002m^?2 min^?1. The rate of reduction was apparently determined by the rate of dissolution of the surface layer of ferrous hydroxide. The rate expression for the precipitation reaction was

d(FeS) dt = kS_{t}^{1} (H^{+})^{1} A_{FeOOH}^{1}

where

d(FeS) dt

was the rate of precipitation of acid extractable iron monosulfide in moles/liter/minute, and k = 82 ± 18 mol^?1l²m^?2 min^?1.A model is proposed with the following steps: protonation of goethite surface layer; exchange of bisulfide for hydroxide in the mobile layer; reduction of surface ferric ions of goethite by dissolved bisulfide species which produces ferrous hydroxide surface layer elemental sulfur and thiosulfate; dissolution of surface layer of ferrous hydroxide; and precipitation of dissolved ferrous specie and aqueous bisulfide ion. 相似文献

20.

Rates of reaction of pyrite and marcasite with ferric iron at pH 2

C.L Wiersma J.D Rimstidt 《Geochimica et cosmochimica acta》1984,48(1):85-92

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction:

FeS_{2} + 14 Fe^{3+} + 8 H_{2} O = 15 Fe^{2+} + 2 SO^{2?}_{4} + 16 H^{+}

was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law:

?dm_{Fe^{3+}} dt = k(A M)m_{Fe^{3+}}

where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and

A M

is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater

A M

ratio, they appear to be more reactive per unit mass than the higher temperature types. 相似文献