Orthoclase dissolution kinetics probed by in situ X-ray reflectivity: effects of temperature, pH, and crystal orientation

Initial dissolution kinetics at orthoclase (001) and (010) cleavage surfaces were measured for ∼2 to 7 monolayers as a function of temperature using in situ X-ray reflectivity. The sensitivity of X-ray reflectivity to probe mineral dissolution is discussed, including the applicability of this approach for different dissolution processes and the range of dissolution rates (∼10⁻¹² to 10⁻⁶ mol/m²/sec) that can be measured. Measurements were performed at pH 12.9 for the (001) surface and at pH 1.1 for the (001) and (010) surfaces at temperatures between 46 and 83°C. Dissolution at pH 12.9 showed a temperature-invariant process with an apparent activation energy of 65 ± 7 kJ/mol for the (001) cleavage surface consistent with previous powder dissolution results. Dissolution at pH 1.1 of the (001) and (010) surfaces revealed a similar process for both surfaces, with apparent activation energies of 87 ± 7 and 41 ± 7 kJ/mol, respectively, but with systematic differences in the dissolution process as a function of temperature. Longer-term measurements (five monolayers) show that the initial rates reported here at acidic pH are greater than steady-state rates by a factor of 2. Apparent activation energies at acidic pH differ substantially from powder dissolution results for K-feldspar; the present results bracket the value derived from powder dissolution measurements. The difference in apparent activation energies for the (001) and (010) faces at pH 1.1 reveals an anisotropy in dissolution kinetics that depends strongly on temperature. Our results imply a projected ∼25-fold change in the ratio of dissolution rates for the (001) and (010) surfaces between 25 and 90°C. The dissolution rate of the (001) surface is higher than that of the (010) surface above 51°C and is projected to be lower below this temperature. These results indicate clearly that the kinetics and energetics of orthoclase dissolution at acidic pH depend on crystal orientation. This dependence may reflect the different manifestation of the Al-Si ordering between the T1 and T2 tetrahedral sites at these two crystal faces and can be rationalized in terms of recent theoretical models of mineral dissolution.     

Internally consistent thermodynamic data for magnesium sulfate hydrates

Experimental studies on the stability of several Mg-sulfate hydrates including epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), starkeyite (MgSO₄·4H₂O), and kieserite (MgSO₄·H₂O) as a function of temperature and relative humidity are in poor agreement with calculations based on thermodynamic properties of these substances taken from the literature. Therefore, we synthesized four different MgSO₄ hydrates and measured their enthalpies of formation by solution calorimetry at T = 298.15 K. The resulting enthalpies of formation from the elements are:

•

Δ_fH⁰₂₉₈ (epsomite) = −3387.7 ± 1.3 kJmol⁻¹

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Δ_fH⁰₂₉₈ (hexahydrite) = −3088.1 ± 1.1 kJmol⁻¹

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Δ_fH⁰₂₉₈ (sanderite, MgSO₄·2H₂O) = −1894.9 ± 1.3 kJmol⁻¹

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Δ_fH⁰₂₉₈ (kieserite) = −1612.4 ± 1.3 kJmol⁻¹

Using mathematical programming (MAP) techniques, standard thermodynamic values consistent both with our calorimetric data and previously published humidity brackets could be derived:

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Epsomite: Δ_fH⁰₂₉₈ = −3388.7 kJmol⁻¹, S⁰₂₉₈ = 371.3 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2871.0 kJmol⁻¹

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Hexahydrite: Δ_fH⁰₂₉₈ = −3087.3 kJmol⁻¹, S⁰₂₉₈ = 348.5 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2632.3 kJmol⁻¹

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Starkeyite: Δ_fH⁰₂₉₈ = −2496.1 kJmol⁻¹, S⁰₂₉₈ = 259.9 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2153.8 kJmol⁻¹

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Kieserite: Δ_fH⁰₂₉₈ = −1611.5 kJmol⁻¹, S⁰₂₉₈ = 126.0 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −1437.9 kJmol⁻¹

Additionally, heat capacity measurements and standard entropy determinations of several magnesium sulfate hydrate minerals from the literature are analyzed and judged against estimates obtained from a linear combination of the heat capacities of MgSO₄ and hexagonal ice. The results of the MAP analysis are compared to these estimates to conclude that heat capacity and entropy correlate well with the number of waters of hydration. However, even the good correlation is not good enough to capture the fine variations in these properties. Consequently, their experimental measurement is inevitable if reliable thermodynamic data are sought. Our MAP thermodynamic data show that epsomite, hexahydrite, and kieserite have stability fields in the T-%RH space. Starkeyite is metastable. Although no MAP data could have been derived for pentahydrite (MgSO₄·5H₂O) and sanderite, their transient existence suggest that both of them are metastable as well.     

Kinetics and mechanisms of the leaching of low Fe sphalerite

The surface speciation and leaching kinetics of 38- to 75-μm sphalerite (0.45 wt.% Fe) particles reacted in O₂ purged perchloric acid (at pH 1.0) at 25, 40, 60, and 85 °C over a leach period of 144 h were investigated. In all cases, an initial rapid leach rate is observed followed by a slower leach rate. These two leach regimes can each be adequately modeled using straight-line interpolation, and thus two activation energies (E_a) have been derived. E_a for the fast and slow Zn dissolution rates were 33 ± 4 kJ mol⁻¹ and 34 ± 4 kJ mol⁻¹ respectively, suggesting the same rate-determining step.     

Surface chemistry of disordered mackinawite (FeS)

Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K_s^app = {Fe²⁺} · {H₂S(aq)} · {H⁺}⁻² = 10^+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10⁻³ mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10⁻³ mol/g FeS, determines surface charge changes. Total site density is 4 sites nm⁻². The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.     

Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

The surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H⁺ and Ca²⁺, and OH⁻ and F⁻ at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO₄) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ∼7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis.     

Self-diffusion of Si and O in diopside-anorthite melt at high pressures

Self-diffusion coefficients for Si and O in Di₅₈An₄₂ liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in ¹⁸O and ²⁸Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of ¹⁸O/¹⁶O and ^29,30Si/²⁸Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10⁻¹¹ m²/s and D(Si) = 3.65 ± 0.37 × 10⁻¹¹ m²/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm³/mol and −2.3 ± 0.4 cm³/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di₅₈An₄₂ liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.     

Effects of pH and temperature on dimerization rate of glycine: Evaluation of favorable environmental conditions for chemical evolution of life

To evaluate favorable environmental conditions for the chemical evolution of life, we studied the effects of pH and temperature on the dimerization rate of glycine (Gly: NH₂-CH₂-COOH), one of the simplest amino acids. Gly dimerizes to form glycylglycine (GlyGly), and GlyGly further reacts to form diketopiperazine (DKP). Gly solutions with pH ranging from 3.1 to 10.9 were heated for 1-14 days at 140 °C, and changes in concentrations of Gly, GlyGly, and DKP were evaluated. At pH 9.8, the experiments were conducted at 120, 140, 160, and 180 °C. The dimerization rate of Gly was nearly constant at pH 3-7 and increased with increasing pH from 7 to 9.8 and then decreased with further increases in pH. To elucidate the reason for this pH dependency, we evaluated the role of the three dissociation states of Gly (cationic state: Gly⁺, zwitterionic state: Gly^±, and anionic state: Gly⁻). For pH >6, the dominant forms are Gly^± and Gly⁻, and the molar fraction of Gly^± decreases and that of Gly⁻ increases with increasing pH. The dimerization rate was determined for each dissociation state. The reaction between Gly^± and Gly⁻ was found to be the fastest; the rate constant of the reaction between Gly^± and Gly⁻ was 10 times the size of that between Gly⁻ and Gly⁻ and 98 times that between Gly^± and Gly^±. The dimerization rate became greatest at pH 9.8 because the molar fractions of Gly^± and Gly⁻ are approximately equal at this pH. The dimerization rate increased with temperature, and an activation energy of 88 kJ mol⁻¹ was obtained. Based on these results and previous reports on the stability of amino acids under hydrothermal conditions, we determined that Gly dimerizes most efficiently under alkaline pH (∼9.8) at about 150 °C.     

Role of molecular oxygen in the dissolution of siderite and rhodochrosite

The dissolution of siderite (FeCO₃) and rhodochrosite (MnCO₃) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10^−8.65 mol m⁻² s⁻¹ for 5.5 < pH < 12 and increases as [H⁺]^0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10⁻¹⁰ mol m⁻² s⁻¹ for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10^−7.5 mol m⁻² s⁻¹, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O₂ or [Fe³⁺(OH)₄]⁻ species and surficial >Fe^II hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10^−4.93[H⁺] + 10^−8.45 mol m⁻² s⁻¹. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10^−8.45 to 10^−9.0 mol m⁻² s⁻¹. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.     

Thermodynamics of iron oxides: Part III. Enthalpies of formation and stability of ferrihydrite (∼Fe(OH)3), schwertmannite (∼FeO(OH)3/4(SO4)1/8), and ε-Fe2O3

Enthalpies of formation of ferrihydrite and schwertmannite were measured by acid solution calorimetry in 5 N HCl at 298 K. The published thermodynamic data for these two phases and ε-Fe₂O₃ were evaluated, and the best thermodynamic data for the studied compounds were selected.Ferrihydrite is metastable in enthalpy with respect to α-Fe₂O₃ and liquid water by 11.5 to 14.7 kJ•mol⁻¹ at 298.15 K. The less positive enthalpy corresponds to 6-line ferrihydrite, and the higher one, indicating lesser stability, to 2-line ferrihydrite. In other words, ferrihydrite samples become more stable with increasing crystallinity. The best thermodynamic data set for ferrihydrite of composition Fe(OH)₃ was selected by using the measured enthalpies and (1) requiring ferrihydrite to be metastable with respect to fine-grained lepidocrocite; (2) requiring ferrihydrite to have entropy higher than the entropy of hypothetical, well-crystalline Fe(OH)₃; and (3) considering published estimates of solubility products of ferrihydrite. The ΔG°_f for 2-line ferrihydrite is best described by a range of −708.5±2.0 to −705.2±2.0 kJ•mol⁻¹, and ΔG°_f for 6-line ferrihydrite by −711.0±2.0 to −708.5±2.0 kJ•mol⁻¹.A published enthalpy measurement by acid calorimetry of ε-Fe₂O₃ was re-evaluated, arriving at ΔH°_f (ε-Fe₂O₃) = −798.0±6.6 kJ•mol⁻¹. The standard entropy (S°) of ε-Fe₂O₃ was considered to be equal to S° (γ-Fe₂O₃) (93.0±0.2 J•K⁻¹•mol⁻¹), giving ΔG°_f (ε-Fe₂O₃) = −717.8±6.6 kJ•mol⁻¹. ε-Fe₂O₃ thus appears to have no stability field, and it is metastable with respect to most phases in the Fe₂O₃-H₂O system which is probably the reason why this phase is rare in nature.Enthalpies of formation of two schwertmannite samples are: ΔH°_f (FeO(OH)_0.686(SO₄)_0.157•0.972H₂O) = −884.0±1.3 kJ•mol⁻¹, ΔH°_f (FeO(OH)_0.664(SO₄)_0.168•1.226H₂O) = −960.7±1.2 kJ•mol⁻¹. When combined with an entropy estimate, these data give Gibbs free energies of formation of −761.3 ± 1.3 and −823.3 ± 1.2 kJ•mol⁻¹ for the two samples, respectively. These ΔG_f° values imply that schwertmannite is thermodynamically favored over ferrihydrite over a wide range of pH (2-8) when the system contains even small concentration of sulfate. The stability relations of the two investigated samples can be replicated by schwertmannite of the “ideal” composition FeO(OH)_3/4(SO₄)_1/8 with ΔG°_f = −518.0±2.0 kJ•mol⁻¹.     

Influence of phosphate on the oxidation kinetics of nanomolar Fe(II) in aqueous solution at circumneutral pH

The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH₂PO₄⁺, FeHPO₄ and FePO₄⁻ have been determined for the first time. FePO₄⁻ was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M⁻¹ s⁻¹. FeH₂PO₄⁺ and FeHPO₄ were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10⁻² M⁻¹ s⁻¹ and (1.2 ± 0.8) × 10⁻¹ M⁻¹ s⁻¹, respectively.     

Interactions of biopolymers with silica surfaces: Force measurements and electronic structure calculation studies

Pull-off forces were measured between a silica colloid attached to an atomic force microscope (AFM) cantilever and three homopolymer surfaces representing constituents of extracellular polymeric substances (EPS). The pull-off forces were −0.84 (±0.16), −0.68 (±0.15), and −2.37 (±0.31) nN as measured in water for dextran, phosphorylated dextran, and poly-l-lysine, respectively. Molecular orbital and density functional theory methods (DFT) were applied to analyze the measured pull-off forces using dimer clusters representing interactions between the three polymers and silica surfaces. Binding energies for each dimer were calculated with basis set superposition error (BSSE) and interpolated using corrections for silica surface hydroxyl density and silica charge density. The binding energies were compared with the normalized pull-off forces with the effective silica surface area contacting the polymer surfaces. The predicted binding energies at a −0.064 C/m² silica surface charge density corresponding to circum-neutral pH were −0.055, −0.029, and −0.338 × 10⁻¹⁸ J/nm² for the dimers corresponding to the silica surface with dextran, phosphorylated dextran, and poly-l-lysine, respectively. Polarizable continuum model (PCM) calculations with different solvents, silanol vibrational frequency calculations, and orbital interaction analysis based on natural bonding orbital (NBO) showed that phosphate groups formed stronger H-bonds with neutral silanols than hydroxyl and amino functional groups of polymers, implying that phosphate containing polymers would play important roles in EPS binding to silica surfaces.     

Diffusion kinetics of proton-induced Ne, He, and He in quartz

A natural quartz sample free of mineral and fluid inclusions was irradiated with a 200 MeV proton beam to produce spallogenic ²¹Ne, ³He and ⁴He. Temperature-dependent diffusivities of these three nuclides were then determined simultaneously by high precision stepped-heating and noble gas mass spectrometry. The outward mobility of proton-induced nuclides reflects diffusion through the quartz lattice. In the studied range of 70 to 400°C the helium diffusion coefficients exceed those of neon by 5-7 orders of magnitude. The implied diffusion parameters E_a = 153.7 ± 1.5 (kJ/mol) and ln(D_o/a²) = 15.9 ± 0.3 (ln(s⁻¹)) and E_a = 84.5 ± 1.2 (kJ/mol) and ln(D_o/a²) = 11.1 ± 0.3 (ln(s⁻¹)) for proton-induced ²¹Ne and ³He, respectively, indicate that cosmogenic neon will be quantitatively retained in inclusion-free quartz at typical Earth surface temperatures whereas cosmogenic helium will not. However, the neon diffusion parameters also indicate that diffusive loss needs to be considered for small (<1 mm) quartz grains that have experienced elevated temperatures. Since natural quartz often contains fluid inclusions which may enhance noble gas retentivity, these parameters likely represent an end-member case of purely solid-state diffusion. The ∼70 kJ/mol higher activation energy for neon diffusion compared to helium diffusion likely represents an energy barrier related to its ∼13% greater diameter and provides a fundamental constraint with which to test theories of solid state diffusion. The diffusion parameters for proton-induced ⁴He are indistinguishable from those for ³He, providing no evidence for the commonly expected inverse square root of the mass diffusion relationship between isotopes. We also find preliminary indication that increased exposure to radiation may enhance neon and helium retentivity in quartz at low temperatures.     

Dissolution of arsenopyrite (FeAsS) and galena (PbS) in the presence of desferrioxamine-B at pH 5

Microorganisms and higher plants produce biogenic ligands, such as siderophores, to mobilize Fe that otherwise would be unavailable. In this paper, we study the stability of arsenopyrite (FeAsS), one of the most important natural sources of arsenic on Earth, in the presence of desferrioxamine (DFO-B), a common siderophore ligand, at pH 5. Arsenopyrite specimens from mines in Panasqueira, Portugal (100-149 μm) that contained incrustations of Pb, corresponding to elemental Pb as determined by scanning electron microscopy-electron diffraction spectroscopy (SEM-EDX), were used for this study. Batch dissolution experiments of arsenopyrite (1 g L⁻¹) in the presence of 200 μM DFO-B at initial pH (pH₀) 5 were conducted for 110 h. In the presence of DFO-B, release of Fe, As, and Pb showed positive trends with time; less dependency was observed for the release of Fe, As, and Pb in the presence of only water under similar experimental conditions. Detected concentrations of soluble Fe, As, and Pb in suspensions containing only water were found to be ca. 0.09 ± 0.004, 0.15 ± 0.003, and 0.01 ± 0.01 ppm, respectively. In contrast, concentrations of soluble Fe, As, and Pb in suspensions containing DFO-B were found to be 0.4 ± 0.006, 0.27 ± 0.009, and 0.14 ± 0.005 ppm, respectively. Notably, the effectiveness of DFO-B for releasing Pb was ca. 10 times higher than that for releasing Fe. These results cannot be accounted for by thermodynamic considerations, namely, by size-to-charge ratio considerations of metal complexation by DFO-B. As determined by SEM-EDX, elemental sample enrichment analysis supports the idea that the Fe-S subunit bond energy is limiting for Fe release. Likely, the mechanism(s) of dissolution for Pb incrustations is independent and occurs concurrently to that for Fe and As. Our results show that dissolution of arsenopyrite leads to precipitation of elemental sulfur, and is consistent with a non-enzymatic mineral dissolution pathway. Finally, speciation analyses for As indicate variability in the As(III)/As(V) ratio with time, regardless of the presence of DFO-B or water. At reaction times <30 h, As(V) concentrations were found to be 50-70%, regardless of the presence of DFO-B. These results are interpreted to indicate that transformations of As are not imposed by ligand-mediated mechanisms. Experiments were also conducted to study the dissolution behavior of galena (PbS) in the presence of 200 μM at pH₀ 5. Results show that, unlike arsenopyrite, the dissolution behavior of galena shows coupled increases in pH with decreases in metal solubility at t > 80 h. Oxidative dissolution mechanisms conveying sulfur oxidation bring about the production of {H⁺}. However, dissolution data trends for arsenopyrite and galena indicate {H⁺} consumption. It is plausible that the formation of Pb species is dependent on {H⁺} and {OH⁻}, namely, stable surface hydroxyl complexes of the form (pH₅₀ 5.8) and for pH values 5.8 or above.     

Activation energy for diffusion of helium in water-saturated, compacted Na-montmorillonite

It is important to clarify the migration behavior of hydrogen gas dissolved in water-saturated, compacted bentonite, which is a promising material for geologic disposal of high-level waste and TRU waste disposal. The diffusion coefficients of helium, which can be detected under extremely low background conditions, in water-saturated, compacted Na-montmorillonite were determined as a function of temperature by a transient diffusion method. The activation energies for diffusion of helium were then obtained. The activation energies were from 6.9 ± 4.8 to 19 ± 2.8 kJ mol^− 1 and were regarded to be independent of dry density. The activation energies of helium in water-saturated Na-montmorillonite were roughly equal to those in bulk water, 14.9 kJ mol^− 1, and in ice, from 11 to 13 kJ mol^− 1. It is possible that helium diffuses not only in pore water but also in interlayer water.     

Tracer diffusion of Mg, Ca, Sr, and Ba in Na-aluminosilicate melts

We employed the thin source technique to investigate tracer diffusion of Mg, Ca, Sr, and Ba in glasses and supercooled melts of albite (NaAlSi₃O₈) and jadeite (NaAlSi₂O₆) compositions. The experiments were conducted at 1 bar and at temperatures between 645 and 1025°C. Typical run durations ranged between 30 min and 35 days. The analysis of the diffusion profiles was performed with the electron microprobe. Diffusivities of Ca, Sr, and Ba were found to be independent of either duration t of the experiment or tracer concentration M, initially introduced into the sample. Mg exhibits a diffusivity depending on run time and concentration and tracer diffusivity is derived by extrapolation to M/√t = 0. Temperature dependence of the diffusivity D can be represented by an Arrhenius equation D = D_o exp(−E_a/RT), yielding the following least-squares fit parameters (with D in m²/s and E_a in kJ/mol): D_Mg = 1.8 · 10⁻⁵ exp(−234 ± 20/RT), D_Ca = 3.5 · 10⁻⁶ exp(−159 ± 6/RT), D_Sr = 3.6 · 10⁻⁶ exp(−160 ± 6/RT), and D_Ba = 6.0 · 10⁻⁶ exp(−188 ± 12/RT) for albite; and D_Mg = 8.3 · 10⁻⁶ exp(−207 ± 18/RT), D_Ca = 3.8 · 10⁻⁶ exp(−153 ± 4/RT), D_Sr = 2.3 · 10⁻⁶ exp(−150 ± 4/RT), and D_Ba = 3.7 · 10⁻⁵ exp(−198 ± 4/RT) for jadeite composition. Ca and Sr diffusivities agree within error in both compositions and exhibit the fastest diffusivities, whereas Mg reveals the lowest diffusivity. The relationship between activation energy and radius shows a minimum at Ca and Sr for albite and jadeite compositions extending the relationship already observed elsewhere for alkalies. With increasing substitution of Si by (Na + Al), diffusivities increase, whereas activation energies decrease. Furthermore, a simple model modified from that of Anderson and Stuart (Anderson O. L. and Stuart D. A., “Calculation of activation energy of ionic conductivity in silica glasses by classical methods,” J. Am. Ceram. Soc.37, 573-580, 1954) is discussed for calculating the activation energies.     

Diffusion kinetics of Fe and Mg in aluminous spinel: experimental determination and applications

The diffusion coefficients of Fe²⁺ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO₂, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe²⁺ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = D₀exp(−Q/RT), where Q is the activation energy, with D₀(Fe) = 1.8 (±2.8) × 10⁻⁵, D₀(Mg) = 1.9 (±1.4) × 10⁻⁵ cm²/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm³/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) ≤0.5 is ∼10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks.     

Effects of seawater carbonate ion concentration and temperature on shell U, Mg, and Sr in cultured planktonic foraminifera

We investigate the sensitivity of U/Ca, Mg/Ca, and Sr/Ca to changes in seawater [CO₃²⁻] and temperature in calcite produced by the two planktonic foraminifera species, Orbulina universa and Globigerina bulloides, in laboratory culture experiments. Our results demonstrate that at constant temperature, U/Ca in O. universa decreases by 25 ± 7% per 100 μmol [CO₃²⁻] kg⁻¹, as seawater [CO₃²⁻] increases from 110 to 470 μmol kg⁻¹. Results from G. bulloides suggest a similar relationship, but U/Ca is consistently offset by ∼+40% at the same environmental [CO₃²⁻]. In O. universa, U/Ca is insensitive to temperature between 15°C and 25°C. Applying the O. universa relationship to three U/Ca records from a related species, Globigerinoides sacculifer, we estimate that Caribbean and tropical Atlantic [CO₃²⁻] was 110 ± 70 μmol kg⁻¹ and 80 ± 40 μmol kg⁻¹ higher, respectively, during the last glacial period relative to the Holocene. This result is consistent with estimates of the glacial-interglacial change in surface water [CO₃²⁻] based on both modeling and on boron isotope pH estimates. In settings where the addition of U by diagenetic processes is not a factor, down-core records of foraminiferal U/Ca have potential to provide information about changes in the ocean’s carbonate concentration.Below ambient pH (pH < 8.2), Mg/Ca decreased by 7 ± 5% (O. universa) to 16 ± 6% (G. bulloides) per 0.1 unit increase in pH. Above ambient pH, the change in Mg/Ca was not significant for either species. This result suggests that Mg/Ca-based paleotemperature estimates for the Quaternary, during which surface-ocean pH has been at or above modern levels, have not been biased by variations in surface-water pH. Sr/Ca increased linearly by 1.6 ± 0.4% per 0.1 unit increase in pH. Shell Mg/Ca increased exponentially with temperature in O. universa, where Mg/Ca = 0.85 exp (0.096*T), whereas the change in Sr/Ca with temperature was within the reproducibility of replicate measurements.     

Solubility of nanocrystalline scorodite and amorphous ferric arsenate: Implications for stabilization of arsenic in mine wastes

Solubility experiments were performed on nanocrystalline scorodite and amorphous ferric arsenate. Nanocrystalline scorodite occurs as stubby prismatic crystals measuring about 50 nm and having a specific surface area of 39.88 ± 0.07 m²/g whereas ferric arsenate is amorphous and occurs as aggregated clusters measuring about 50–100 nm with a specific surface area of 17.95 ± 0.19 m²/g. Similar to its crystalline counterpart, nanocrystalline scorodite has a solubility of about 0.25 mg/L at around pH 3–4 but has increased solubilities at low and high pH (i.e. <2 and >6). Nanocrystalline scorodite dissolves incongruently at about pH > 2.5 whereas ferric arsenate dissolution is incongruent at all the pH ranges tested (pH 2–5). It appears that the solubility of scorodite is not influenced by particle size. The dissolution rate of nanocrystalline scorodite is 2.64 × 10⁻¹⁰ mol m⁻² s⁻¹ at pH 1 and 3.25 × 10⁻¹¹ mol m⁻² s⁻¹ at pH 2. These rates are 3–4 orders of magnitude slower than the oxidative dissolution of pyrite and 5 orders of magnitude slower than that of arsenopyrite. Ferric arsenate dissolution rates range from 6.14 × 10⁻⁹ mol m⁻² s⁻¹ at pH 2 to 1.66 × 10⁻⁹ mol m⁻² s⁻¹ at pH 5. Among the common As minerals, scorodite has the lowest solubility and dissolution rate. Whereas ferric arsenate is not a suitable compound for As control in mine effluents, nanocrystalline scorodite that can be easily precipitated at ambient pressure and temperature conditions would be satisfactory in meeting the regulatory guidelines at pH 3–4.     

Competitive adsorption of strontium and fulvic acid at the muscovite-solution interface observed with resonant anomalous X-ray reflectivity

Molecular-scale distributions of Sr²⁺ and fulvic acid (FA) adsorbed on the muscovite (0 0 1) surface were investigated using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). The total amount of Sr²⁺ adsorbed from a 1 × 10⁻² mol/kg SrCl₂ and 100 mg/kg Elliott Soil Fulvic Acid II (ESFA II) solution at pH 5.5 compensated 81 ± 5% of the muscovite surface charge, less than previously measured (118 ± 5%) in an ESFA II-free solution with the same Sr concentration and pH. Inner-sphere (IS) and outer-sphere (OS) Sr²⁺ constituted 87% of the total adsorbed species in IS:OS proportions of 19:81 compared to 42:58 in the solution without FA, suggesting that adsorbed FA competes with the IS Sr²⁺ for surface sites. The coverage of both IS and OS Sr²⁺ decreased even more in a pH 3.5 solution containing the same concentration of FA and 0.5 × 10⁻² mol/kg Sr(NO₃)₂, whereas a significant amount of Sr²⁺ accumulated farther from the surface in the FA layer. The amount of Sr²⁺ incorporated in the ∼10 Å thick FA layer decreased by 79% with decreasing FA concentration (100 → 1 mg/kg) and increasing Sr²⁺ concentration (0.5 × 10⁻² → 1 × 10⁻² mol/kg) and pH (3.5 → 3.6). These results indicate not only that adsorbed FA molecules (and perhaps also H₃O⁺) displace Sr²⁺ near the muscovite surface, but also that the sorbed FA film provides binding sites for additional Sr²⁺ away from the surface. When a muscovite crystal pre-coated with FA after reaction in a 100 mg/kg ESFA II solution for 50 h was subsequently reacted with a 0.5 × 10⁻² mol/kg Sr(NO₃)₂ and 100 mg/kg ESFA II solution at pH 3.7, a significant fraction of Sr²⁺ was distributed in the outer part of the FA film similar to that observed on fresh muscovite reacted at pH 3.5 with a pre-mixed Sr-FA solution at the same concentrations. However, this Sr²⁺ sorbed in the pre-adsorbed organic film was more widely distributed and had a lower coverage, suggesting that pre-sorbed FA may undergo fractionation and/or conformational changes that diminish its capacity, and that of the muscovite (0 0 1) surface, for adsorbing the aqueous Sr cation.     

Solubility of cyclooctasulfur in pure water and sea water at different temperatures

The solubility of cyclooctasulfur in water and sea water at various temperatures in the range between 4 and 80 °C was determined. Cyclooctasulfur in equilibrium with rhombic sulfur reacted with hot acidic aqueous potassium cyanide to form thiocyanate anion which was measured by anion chromatography. Sulfur solubility in pure water was found to increase with temperature by more than 78 times: from 6.1 nM S₈ at 4 °C to 478 nM S₈ at 80 °C. The following thermodynamic values for solubilisation of S₈ in water were calculated from the experimental data: K° = 3.01 ± 1.04 × 10⁻⁸, ΔG_r° = 42.93 ± 0.73 kJ mol⁻¹, ΔH_r° = 47.4 ± 3.6 kJmol⁻¹, ΔS_r° = 15.0 ± 11.7 J mol⁻¹ K⁻¹). Solubility of cyclooctasulfur in sea water was found to be 61 ± 13% of the solubility in pure water regardless of the temperature.