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

Sang Soo Lee  Changyong Park  Neil C. Sturchio 《Geochimica et cosmochimica acta》2010,74(6):1762-9581

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.  相似文献   

Adsorption of Sr and Ba at the cleaved mica-water interface: Free energy profiles and interfacial structure     

Artur Meleshyn 《Geochimica et cosmochimica acta》2010,74(5):1485-12976

Monte Carlo simulations show that the adsorption position of the Sr²⁺ or Ba²⁺ ion on the cleaved muscovite surface is determined by the radius of the ion’s first hydration shell, hydrogen bonding of the first shell water molecules with the basal oxygens of muscovite as well as a requirement of minimization of the number of muscovite’s lattice cations in the ion’s first coordination shell. Accordingly, Sr²⁺ or Ba²⁺ adsorbs in ditrigonal cavities at a distance of 1.12 Å or 1.35 Å, respectively, from the basal surface on dehydrated mica and above tetrahedral substitutions at a height of 1.93 ± 0.02 Å or 2.15 ± 0.03 Å, respectively, at the highest simulated water coverage of 28 H₂O per ion. The ion’s displacement from a ditrigonal cavity occurs upon adsorption of 2 H₂O per ion for Sr²⁺ and 3 H₂O per ion for Ba²⁺. At a coverage of 28 H₂O per ion, outer-sphere adsorption of Sr²⁺ or Ba²⁺ at a height of 3.9 ± 0.2 Å or 4.17 ± 0.07 Å, respectively, is possible albeit unfavorable on the free energy scale by 107 ± 7 kJ/mol or 89 ± 13 kJ/mol, respectively, as compared to inner-sphere adsorption. Activation energies for the transformation between inner-sphere and outer-sphere adsorptions are calculated to be 121 ± 3 kJ/mol for Sr²⁺ and 99 ± 10 kJ/mol for Ba²⁺. A comparison of these values with those reported recently for Mg²⁺ and Ca²⁺ results in an adsorption affinity sequence Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ in agreement with the sequence predicted recently for low dielectric constant solids (which include mica) (Sverjensky, 2006). A recent resonant anomalous X-ray reflectivity study of Sr²⁺ adsorption on muscovite (Park et al., 2006) has questioned the common assumption (Stumm, 1992), which is supported by the present simulation results, that inner-sphere adsorption is stronger than outer-sphere adsorption. A modification of the cleaved muscovite surface as a result of Sr²⁺ adsorption in muscovite’s ditrigonal cavities and related destabilization of muscovite’s hydroxyl groups is proposed as a possible reason for this controversy.  相似文献   

Structure and dynamics of water on muscovite mica surfaces     

Hiroshi Sakuma  Katsuyuki Kawamura 《Geochimica et cosmochimica acta》2009,73(14):4100-16115

The structure and dynamics of water on muscovite mica (0 0 1) surfaces have been investigated by molecular dynamics simulations. X-ray reflectivity profiles highly reflecting the interfacial structure are directly calculated and compared with those of experiments. The direct comparison has validated the usefulness of MD simulations to understand the real interfacial structure of the mica−water system. We observed five distinguished peaks in the density profile of oxygen present in water, and these peaks are attributable to the water molecules directly adsorbed on mica, hydrated to the K⁺ ions on the mica surface, and ordered due to hydrogen bonds between hydrated K⁺ ions. The hydrated K⁺ ions make an inner-sphere complex and have an explicit first hydration shell with a radius of 3.6 Å and a hydration number of 2.9. The change of the viscosity of water located above 1 nm apart from the mica surface was not observed. This feature is in good agreement with a recent experimental study in which the shear measurement was conducted using a surface forces apparatus. The increase of the viscosity by a factor of ca. 2-3 relative to that of the bulk water was observed at water located within 1 nm from the isolated mica surface.  相似文献   

Interaction of muscovite (0 0 1) with Pu bearing solutions at pH 3 through ex-situ observations     

P. Fenter  S.S. Lee  L. Soderholm  O. Schwindt 《Geochimica et cosmochimica acta》2010,74(24):6984-6995

The interaction of Pu³⁺ bearing solutions with the muscovite (0 0 1) basal plane is explored using a combination of ex-situ approaches including alpha-counting, to determine the Pu³⁺ adsorption isotherm, and X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR), to probe the interfacial structure and Pu-specific distribution, respectively. Pu uptake to the muscovite (0 0 1) surface from Pu³⁺ solutions in a 0.1 M NaClO₄ background electrolyte at pH 3 follows an approximate Langmuir isotherm with an apparent adsorption constant, K_app = 5 × 10⁴ M⁻¹, and with a maximum coverage that is consistent with the amount needed to fully compensate the surface charge by trivalent Pu. The XR results show that the muscovite surface reacted with a 10⁻³ M Pu³⁺ solution (at pH 3 with 0.1 M NaClO₄) and dried in the ambient environment, maintains a 30-40 Å thick layer, indicating the presence of a residual hydration layer (possibly including adventitious carbon). The RAXR results indicate that Pu sorbs on the muscovite surface with an intrinsically broad distribution with an average height of 18 Å, substantially larger than heights expected for any specifically adsorbed inner- or outer-sphere complexes. These results are discussed in the context of recent studies of cation adsorption trends on muscovite and the possible roles of Pu hydrolysis species in controlling the Pu-muscovite interactions.  相似文献   

Structure and dynamics of water on Li-, Na-, K-, Cs-, H₃O-exchanged muscovite surfaces: A molecular dynamics study     

Hiroshi Sakuma  Katsuyuki Kawamura 《Geochimica et cosmochimica acta》2011,75(1):63-23407

The distribution and dynamics of water molecules and monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺, and H₃O⁺) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs⁺ and Cl⁻ ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li⁺ ions near the muscovite surface. For Na⁺, K⁺, Cs⁺, and H₃O⁺ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li⁺, Na⁺, K⁺, and H₃O⁺ ions adsorbed on the muscovite surfaces. The water molecules around Cs⁺ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs⁺ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li⁺ ions and decreased with an increase in the ionic radius.  相似文献   

Adsorption of Rb and Sr at the orthoclase (0 0 1)-solution interface     

P. Fenter  C. Park 《Geochimica et cosmochimica acta》2008,72(7):1848-1863

Adsorption of Rb⁺ and Sr²⁺ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb⁺ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K⁺ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å²). These results are consistent with an ion-exchange reaction in which Rb⁺ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO₃)₂ solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr²⁺ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr²⁺ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb⁺, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr²⁺ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr²⁺ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr²⁺ on orthoclase. Differences in the partitioning of Sr²⁺ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.  相似文献   

Cation sorption on the muscovite (0 0 1) surface in chloride solutions using high-resolution X-ray reflectivity     

Michel L. Schlegel  Kathryn. L. Nagy  Paul Fenter  Neil C. Sturchio 《Geochimica et cosmochimica acta》2006,70(14):3549-3565

The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl₂ and 0.01 m BaCl₂ solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M₁ muscovite were small (?0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl₂ and BaCl₂ solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water.  相似文献   

Adsorption of barium ions by β-manganese dioxide and its activation in oleate flotation     

M.A Arafa  A.A Yousef  M.A Malati 《International Journal of Mineral Processing》1974,1(3):267-275

The Ba²⁺ ion adsorption isotherms on β-MnO₂ were of the Langmuir type. The endothermic heat of adsorption (40 kJ mol^?1) is ascribed to entropy contributions associated with the $\text{Na}{}^{\mathrm{+}}\text{Ba}{}^{\mathrm{2+}}$ ion-exchange mechanism. The Ba²⁺ ion adsorption density was higher at pH 10 than that at pH 7, due to the more negative surface charge at the higher pH. Ba²⁺ ions were found to reverse the sign of the ζ potential of the MnO₂ particles.More oleate was adsorbed by β-MnO₂ in the presence of Ba²⁺ ions than in their absence. The oleate adsorption isotherms on Ba²⁺-activated MnO₂ were of the Freundlich type and indicated an exothermic process. Hallimond flotation recovery of Ba²⁺-activated MnO₂ was higher at pH 10 than at pH 7, although less oleate was adsorbed at the higher pH. At pH 7, Mn²⁺-activation led to higher recoveries than Ba²⁺-activation. It seems that the attraction between the surface and the activator plays an important rôle in determining the flotation recovery.  相似文献   

The mechanism and kinetics of DTPA-promoted dissolution of barite     

Christine V. Putnis  Magdalena KowaczAndrew Putnis 《Applied Geochemistry》2008

The dissolution rate of natural barite, BaSO₄, was measured in solutions of DTPA (diethylene triamine penta-acetic acid) to investigate the mechanism of ligand-promoted dissolution using a strong chelating agent. Experiments were carried out over a range of DTPA concentrations 0.5–0.0001 M solutions, at room temperature (22 °C), as well as a range of temperatures, 22–80 °C at 1 atm. The dissolution rate is inversely related to the DTPA concentration in solution. A more dilute DTPA solution is shown to be more efficient as a solvent in terms of the approach to the equilibrium saturation value for the dissolution of Ba²⁺. An analysis of the temperature dependence of the dissolution rate at high pH by the determination of activation energies indicates that the reaction is probably controlled by the pre-exponential term in the rate constant. This indicates that reaction frequency mostly controls differences in reactivity and suggests an explanation for the results in terms of stearic hindrance due to adsorbed DTPA molecules at the barite surface. The effect of DTPA on the solvation of the Ba²⁺ ion may also influence the dissolution rate.  相似文献   

Ar loss in experimentally deformed muscovite and biotite with implications for Ar/Ar geochronology of naturally deformed rocks     

Michael Cosca  Holger Stunitz  John P. Lee 《Geochimica et cosmochimica acta》2011,75(24):7759-7778

The effects of deformation on radiogenic argon (⁴⁰Ar^∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO₂) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating ⁴⁰Ar^∗ loss of 0-35% in muscovite and 2-3% ⁴⁰Ar^∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation ⁴⁰Ar/³⁹Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% ⁴⁰Ar^∗ loss relative to the undeformed muscovite. The in situ UV laser ablation ⁴⁰Ar/³⁹Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% ⁴⁰Ar^∗ loss. No spatial correlation is observed between in situ⁴⁰Ar/³⁹Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed ⁴⁰Ar^∗ loss in the experimentally treated muscovite can be utilized to predict average ⁴⁰Ar^∗ diffusion dimensions. Maximum ⁴⁰Ar/³⁹Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for ⁴⁰Ar^∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental ⁴⁰Ar/³⁹Ar ages indicating ⁴⁰Ar^∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale ⁴⁰Ar^∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for ⁴⁰Ar^∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both ⁴⁰Ar^∗ retention and intragrain distribution in deformed mica. Intragrain ⁴⁰Ar/³⁹Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.  相似文献   

Effects of substrate structure and composition on the structure, dynamics, and energetics of water at mineral surfaces: A molecular dynamics modeling study     

Jianwei Wang  Andrey G. Kalinichev  R. James Kirkpatrick 《Geochimica et cosmochimica acta》2006,70(3):562-582

Molecular dynamics computer simulations of the molecular structure, diffusive dynamics and hydration energetics of water adsorbed on (0 0 1) surfaces of brucite Mg(OH)₂, gibbsite Al(OH)₃, hydrotalcite Mg₂Al(OH)₆Cl · 2H₂O, muscovite KAl₂(Si₃Al)O₁₀(OH)₂, and talc Mg₃Si₄O₁₀(OH)₂ provide new insight into the relationships between the substrate structure and composition and the molecular-scale structure and properties of the interfacial water. For the three hydroxide phases studied here, the differences in the structural charge on the octahedral sheet, cation occupancies and distributions, and the orientations of OH groups all affect the surface water structure. The density profiles of water molecules perpendicular to the surface are very similar, due to the prevalent importance of H-bonding between the surface and the water and to their similar layered crystal structures. However, the predominant orientations of the surface water molecules and the detailed two-dimensional near-surface structure are quite different. The atomic density profiles and other structural characteristics of water at the two sheet silicate surfaces are very different, because the talc (0 0 1) surface is hydrophobic whereas the muscovite (0 0 1) surface is hydrophilic. At the hydrophilic and electrostatically neutral brucite and gibbsite (0 0 1) surfaces, both donating and accepting H-bonds from the H₂O molecules are important for the development of a continuous hydrogen bonding network across the interfacial region. For the hydrophilic but charged hydrotalcite and muscovite (0 0 1) surfaces, only accepting or donating H-bonds from the water molecules contribute to the formation of the H-bonding network at the negatively and positively charged interfaces, respectively. For the hydrophobic talc (0 0 1) surface, H-bonds between water molecules and the surface sites are very weak, and the H-bonds among H₂O molecules dominate the interfacial H-bonding network. For all the systems studied, the orientation of the interfacial water molecules in the first few layers is influenced by both the substrate surface charge and the ability by the surfaces to facilitate H-bond formation. The first layer of water molecules at all surfaces is well ordered in the xy plane (parallel to the surface) and the atomic density distributions reflect the substrate crystal structure. The enhanced ordering of water molecules at the interfaces indicates reduced orientational and translational entropy. In thin films, water molecules are more mobile parallel to the surface than perpendicular to it due to spatial constraints. At neutral, hydrophilic substrates, single-monolayer surface coverage stabilizes the adsorbed water molecules and results in a minimum of the surface hydration energy. In contrast, at the charged and hydrophilic muscovite surface, the hydration energy increases monotonically with increasing water coverage over the range of coverages studied. At the neutral and hydrophobic talc surface, the adsorption of H₂O is unfavorable at all surface coverages, and the hydration energy decreases monotonically with increasing coverage.  相似文献   

The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology     

P. S. Dahl 《Contributions to Mineralogy and Petrology》1996,123(1):22-39

 Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of ⁴⁰Ar and ⁸⁷Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)⁺ in biotite and of small La³⁺ and Na⁺ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H₂O/H₃O⁺-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba²⁺ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K⁺, Ba²⁺)-H⁺ repulsion thereby induced by the bare proton both destabilizes Ba²⁺ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba²⁺ substitution since its O−H bonds are directed into the vacant M ₁ octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)₋₁ exchange enhances Ba²⁺ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z _i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z _i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger ⁴⁰Ar/³⁹Ar and Rb/Sr ages in natural biotite (high Z _i ) relative to coexisting muscovite (lower Z _i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model. Received: 9 February 1995 / Accepted: 8 September 1995  相似文献   

Structure, charge distribution, and electron hopping dynamics in magnetite (Fe₃O₄) (1 0 0) surfaces from first principles     

Frances N. Skomurski  Sebastien Kerisit  Kevin M. Rosso 《Geochimica et cosmochimica acta》2010,74(15):4234-7290

For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum-mechanical calculations were applied to predict Fe²⁺ availability and electron hopping rates at magnetite (1 0 0) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½-Fe_tet layer relaxed into the Fe_oct (1 0 0) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe²⁺ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe²⁺-Fe³⁺ valence interchange small polaron hopping, calculated at 10¹⁰-10¹² hops/s for bulk and bulk-like (i.e., near-surface) environments. This process is envisioned to control sustainable overall rates of interfacial redox reactions. These rates decrease by up to three orders of magnitude (10⁹ hops/s) at the (1 0 0) surface, and no significant difference is observed for vacuum-terminated versus hydrated cases. Slower hopping rates at the surface appear to arise primarily from larger reorganization energies associated with octahedral Fe²⁺-Fe³⁺ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe²⁺-Fe³⁺ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe²⁺ availability and its replenishment rate, bulk electron hopping mobility is an upper-limit for magnetite and slower surface rates may need to be considered as potentially rate-limiting. They also suggest that slower hopping mobilities calculated for surface environments may be amenable to Fe²⁺-Fe³⁺ site discrimination by conventional spectroscopic probes.  相似文献   

Variations in composition and abundance of white mica in the hydrothermal alteration system at Hellyer,Tasmania, as revealed by infrared reflectance spectroscopy   总被引：2，自引：0，他引：2  

K. Yang  J.F. Huntington  J.B. Gemmell  K.M. Scott 《Journal of Geochemical Exploration》2011

Short-wave infrared (SWIR) spectral reflectance of hydrothermally altered volcanic rocks in the footwall of the Hellyer massive sulfide deposit was measured with a portable PIMA-II infrared spectrometer. The Al–OH band was used to derive information on the octahedral Al content and the abundance of white mica (sericite) in the hydrothermal alteration and mineralization system. The range of the Al–OH band wavelength from 2192 nm to 2222 nm corresponds to the number of octahedral Al (Al^vi) in white mica approximately from 3.9 to 3.0 (based on 4 octahedral cations per formula). This Al^vi range represents a significant compositional variation, covering most of the compositional region between muscovite (Al^vi = 4.0) and phengite (Al^vi = 3.0). Furthermore, the spectral reflectance data show that the compositional variation of white mica is spatially related to hydrothermal alteration zoning, such that phengitic white mica tends to occur in 1) main upflow fluid channel, 2) intensely altered volcanic rocks, and 3) Pb–Zn mineralization, whereas muscovitic white mica was formed preferentially distal to massive sulfide mineralization on the margin of the footwall alteration system. The results suggest that the Al–OH band wavelength, and therefore the octahedral Al content, of white mica can be used as vectors to mineralization to map the hydrothermal system at Hellyer.  相似文献   

Dissolution of uranophane: An AFM, XPS, SEM and ICP study     

Michael Schindler  Michael Freund  Peter C. Burns 《Geochimica et cosmochimica acta》2009,73(9):2510-202

Dissolution experiments on single crystals of uranophane and uranophane-β, Ca(H₂O)₅[(UO₂)(SiO₃(OH)]₂, from the Shinkolobwe mine of the Democratic Republic of Congo, were done in an aqueous HCl solution of pH 3.5 for 3 h, in HCl solutions of pH 2 for 5, 10 and 30 min, and in Pb²⁺-, Ba-, Sr-, Ca- and Mg-HCl solutions of pH 2 for 30 min. The basal surfaces of the treated uranophane crystals were examined using atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Solutions after dissolution experiments on single crystals and synthetic powders were analysed with inductively coupled plasma-optical emission spectroscopy (ICP-OES) and mass spectroscopy (ICP-MS). The morphology of the observed etch pits (measured by AFM) were compared to the morphology, predicted on the basis of the bond-valence deficiency of polyhedron chains along the edges of the basal surface. Etch pits form in HCl solutions of pH 2. Their decrease in depth with the duration of the dissolution experiment is explained with the stepwave dissolution model, which describes the lowering of the surrounding area of an etch pit with continuous waves of steps emanated from the etch pit into the rest of the crystal surface. Hillocks form in an HCl solution of pH 3.5, and the chemical composition of the surface (as indicated by XPS) shows that these hillocks are the result of the precipitation of a uranyl-hydroxy-hydrate phase. Well-orientated hillocks form on the surface of uranophane in a SrCl₂-HCl solution of pH 2. They are part of an aged silica coating of composition Si₂O₂(OH)₄(H₂O)_n. An amorphous layer forms on the surface of uranophane in a MgCl₂-HCl solution of pH 2, which has a composition and structure similar to silicic acid. Small crystallites of uranyl-hydroxy-hydrate phases form on the surface of uranophane after treatment in Pb(NO₃)₂-HCl and BaCl₂-HCl solutions of pH 2. Dissolution experiments on synthetic uranophane powders show that in the early stage of the experiments, the dissolution rate of uranophane increase in the sequence Pb(NO₃)₂-HCl < BaCl₂-HCl < CaCl₂-HCl < HCl < SrCl₂-HCl < MgCl₂-HCl, indicating that the dissolution of uranophane is more enhanced in solutions containing divalent cations of small ionic radii and high Lewis acidity (Mg, MgCl⁺).  相似文献   

Co-precipitation of radium in high ionic strength systems: 1. Thermodynamic properties of the Na-Ra-Cl-SO₄-H₂O system - Estimating Pitzer parameters for RaCl₂     

Yoav O. Rosenberg  Volker Metz 《Geochimica et cosmochimica acta》2011,75(19):5389-5402

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Transmetallation of Gd-DTPA by Cu,Y and lanthanides and its impact on the hydrosphere     

Peter Möller  Peter Dulski 《Applied Geochemistry》2010

The concurrent exchange of REE³⁺ and Y³⁺ (combined to M³⁺) and Cu²⁺ for Gd³⁺ in Gd-DTPA (Gd-diethylenetriaminepentaacetic acid or gadopentetic acid) in the presence of clay is a very slow process if the concentrations of M³⁺, Cu²⁺ and Gd-DTPA in solution are in the range of 0.01–22 nmol/L. The kinetics of transmetallation was followed for 1033 h without reaching equilibrium, although the release of metal ions from the clay pool is a fast process. The sum of all newly formed mono-nuclear M-DTPA species is less than the difference [Gd-DTPA_o] − [Gd-DTPA] even after 1033 h but the sum of all derived M-DTPA + Cu-DTPA chelates exceeds this difference indicating that within this time span poly-nuclear chelates of Cu also formed. Formation of CuGd-DTPA chelates is the fastest process followed by formation of less stable MGd-DTPA chelates. With progress of formation of CuGd-DTPA the concentration of Gd-DTPA is lowered and consequently MGd-DTPA decomposes. Furthermore Cu²⁺ reacts with MGd-DTPA to form CuM-DTPA. The observed rate constants vary from species to species, whereas the pseudo-first-order-rate constants k_M are nearly the same for all lanthanides. The observed rate constant for k_Cu exceeds those of k_M because Cu concentrations are higher than M. The changes in M speciation under the influence of DTPA are estimated for a typical composition of surface water. Input of Gd-DTPA leaves only La and, to a lesser degree, Ce unaffected by transmetallation. The total concentrations of both Cu and intermediate to heavy REE increase, whereas total Gd decreases because released Gd³⁺ is adsorbed by clay minerals. Depending on Cu²⁺ and GdL²⁻ concentrations in natural surface and groundwaters, Gd-DTPA decreases by about 10% within a year. Equilibrium is theoretically reached only after more than 70 a.  相似文献   

Determination of hydrogen content in geological samples using elastic recoil detection analysis (ERDA)     

Hélène Bureau  Caroline Raepsaet  Anna Carraro 《Geochimica et cosmochimica acta》2009,73(11):3311-3322

The present study illustrates the interest of using the elastic recoil detection analysis (ERDA) method to characterize any geological sample matrix with respect to hydrogen. ERDA is combined with Rutherford back scattering (RBS) and particle induced X-ray emission (PIXE), allowing the simultaneous characterization of the matrix with respect to major and trace elements (Z > 15). Analyses are performed by mapping of a 4 × 16 μm² incident beam of ⁴He⁺ on large areas (50 × 200 μm²). The method is almost not destructive and requires no calibration with respect to well known hydrous samples. Hydrous and nominally anhydrous phases in contact with each other in the same sample may both be characterized. The depth of the analyses is limited to several μm beneath the surface, allowing tiny samples to be investigated, provided their sizes are larger than the incident beam. Our setup has been improved in order to allow H determination on a micrometric scale with a 5-15% relative uncertainty and a detection limit of 94 wt ppm H₂O. We present multi-elemental mappings on a large panel of samples: (1) natural and analogue synthetic glasses from Stromboli volcano (0.44-4.59 wt% H₂O), natural rhyolitic glasses (1466-1616 wt ppm H₂O); (2) magmatic rhyolitic melt inclusions from Guadeloupe Island (4.37-5.47 wt% H₂O) and their quartz host crystal (2020 ± 230 wt ppm H₂O); (3) nominally anhydrous natural (82-260 wt ppm H₂O) and experimentally hydrated (240-790 wt ppm H₂O) olivines; natural clinopyroxenes (159-716 wt ppm H₂O); natural orthopyroxenes (201-452 wt ppm H₂O); a natural garnet (90 wt ppm H₂O). Results show that ERDA is a strong and accurate reference method that can be used to characterize geological sample from various matrix compositions from high to low water contents. It can be used to calibrate other methods of microanalysis such as Fourier Transform Infrared Spectroscopy (FTIR) or secondary ion mass spectrometry (SIMS).  相似文献   

Quantification of drying induced acidity at the mineral-water interface using ATR-FTIR spectroscopy     

C.E. Clarke  J. Aguilar-Carrillo  A.N. Roychoudhury 《Geochimica et cosmochimica acta》2011,75(17):4846-4856

Drying induced pH changes were quantified on the surface of Na⁺, Ca²⁺, Mg²⁺ and Al³⁺ saturated smectite and kaolinite clays. This was achieved using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to measure real time changes to a pH indicator, sorbed to the clay mineral surface, during wetting and drying events. Using this technique it was possible to measure how low the pH of the surface drops during dehydration, the critical water content at which acidification of the surface begins and lastly how reversible the pH decrease is. The results show that only Al³⁺-smectite shows acidification below pH 4.8 with drying. The pH starts to decrease on the Al³⁺-smectite surface even when significantly hydrated (gravimetric water content ∼ 125 mg/m²), and falls to between 1.2 and 1.4 when completely air dry. The drying induced pH decrease is completely reversible on rewetting, suggesting large pH oscillations may occur on smectite surfaces with appreciable exchangeable Al³⁺. Aluminium saturated kaolinite did not show significant acidification in response to drying (pH > 3.5), however, a 0.1 M AlCl₃ solution evaporated to a final pH of 2.8. The enhanced acidification observed on an Al-smectite clay compared to a solution containing free Al³⁺ ions highlights the role of highly charged surfaces in the hydrolysis reaction that occurs within the hydration shell of exchangeable Al³⁺ ions.  相似文献   

Geochemistry of waters associated with the old mine workings at Fonte Santa (NE of Portugal)     

M.E.P. Gomes  I.M.H.R. Antunes  P.B. Silva  A.M.R. Neiva  F.A.L. Pacheco 《Journal of Geochemical Exploration》2010

The quartz veins containing scheelite from Fonte Santa mine cut the Lower Ordovician quartzites. A muscovite–biotite granite (G1) and a muscovite granite (G2), both S-type, crop out close to the Fonte Santa mine and are related to the Moncorvo–Bemposta shear zone. The most altered samples of G2 show intense muscovitization and microclinization and contain chlorite, columbite–tantalite, wolframite, W-ixiolite and Fe-oxides. The tin-bearing granites contain 18 ppm (G1) and 73 ppm (G2) Sn. The most altered samples of G2 correspond to a tungsten granite. The quartz veins contain muscovite, chlorite, tourmaline, scheelite, pyrrhotite, pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, iron oxides, Fe sulfates, phosphates of Pb, Fe and Al. The Fonte Santa mine area was exploited for W between 1942 and 1982. At the end of November 2006, a flood event damaged the tailings dam of Fonte Santa mine, releasing contaminated material and increasing contaminant levels in water within the area of influence of the mine. The waters related to the Fonte Santa mine are poorly mineralized, with electrical conductivity of < 965 µS/cm, and of a mixed type or HCO₃⁻ and SO₄²⁻ types. Most pH values (5.0−8.5) indicate that there is no significant acidic drainage in the region, as found in other areas. More acidic values (pH = 3.4) were found in the mine's lagoon. Waters associated with mineralized veins and old mine activities have Fe and Mn concentrations that forbid their use for human consumption and agriculture. Natural Na, Mg and K water contents are associated with the alteration of albite, chlorite and muscovite of country rock, while Ca with the W-bearing quartz veins. Weathering agents are carbonic and sulphuric acids and the latter has a strong influence in areas draining fine-grained mine tailings.  相似文献