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1.
The basal plane reactivities of the sheet silicates apophyllite and phlogopite have been studied by hydrothermal atomic force microscopy (HAFM) in situ in aqueous solutions at temperatures from 20 to 140 °C. At pH 4-5.6 (T = 20-100 °C), the apophyllite basal surface undergoes a swelling process which forms square hillocks on the surface. The reaction comprises three sequential morphological transformations that cause swelling to increase from 0.15 to 2.5 nm. In the first two transformations, interlayer cations are replaced by hydronium ions from the solution; the third transformation involves a depolymerization and partial cross-linking of the protonated silicate sheets. The reaction of phlogopite with acidic aqueous solutions (pH 1.5-2) at high temperature (T = 100-140 °C) causes the nucleation of numerous monolayer etch pits on the pristine surface. Where the 2D pits recur at the same lateral position, they can accumulate to a total pit depth of up to 50 nm. The formation of an altered layer has also been detected at these conditions. The alteration affects the uppermost 4-5 layers. The layers are expanded, corrugated, highly unstable, and readily peel off the surface. Etch pit formation has been detected even underneath the altered layer. On the basis of HAFM data, dissolution rates and activation energies were calculated. The presented data show that the basal surface of phlogopite plays an important role in the dissolution process at least at elevated temperatures and that the absolute amount of released material has comparable contributions from both basal surfaces and edge surfaces.  相似文献   

2.
Examination of the surface morphology (via scanning electron microscopy) and surface composition (via X-ray photoelectron spectroscopy) of sodic plagioclase and potash feldspar grains taken from four different soils, provides little or no evidence for the existence of a tightly adhering protective surface layer of altered composition on the feldspar surface. Grains, from which all adhering clay has been removed by ultrasonic cleaning, exhibit the same chemical composition in the outermost few tens of angstroms as the underlying feldspar. Aluminum-rich ‘clay’ coatings which continue to adhere to the grains after ultrasonic treatment are patchy, highly hydrous, and unlikely to act as major diffusion-limiting, and thus protective, barriers. Attack by dissolution of the feldspar surface is non-uniform and follows a definite etching sequence characterized by the development and growth of distinctive etch pits. This dissolution sequence can be reproduced by treating fresh feldspars in the laboratory with strong HF-H2SO4? solutions and, thus, the sequence is unaffected by the composition of the attacking solution. All of our results suggest that the dissolution of feldspar during weathering is controlled by selective chemical reaction at the feldspar-solution interface and not by uniform diffusion through a protective surface layer.  相似文献   

3.
Surface coatings are very common on mineral grains in soils but most laboratory dissolution experiments are carried out on pristine, uncoated mineral grains. An experiment designed to unambiguously isolate the effect of surface coatings on mineral dissolution from any influence of solution saturation state is reported. Two aliquots of 53 to 63 μm anorthite feldspar powder were used. One was dissolved in pH 2.6 HCl, the other in pH 2.6 FeCl3 solution, both for ∼6000 h in flow-through reactors. An amorphous Fe-rich, Al-, Ca- and Si-free orange precipitate coated the anorthite dissolved in the FeCl3 solution. BET surface area of the anorthite increased from 0.16 to 1.65 m2 g−1 in the HCl experiment and to 3.89 m2 g−1 in the FeCl3 experiment. The increase in surface area in the HCl experiment was due to the formation of etch pits on the anorthite grain surface whilst the additional increase in the FeCl3 experiment was due to the micro- and meso-porous nature of the orange precipitate. This precipitate did not inhibit or slow the dissolution of the anorthite. Steady state dissolution rates for the anorthite dissolved in the HCl and FeCl3 were ∼2.5 and 3.2 × 10−10 molfeldspar m−2 s−1 respectively. These rates are not significantly different after the cumulative uncertainty of 17% in their value due to uncertainty in the inputs parameters used in their calculation is taken into account. Results from this experiment support previous theoretical and inference-based conclusions that porous coatings should not inhibit mineral dissolution.  相似文献   

4.
The influence of background electrolytes on the mechanism and kinetics of calcite dissolution was investigated using in situ Atomic Force Microscopy (AFM). Experiments were carried out far from equilibrium by passing alkali halide salt (NaCl, NaF, NaI, KCl and LiCl) solutions over calcite cleavage surfaces. This AFM study shows that all the electrolytes tested enhance the calcite dissolution rate. The effect and its magnitude is determined by the nature and concentration of the electrolyte solution. Changes in morphology of dissolution etch pits and dissolution rates are interpreted in terms of modification in water structure dynamics (i.e. in the activation energy barrier of breaking water-water interactions), as well as solute and surface hydration induced by the presence of different ions in solution. At low ionic strength, stabilization of water hydration shells of calcium ions by non-paired electrolytes leads to a reduction in the calcite dissolution rate compared to pure water. At high ionic strength, salts with a common anion yield similar dissolution rates, increasing in the order Cl− < I− < F− for salts with a common cation due to an increasing mobility of water around the calcium ion. Changes in etch pit morphology observed in the presence of F− and Li+ are explained by stabilization of etch pit edges bonded by like-charged ions and ion incorporation, respectively. As previously reported and confirmed here for the case of F−, highly hydrated ions increased the etch pit nucleation density on calcite surfaces compared to pure water. This may be related to a reduction in the energy barrier for etch pit nucleation due to disruption of the surface hydration layer.  相似文献   

5.
The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 μm in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 μm during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10−11 mol cm−2 s−1 over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an “average” rate of 1.08 × 10−11 mol cm−2 s−1 with a standard deviation of 0.3 × 10−11 mol cm−2 s−1 based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature.A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state.  相似文献   

6.
Using an approach combining high-resolution and energy-filtered transmission electron microscopy (HRTEM and EFTEM), we have studied with Å to nm-spatial resolution the interfacial region that delimits the near-surface altered zone and non-altered labradorite feldspar after dissolution under acid pH conditions. The interface is characterized by extremely sharp and spatially coincident changes in structure and chemistry. The 500-nm-thick altered zone is depleted in interstitial cations (Ca, Na, K) and Al, a framework element, whereas it is enriched in H, O, and Si. Modeling H+-alkali interdiffusion within a 500-nm-thick altered zone shows that volume interdiffusion cannot reproduce the sharp chemical interfaces measured by EFTEM. Based on these new data, we propose that the near-surface altered zone is a result of interfacial dissolution-reprecipitation, and not of preferential leaching of cations and interdiffusion with H+. This implies an intrinsic dissolution process that is stoichiometric, where the breaking of bonds and release of interstitial cations and framework elements (Al, Si, and O) to solution occur contemporaneously at equal relative rates from the original fluid–mineral interface.  相似文献   

7.
The paucity of weathering rates for quartz in the natural environment stems both from the slow rate at which quartz dissolves and the difficulty in differentiating solute Si contributed by quartz from that derived from other silicate minerals. This study, a first effort in quantifying natural rates of quartz dissolution, takes advantage of extremely rapid tropical weathering, simple regolith mineralogy, and detailed information on hydrologic and chemical transport. Quartz abundances and grain sizes are relatively constant with depth in a thick saprolite. Limited quartz dissolution is indicated by solution rounding of primary angularity and by the formation of etch pits. A low correlation of surface area (0.14 and 0.42 m2 g−1) with grain size indicates that internal microfractures and pitting are the principal contributors to total surface area.Pore water silica concentration increases linearly with depth. On a molar basis, between one and three quarters of pore water silica is derived from quartz with the remainder contributed from biotite weathering. Average solute Si remains thermodynamically undersaturated with respect to recently revised estimates of quartz solubility (<180 μM) but exceeds estimated critical saturation concentrations controlling the initiation of etch pit formation (>17–81 μM). Etch pitting is more abundant on grains in the upper saprolite and is associated with pore waters lower in dissolved silica. Rate constants describing quartz dissolution increase with decreasing depth (from 10−14.5–10−15.1 mol m−2 s−1), which correlate with both greater thermodynamic undersaturation and increasing etch pit densities. Unlike for many aluminosilicates, the calculated natural weathering rates of quartz fall slightly below the rate constants previously reported for experimental studies (10−12.4–10−14.2 mol m−2 s−1). This agreement reflects the structural simplicity of quartz, dilute solutes, and near-hydrologic saturation.  相似文献   

8.
Obsidian glass alteration experiments under near hydrothermal conditions were performed to study mechanism and conditions of formation of altered minerals. X-ray diffraction patterns and cell dimensions of the specimens treated at 150, 200 and 300°C (pH = 8.03) revealed appearance of three main minerals — illite (9.5–10 Å), chlorite (7.04 Å) and halloysite (10.25Å). Further increase in the pH favours matrix dissolution with the formation of secondary altered layers. SEM-EDS study show that the alteration causes smoothing of the grain surfaces. These surfaces exhibits etch pits and series of depressions, formed by the process of dissolution. SEM — Back Scattered Electron images of obsidian specimens show thin laminae of smectite, with foliated bulky rims and cellular honeycomb texture, formed by precipitation from the solution as well as by direct transformation of glass during alteration. This mechanism is resulting from the alteration of alkalis by ionic inter-diffusion with H3O+ and H+ and inward diffusion of H2O, leading to free diffusion of silica into solution and then to a local rearrangement of the glass framework. Thus, a direct transformation of glass into clay minerals is the major reaction mechanism as evidenced by the mechanism of glass dissolution and subsequent mineral precipitation.  相似文献   

9.
The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the <hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10−2 nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10−2 nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the <hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution.Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10−2 nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10−2 nm/s for those from other line defects).The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface.  相似文献   

10.
《Applied Geochemistry》2004,19(6):835-841
Experiments on dissolution kinetics of galena were performed in 1 mol l−1 NaCl solutions at pH 0.43–2.45 and 25–75 °C. When the dissolution reaction is far from equilibrium, a linear relation exits between the dissolution rate, r, and the H+ ion activity, [H+]. The rate law for galena dissolution is given by the following equation: r=k[H+]. With respect to H+, the dissolution reaction is in the first order. The apparent rate constant, k, has values of 2.34×10−7 mol m−2 s−1 at 25 °C, 1.38×10−6 mol m−2 s−1 at 50 °C, and 7.08×10−6 mol m−2 s−1 at 75 °C. The activation energy of dissolution reaction is 43.54 kJ mol−1. The mechanism of dissolution is suggested to be surface chemical reaction, and the rate determining step is the dissociation of the Pb–S bond of the surface complex, which releases Pb2+ into the solution.  相似文献   

11.
Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E a = 332 kJ mol-1, and the apparent rate constant, k = 1041.2 mol diopside cm-2 s-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m-2) than the pits nucleated at defects (39 to 65 mJ m-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (Eb-homogeneous = 2.59 × 10-16 mJ K-1) were lower than the pits nucleated at defects (Eb-defect assisted = 8.44 × 10-16 mJ K-1).  相似文献   

12.
《Applied Geochemistry》2000,15(4):425-438
The dissolution of silica and diffusion of reactive dissolved Si in the porewaters of river sediments are investigated using sediments of different physical and chemical properties. Three sediments are considered: (a) from sectioned cores taken from a river-bed, (b) fine organic-rich surface sediment (<5 cm depth) installed in a fluvarium channel and, (c) coarse river sediment of low organic matter content also installed in a fluvarium channel. Dissolution rates of silica are measured at 10°C using batches of suspended material. The derived dissolution rate constants show large differences between the sediments. The river bed-sediment cores had vertical concentration profiles of dissolved Si that are consistent with the diffusion and dissolution of biogenic silica. Experiments in a fluvarium channel enabled Si fluxes to be calculated from a mass-balance of the overlying solution. The results are consistent with the attainment of a steady-state concentration profile of dissolved Si in the sediment. There are no discernible effects of water velocity over the sediment between 5 and 11 cm s−1. However, at 20 cm s−1, the flux increases as a result of either entrainment of fine particles at the surface or advective effects in the surface sediment. A fluvarium experiment with the fine sediment (<125 μm) over 61 days, produced a concentration profile with the highest concentration of 1025 μmol dm−3 at a depth of 4–5 cm in the sediment. A FORTRAN program is used to model the results of the increase in dissolved Si in the overlying water and development of a concentration profile in the porewater. This leads to a sediment diffusion coefficient of 1.21×10−9 m2 s−1 at 8.8°C at the beginning of the experiment and rate constant k=13.1×10−7 s−1 at pH=7.82 and average temperature of 7.6°C for the entire experiment. Fluxes measured at the sediment–surface interface and calculated assuming steady-state profiles had developed are typically 0.01–0.04 μmol m−2 (of river bed) s−1. The approach enables the efflux of dissolved Si from bottom-sediments to be estimated from dissolution rates measured using suspensions of bed-sediment.  相似文献   

13.
The dissolution of prismatic and rhombohedral quartz surfaces by KOH/H2O solutions was investigated by atomic force microscopy. Rates of dissolution of different classes of surface features (e.g., steps, voids, and dislocation etch pits) were measured. The prismatic surface etched almost two orders of magnitude faster than the rhombohedral surface, mostly due to the difference in the number and the rate of dissolution of extended defects, such as dislocations. Because of the presence of imperfect twin boundaries, defect densities on the prismatic surface were estimated at 50-100 μm−2, whereas the rhombohedral surface possessed only ∼0.5-1.0 μm−2, mostly in the form of crystal voids. Crystal voids etched almost one order of magnitude faster on the prismatic surface than on the rhombohedral surface due to differences in the number and the density of steps formed by voids on the different surfaces. In the absence of extended defects, both surfaces underwent step-wise dissolution at similar rates. Average rates of step retreat were comparable on both surfaces (∼3-5 nm/h on the prismatic surface and ∼5-10 nm/h on the rhombohedral surface). Prolonged dissolution left the prismatic surface reshaped to a hill-and-valley morphology, whereas the rhombohedral surface dissolved to form coalescing arrays of oval-shaped etch pits.  相似文献   

14.
The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.  相似文献   

15.
Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6C0 and the measured Ccrit, was 0.75 ± 0.15C0 (C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting.Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth.These results indicate that for C >Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ⪡ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.  相似文献   

16.
《Applied Geochemistry》1998,13(7):905-916
Experiments measuring kaolinite and smectite dissolution rates were carried out using batch reactors at 35° and 80°C. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of clay of the ≤2 μm fraction and 80, 160 or 240 ml of 0.1–4 M KOH solution. An untreated but sized kaolinite from St. Austell and two treated industrial smectites were used in the experiments. One smectite is a nearly pure montmorillonite, while the second has a significant component of beidellitic charge (35%). The change in solution composition and mineralogy was monitored as a function of time. Initially, the 3 clays dissolved congruently. No new formed phases were observed by XRD and SEM during the pure dissolution stage. The kaolinite dissolution is characterized by a linear release of silica and Al as a function of the log of time. This relationship can be explained by a reaction affinity effect which is controlled by the octahedral layer dissolution. Far from equilibrium, dissolution rates are proportional to a0.56±0.12OH− at 35°C and to a0.81±0.12OH− at 80°C. The activation energy of kaolinite dissolution increases from 33±8 kJ/mol in 0.1 M KOH solutions to 51±8 kJ/mol in 3 M KOH solutions. In contrast to kaolinite, the smectites dissolve at much lower rates and independently of the aqueous silica or Al concentrations. The proportionality of the smectite dissolution rate constant at 35 and 80°C was a0.15±0.06OH−. The activation energy of dissolution appears to be independent of pH for smectite and is found to be 52±4 kJ/mol. The differences in behavior between the two kinds of minerals can be explained by structural differences. The hydrolysis of the tetrahedral and the octahedral layer appears as parallel reactions for kaolinite dissolution and as serial reactions for smectite dissolution. The rate limiting step is the dissolution of the octahedral layer in the case of kaolinite, and the tetrahedral layer in the case of smectite.  相似文献   

17.
In situ dissolution experiments on a set of pure, optical quality Iceland spar calcite samples from four different localities showed etch pit step retreat rates to be inversely proportional to total inherent trace cation composition. Atomic absorption spectroscopy (AAS) revealed Fe2+, Mg2+, Mn2+ and Sr2+ in amounts varying from a few to hundreds of ppm. We used a very simple experimental set-up, with an Atomic Force Microscope (AFM) fluid cell and a droplet of MilliQ water. As the calcite dissolved and approached equilibrium with the solution, trace cations were released, which were then present for interaction with the dissolving surface. We monitored continuous free-drift dissolution, in situ, on fresh cleavage surfaces for up to 40 min. Dissolution produced one-layer-deep, rhombic etch pits that continually expanded as we collected images. The rhombohedral symmetry of calcite defines two obtuse and two acute edges on the cleavage surface of etch pits and these, as expected from previous work, had different dissolution rates. Despite identical experimental conditions for all samples, we observed lower step retreat rates for both obtuse and acute edges on calcite characterised by relatively high trace cation composition. Increased cation concentration, particularly Mn, was also correlated with rounding of obtuse-obtuse corners, resulting in obtuse step retreat rates similar to those for acute sides. Physcial limitations of the AFM technique were taken into account when measuring step rate retreat and results were collected only from single-layer etch pits, which represent crystalline calcite with minimal defects. Dissolution rates presented here are thus lower than previous reports for studies of deep etch pits and where the physical limitations of imaging may not have been considered. In addition to molecular-level proof that divalent cations inherent at ppm levels in the calcite affect the dissolution process, these results show that pure, optical quality Iceland spar calcite should not be considered pure in the chemical sense. The results imply that dissolution rates determined for ideal systems with pure, synthetic or natural, materials may be considered as the boundary condition for dissolution in real systems in nature, where cations are always present both in the solution and in the initial solid.  相似文献   

18.
The dissolution rate of albite has been measured as a function of pH and time at 25°C and 70°C in a single-pass flow-through leaching apparatus. Run times extended to 50 days in each experiment. Limited data were obtained at 25°C in the pH range 4–10. More extensive data were obtained at 70°C over the pH range 1.39–11.75.Dissolution rates were defined by release of Si, and in some cases also by Al and Na releases. Speciationsolubility calculations indicate the solutions were well undersaturated for all the likely possible secondary minerals. The fluid was maintained far from equilibrium with albite in all runs. Analysis of the data shows a general consistency with the transition state theory model of Helgesonet al. (1984).Feldspars leached at low and high pH at 70°C showed extensive development of prismatic etch pits demonstrating a surface reaction-controlled dissolution process.  相似文献   

19.
A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10−10.95 mol/cm2/s (PCO2 10−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10−11.68 mol/cm2/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10−12.4 mol/cm2/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm−1.  相似文献   

20.
《Geochimica et cosmochimica acta》1999,63(13-14):2043-2059
Effects of the organic acid (OA) anions, oxalate and citrate, on the solubility and dissolution kinetics of feldspars (labradorite, orthoclase, and albite) at 80°C and of quartz at 70°C were investigated at pH 6 in separate batch experiments and in media with different ionic strength (0.02–2.2 M NaCl). Although it has been shown that OAs can increase rates of feldspar dissolution, prior experiments have focused primarily on dilute, highly undersaturated and acidic conditions where feldspar dissolution kinetics are dominated by H+ adsorption and exchange reactions. Many natural waters, however, are only weakly acidic and have variable ionic strength and composition which would be expected to influence mineral surface properties and mechanisms of organic ligand-promoted reactions.Oxalate and citrate (2–20 mM) increased the rate of quartz dissolution by up to a factor of 2.5. Quartz solubility, however, was not increased appreciably by these OAs, suggesting that Si–OA complexation is not significant under these conditions. The lack of significant OA–SiO2 interaction is important to understanding the effects of OAs on the release of both Si and Al from feldspars. In contrast to quartz, both the rates of dissolution and amounts of Si and Al released from the three feldspars studied increased regularly with increasing OA concentration. Feldspar dissolution was congruent at all but the lowest OA concentrations. Total dissolved Al concentrations increased by 1–2 orders of magnitude in the presence of oxalate and citrate, and reached values as high as 43 mg/l (1.6 mM). Si concentrations reached values up to 65 mg/l (2.3 mM) in feldspar–OA experiments. Precipitation of authigenic clays was observed only in experiments without or at very low concentrations of OAs. The high concentrations of dissolved Si attained during dissolution of feldspars in OA solutions, relative to Si concentrations in quartz–OA experiments, is attributed to concomitant release of Si driven by strong Al–OA interactions.Modeling of the dependence of feldspar dissolution rates on OA concentration in natural diagenetic environments is complicated by the competing effects of overall solution chemistry and ionic strength on the dissolution mechanism. Results of experiments using labradorite (An70) indicate that in OA-free solutions, dissolution is progressively slower at increasing NaCl concentrations (up to 2.2 M), in agreement with prior experiments on the effects of alkali metals on feldspar dissolution. The combined effects of oxalate and NaCl on labradorite dissolution rates are such that the rate increase due to oxalate is suppressed by the addition of NaCl. Thus, feldspar dissolution kinetics should be most significantly affected by a given concentration of OAs in low ionic strength solutions.  相似文献   

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