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1.
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.  相似文献   

2.
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   

3.
Olivine dissolution in basaltic melt   总被引：1，自引：0，他引：1  
The main purpose of this work is to understand and quantify diffusive and convective olivine dissolution in basaltic melt. Crystal dissolution and growth in a magma chamber is often accompanied by the descent or ascent of the crystal in the chamber due to gravity. The motion induces convection that enhances mass transport. Such convective dissolution and growth rates have not been quantified before. MgO diffusivity in the melt (DMgO), MgO concentration of the interface melt (C0) and the effective thickness of the compositional boundary layer (δ) are necessary parameters to model the convective dissolution. Experiments of non-convective olivine dissolution in a basaltic melt were conducted at 1271-1480 °C and 0.47-1.42 GPa in a piston-cylinder apparatus. At specific temperature and pressure conditions, multiple experiments of different durations show that the interface melt reaches near-saturation within 2 min. Therefore, diffusion, not interface reaction, is the rate-controlling step for non-convective olivine dissolution in basaltic melt. The compositional profile length and olivine dissolution distance are proportional to the square root of experimental duration, consistent with diffusive dissolution. DMgO and C0 are obtained from the experimental results. DMgO displays Arrhenian dependence on temperature, but the pressure dependence is small and not resolved. C0 increases with increasing temperature and decreases with increasing pressure. Comparison with literature data shows that DMgO depends strongly on the initial melt composition, while C0 does not. δ is estimated from fluid dynamics. DMgO/δ, which characterizes the kinetic and dynamic aspects of convective crystal dissolution, is parameterized as a function of temperature, pressure, and olivine composition. Convective olivine dissolution rate in basaltic melt can be conveniently calculated from the model results. Application to convective crystal growth and xenolith digestion is discussed.  相似文献   

4.
If water was ever present on Mars, as suggested by geomorphological features, then much of the surface and subsurface may have experienced chemical weathering. Among those materials most readily altered is olivine, which has been identified on the Martian surface with IR spectroscopy and Mossbauer techniques and occurs in Martian meteorites. We use geochemical models of olivine dissolution kinetics to constrain the residence time of olivine on the surface of Mars in the presence of liquid water. From these models, we have calculated maximum dissolution rates and minimum residence times for olivine as a function of temperature, pH, Fe-composition, and particle size. In general, the most favorable conditions for olivine dissolution are fayalite-rich compositions, small particle sizes, high temperatures, and acidic solutions that are far from equilibrium. The least favorable conditions for olivine dissolution are forsterite-rich compositions, large particle sizes, ultra-low temperatures, and a neutral pH solution near equilibrium. By using kinetic models of olivine dissolution to bound dissolution rates and residence times, we can make inferences about the temporal extent of aqueous alteration on the surface of Mars. Under favorable conditions (pH 2, 5 °C, and far from equilibrium) a relatively large 0.1 cm (radius) particle of Fo65 composition can completely dissolve in 370 years. Particles may last 102–104 times longer under less favorable conditions. However, residence times of a few million years or less are small compared to the age of most of the Martian surface. The survival of olivine on the surface of Mars, especially in older terrains, implies that contact with aqueous solutions has been limited and wet periods on Mars have been short-lived.  相似文献   

5.
Equilibrium-kinetic model of water-rock interaction   总被引：1，自引：0，他引：1  
A computer model was developed for chemical interaction in water-rock systems. The model is based on the concept of partial equilibrium [1] and combines the calculation of chemical equilibria in multicomponent systems with accounting for the kinetics of the congruent dissolution of minerals as a function of pH (zeroth order kinetic reactions). The development of the process in time is simulated as a series of sequential partial equilibria, and the bulk chemical composition of the system is calculated at each time step from the chemical composition of aqueous solution at the beginning of the step and masses of minerals dissolved during time Δt. The dissolution rates of individual minerals are calculated at each time step for the given temperature, current pH value, and the degree of solution saturation with respect to minerals. Variations in the surface area of minerals due to precipitation and dissolution are accounted for. Model application is exemplified by the calculation of chemical equilibria in the water-granite system. The model may be useful for understanding the character of low-temperature interactions in water-rock systems under stagnant conditions, in particular, the multistage development of groundwater chemistry, interaction of liquid radioactive waste injected into underground repositories, etc.  相似文献   

6.
The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.  相似文献   

7.
This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe3+ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.  相似文献   

8.
We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (Ea,app) for the process, via mass-loss calculations from image analysis. We obtain a value of Ea,app = 49 ± 2 kJ mol−1, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of Ea,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to Ea,app = 140 kJ mol−1, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of Ea,app represents the slow formation of etch pits into the (0 0 1) surface.  相似文献   

9.
The dolomite problem has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite.We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase.Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.  相似文献   

10.
Dissolution experiments of a tholeiite basalt glass carried out at different pH and T (up to 300°C) using a rotatingdisc apparatus show that, depending on pH and T, dissolution can be controlled by one of the following steps: (1) surface reaction; (2) transport of reactants in solution; and (3) mixed reaction. The activation energies of these different processes were found to be 60, 9 and 15–50 kJ mol−1, respectively. Taking account of these results, it appears likely that surface reactions are not rate limiting for the hydrolysis of most crystalline silicate minerals in hydrothermal and metamorphic processes, and that caution should be exercised when predicting rate of reactions at high temperatures solely on the basis of activation energies measured at low temperatures.

Comparison of experimental and theoretical potentiometric titrations of the basalt glass and its constituent oxides indicates that the adsorption of H+ and OH− ions at the basalt surface is metal cation specific and that the net adsorption can be predicted from the sole knowledge of the acidity constants of the network-forming constituent oxides. We found that in the acidic pH region dissolution is promoted by the adsorption of H+ on al and Fe surface sites while in the basic region, dissolution is promoted by the adsorption of OH− on Si sites. The combination of the two distinct types of surface sites, Al and Fe on the one hand, and Si on the other hand, results in a dissolution rate minimum at a pH-value between the pHzpc of the two groups of oxide components. Linear regressions with a slope n=3.8 are observed both in acid and alkaline solutions in logarithmic plots of the rate of dissolution vs. the surface charge. The value of n, which represents the number of protonation or hydroxylation steps prior to metal detachment, has been found equal to the mean valence of the network-forming metals.

Combining concepts of surface coordination chemistry with transition state theory afforded characterisation of the activated complexes involved in basalt dissolution processes. From the values obtained for the thermodynamic properties of activation for basalt dissolution it is assumed that the activated complexes formed during the H2O-promoted dissolution of the basalt glass are more tightly bonded than those formed during H+- or OH−-promoted dissolution.  相似文献   


11.
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   

12.
Dissolution of celestite (0 0 1) was studied by atomic force microscopy as a function of solution undersaturation. In solutions near saturation with respect to celestite, dissolution of the mineral took place exclusively by removal of ions from existing step edges. The onset of etch pit nucleation was observed at a critical saturation state of . Below this saturation state, dissolution took place both at existing step edges and via the creation of new steps surrounding the etch pits. The dissolution rates of celestite exhibited a non-linear dependence on saturation state. Basic crystal dissolution/growth models were inadequate to describe the non-linearity, but a model that incorporates a critical undersaturation provided an improved fit to data collected at high undersaturation. A simple model for dissolution at low undersaturation also fit the rate data well, but in light of the conditions necessary to produce new step edges, the rate coefficient in this model is poorly constrained due to the effects of sample surface history. Consideration of the process of topographic relaxation, consisting of changes in the surface microtopography (i.e., step density) resulting from changes in solution conditions, led to predicted relaxation times on the order of days for the celestite-water interface.  相似文献   

13.
Halite single crystals in saturated solution were used to study dissolution precipitation creep (DPC) at conditions where plastic deformation is negligible. Specifically, the free unloaded surfaces of these crystals were investigated by a novel Linnik-based phase shift interference microscope. The method allows observations of the crystal surface in-situ and with an axial resolution in the nanometer scale. Transport phenomena in open systems, temperature gradients, and gradients in strain energy density were found to cause morphological changes on the free crystal surface by dissolution/reprecipitation. We did not find evidence for DPC by applying a homogeneous stress field to the crystal as long as plastic deformation was avoided. These findings suggest that deformation of rocks by DPC in situations where dislocation creep is not activated, but is rather promoted by fluid transport through the rock or by episodic changes of extensive parameters affecting solubility than by homogeneous stress alone.Editorial responsibility: J. Hoefs  相似文献   

14.
Fernando Corfu 《Lithos》2000,53(3-4):279-291
Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old 207Pb/206Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young 207Pb/206Pb ages, followed by a 4 1/2 h HF dissolution step in an oven at 190°C that liberates the excessively old Pb. The final residue yields in most cases U–Pb data that are consistent with the results obtained by the normal selection, abrasion, and total dissolution procedure. In these examples, the too-old ages cannot be easily explained by the presence of an inherited core component but are more likely to indicate segregation of Pb in zircon sub-domains during thermal annealing episodes early in its history, as has been proposed by other workers. Aside from shedding light on these particular aspects of zircon U–Pb systematics, the combined results also provide geologically relevant information concerning the regional evolution of the western Superior Province in Ontario. An age of 2718±3 Ma for a gabbroic unit from the Quetico Subprovince shows that this was coeval to 2722–2718 Ma ultramafic to felsic igneous rocks in the adjacent Shebandowan greenstone belt, including a gabbro body dated in this study at 2725+17/−11 Ma. These age relationships suggest that volcanic units of the Shebandowan greenstone belt were tectonically imbricated in younger sedimentary rocks of the Quetico basin during late Archean convergence. The other three samples represent felsic intrusive units from Geraldton in the Wabigoon subprovince. An age of 2699±1 Ma for an Au-mineralized feldspar porphyry dyke and identical ages of 2690±1 Ma for two phases of the syn-tectonic Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt.  相似文献   

15.
The equations of soil freezing are established where the soil is partially water-saturated, i.e., when it contains air. We choose a macroscopic viewpoint, using leveled parameters (averages on a “small” volume surrounding the considered point). We assume that water can exist at a temperature below 0°C. Mechanical energy (kinetic energy and power of inner forces) is neglected with respect to thermal energy; radiation is also neglected. The establishment of the equations is based upon the expression (1) of the mass and energy conservation laws; and (2) of constitutive laws such as Fourier's law, Darcy's law, the curve of capillary pressure in terms of the saturation degree, etc.

We obtain a system of nonlinear partial differential equations with a free surface; the unknowns are the temperature, the saturation degree and the water pressure at each point and at each time; these unknowns are coupled in the differential equations.  相似文献   


16.
周自立 《沉积学报》1988,6(1):13-20
本文应用扫描电子显微镜及偏光显微镜,研究重矿物阶状石榴石表面的显微结构,确定它是自生成因的铁铝石榴石。根据阶状石榴石在各探井剖面中的纵向分布规律及特征变化,可以推断它的形成与埋藏深度或地温关系密切。通过计算确定,阶状石榴石初始形成的温度为92±5℃,可作为成岩矿物地温计。这对评价新探区的生油岩及储集岩均有实际意义。  相似文献   

17.
Dissolution of magnesite in acetic acid solutions was investigated. The influence of various parameters such as reaction temperature, particle size and acid concentration was studied in order to elucidate the kinetics of magnesium carbonate. The leaching rate increased with decreasing particle size and with increasing temperature. Initially, the dissolution in terms of acid concentration increased until a definite concentration and then fell with increasing concentration. A kinetic model was researched to describe the dissolution and to analyse the kinetic data, basically. Dissolution curves were evaluated in order to test shrinking core models for fluid–solid systems. Consequently, it was determined that the dissolution of natural magnesite was controlled by chemical reaction, i.e., 1−(1−x)1/3=kt. The apparent activation energy of leaching process was found as 78.40 kJ mol−1.  相似文献   

18.
Pore size is usually thought to control the rate of crystal growth in porous geological media by determining the ratio of mineral surface area to fluid volume. However, theory suggests that in micron-scale to nanometer scale pores, interfacial energy (surface energy) effects can also become important. Interfacial energy typically increases the solubility of very small crystals growing in tiny pores, and when the fluid is close to equilibrium - as is often the case in geological systems - mineral precipitation could occur in relatively large pores, while in very small adjacent pores crystal growth might be suppressed. Such a mechanism would effectively restrict the reactive surface area of the porous medium, thereby reducing the bulk reaction rate. We investigated the pore size distributions in naturally cemented sandstone adjacent to an isolated stylolite and found that quartz precipitation was inhibited in pores smaller than 10 μm in diameter. Furthermore, we demonstrate that kinetic formulations which assume constant solubility cannot reproduce the observed pore size patterns in mineralized samples; by contrast, excellent fits with the data are obtained when interfacial energy effects are taken into account. Reaction rates in geological media determined in field studies can be orders of magnitude lower than those measured in laboratory experiments, and we propose that reduced reaction rates in porous media with micron and submicron-scale porosity could account for much of the apparent paradox.  相似文献   

19.
 借助原位液槽原子力显微镜（in situ AFM）的观察,通过Cd2+,Pb2+替代方解石最外层晶格Ca2+生长模式的实验研究, 探讨了Cd2+与Pb2+作用下方解石表面溶解与结晶行为。在液体反应槽中,分别将含不饱和Cd2+与Pb2+溶液流经方解石{101 _ 4}解理面,结果发现:（1）Cd2+的存在不影响方解石沿<4_41> 晶向台阶的溶解,而Pb2+的存在则强烈阻碍了方解石沿<441>+晶向台阶的溶解;（2）停止输入溶液含Cd2+,Pb2+溶液后,随着方解石表面与溶液达到平衡,溶解过程逐渐转变为结晶过程。结果显示在Cd2+存在时,单分子生长层具有方解石原有的定向性,而在Pb2+存在时的生长则不具任何定向性。尽管有此差异, 但（Ca,Cd）CO3 和（Ca,Pb）CO3 固溶体都受控于单分子层外延生长这一结晶机理。 　　含Cd2+和Pb2+溶液对方解石溶解动力学的作用与选择性吸附的阳离子半径大小、吸附复合体的几何形状及其结晶学取 向有关。Cd2+离子倾向于优先进入更狭小的<4_41>- 晶向的微台阶上,而Pb2+则倾向于形成扭曲的八面体络合物吸附在更开 阔的<4_41>+ 晶向台阶上。因此,Pb2+存在下方解石表面生长方向无序可认为是白铅矿和方解石结构差异的原因。  相似文献   

20.
Barium sulfate is used as a model system to illustrate how solution composition can affect processes of crystal dissolution and growth. Rates and modes of reactions as well as morphological features can be modified by the introduction of simple ionic salts (KCl, NaCl, LiCl, CsCl, NaF, NaNO3), due to the effects of these electrolytes on water structure dynamics and solute hydration. Based on the results of AFM in situ experiments performed at supersaturation (Ω) = 10.6 ± 0.1 and ionic strength (IS) in the range of 0.005-0.1 M we show that growth and dissolution behavior of barite changes under conditions of constant thermodynamic driving force (Ω) and constant IS in a systematic way depending on the specific background electrolyte used to adjust IS. The results are interpreted in terms of the relationships between solution composition, ion properties and the consequent growth and dissolution behavior. Island spreading rate is affected by salt-specific effects on the activation energy barrier of expelling water molecules from solvation shells of barite building units. Dissolution kinetics depends on the balance between the energy expended on breaking solvent structure and the energy gain on hydrating Ba2+ and ions, which are specific for different electrolyte solutions. Nucleation rates are determined by the frequency of water exchange around a barium cation which also depends on solution composition. Relating the structure of the solution to its composition can help to understand phenomena such as growth and dissolution in the presence of organic additives or impurity incorporation.  相似文献   

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