Variation in calcite dissolution rates:: A fundamental problem?

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.     

The role of background electrolytes on the kinetics and mechanism of calcite dissolution

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.     

In situ atomic force microscopy measurements of biotite basal plane reactivity in the presence of oxalic acid

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 (E_a,app) for the process, via mass-loss calculations from image analysis. We obtain a value of E_a,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 E_a,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 E_a,app = 140 kJ mol⁻¹, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of E_a,app represents the slow formation of etch pits into the (0 0 1) surface.     

An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions

In situ Atomic Force Microscopy, AFM, experiments have been carried out using calcite cleavage surfaces in contact with solutions of MgSO₄, MgCl₂, Na₂SO₄ and NaCl in order to attempt to understand the role of Mg²⁺ during calcite dissolution. Although previous work has indicated that magnesium inhibits calcite dissolution, quantitative AFM analyses show that despite the fact that Mg²⁺ inhibits etch pit spreading, it increases the density and depth of etch pits nucleated on calcite surfaces and, subsequently, the overall dissolution rates: i.e., from 10^−11.75 mol cm⁻² s⁻¹ (in deionized water) up to 10^−10.54 mol cm⁻² s⁻¹ (in 2.8 M MgSO₄). Such an effect is concentration-dependent and it is most evident in concentrated solutions ([Mg²⁺] >> 50 mM). These results show that common soluble salts (especially Mg sulfates) may play a critical role in the chemical weathering of carbonate rocks in nature as well as in the decay of carbonate stone in buildings and statuary.     

In situ Atomic Force Microscopy study of dissolution of the barite (0 0 1) surface in water at 30 °C

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.     

Defect distribution and dissolution morphologies on low-index surfaces of α-quartz

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.     

Interferometric study of the dolomite dissolution: a new conceptual model for mineral dissolution

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.     

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

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⁺).     

Calcite dissolution: Effects of trace cations naturally present in Iceland spar calcites

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.     

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg²⁺ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω_calcite<0.2, whereas a sudden inhibition from Mg²⁺ was activated at Ω_calcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg²⁺.     

Dissolution kinetics of calcite at 50-70 °C: An atomic force microscopic study under near-equilibrium conditions

Direct measurements of calcite faces were performed using in situ atomic force microscopy (AFM) to reveal the dissolution processes as a function of solution saturation state and temperature. Time-sequential AFM images demonstrated that step velocities at constant temperature increased with increasing undersaturation. The anisotropy of obtuse and acute step velocities appeared to become more significant as solutions approached equilibrium and temperature increased. At saturation state Ω > 0.02, a curvilinear boundary was formed at the intersection of two acute steps and the initially rhombohedral etch pit exhibited a nearly triangular shape. This suggests that the and steps may not belong to the calcite-aqueous solution equilibrium system. Further increase in the saturation state (Ω ? 0.3) led to a lack of etch pit formation and dissolution primarily occurred at existing steps, in accordance with Teng (2004). Analysis of step kinetics at different temperatures yielded activation energies of 25 ± 6 kJ/mol and 14 ± 13 kJ/mol for obtuse and acute steps, respectively. The inconsistencies in etch pit morphology, step anisotropy, and step activation energies from the present study with those of studies far-from-equilibrium can be explained by increased influence of the backward reaction, or growth, near-equilibrium. We propose that the backward reaction occurs preferentially at the acute-acute kink sites. The kinetics and effective activation energies of near-equilibrium calcite dissolution presented in this work provide accurate experimental data under likely CO₂ sequestration conditions, and thus are crucial to the development of robust geochemical models that predict the long-term performance of mineral-trapped CO₂.     

Does reactive surface area depend on grain size? Results from pH 3, 25 °C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite

Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 mol_feldspar g⁻¹ s⁻¹. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10⁻¹² mol_biotite m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10⁻¹² mol_biotite g⁻¹ s⁻¹. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.     

Dissolution kinetics and topographic relaxation on celestite (0 0 1) surfaces: The effect of solution saturation state studied using Atomic Force Microscopy

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.     

Influence of pH and temperature on the early stage of mica alteration

Mineral dissolution and precipitation reactions actively participate in controlling fluid chemistry during water–rock interaction. In this study, the changes in the biotite and muscovite basal surface nano-morphology were evaluated during interaction with fluids of different pH (pH = 1.1, 3.3 and 5.7) at different temperatures (T = 25°, 120°, and 200 °C). Results show that at the nanometre scale resolution of the atomic force microscope (AFM), dissolution generates etch pits with a stair-shaped pattern over the (0 0 1) surface. The flux of dissolved elements decreases when pH increases. However, at pH 5.7, a change was found in the flux after 42 h of reaction when abundant gibbsite and kaolinite coat the dissolving mineral surface. This phenomenon was widely observed at edges of the etch pits by AFM. It was also found that an increase in temperature produces an enhancement in the elemental flux in both micas. Dissolution regime changes after less than one day of interaction at high temperature because of abundant coating formation over the etch pits and edges. The results demonstrate the key role of nanometre size neogenic phases in the control of elemental flux from mica surfaces to solution. The formation of nanometre size coatings, blocking the sites active for dissolution, appears to control the alteration of phyllosilicates even at the early stage of the interaction.     

Ferrihydrite dissolution by pyridine-2,6-bis(monothiocarboxylic acid) and hydrolysis products

Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10⁻⁶ mol m⁻² h⁻¹. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H₂S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.     

In situ AFM study on barite (0 0 1) surface dissolution in NaCl solutions at 30 °C

This paper reports in situ observations on barite (0 0 1) surface dissolution behavior in 0.1–0.001 M NaCl solutions at 30 °C using atomic force microscopy (AFM). The step retreating on barite (0 0 1) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (⩾0.01 M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial 〈h k 0〉 one-layer steps (7.2 Å) into two half-layer steps (3.6 Å) occurred in all NaCl solutions, while that of the initial [0 1 0] one-layer steps observed only in the 0.1 M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [0 1 0] direction was bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces in a 0.001 M NaCl solution, and a rhombic form was bounded by the {5 1 0} and (0 0 1) faces in 0.01 M and 0.1 M NaCl solutions. An intermediate form was observed in a 0.005 M NaCl solution, which was defined by {1 0 0}, a curved face tangent to the [0 1 0] direction, {3 1 0}, and (0 0 1) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8 °C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na⁺ and Cl⁻ ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba²⁺ and SO₄²⁻ (through an increase in water temperature).     

Albite dissolution kinetics as a function of distance from equilibrium: Implications for natural feldspar weathering

Determining the kinetics of many geologic and engineering processes involving solid/fluid interactions requires a fundamental understanding of the Gibbs free energy dependency of the system. Currently, significant discrepancies seem to exist between kinetic datasets measured to determine the relationship between dissolution rate and Gibbs free energy. To identify the causes of these discrepancies, we have combined vertical scanning interferometry, atomic force microscopy, and scanning electron microscopy techniques to identify dissolution mechanisms and quantify dissolution rates of albite single crystals over a range of Gibbs free energy (−61.1 < ΔG < −10.2 kJ/mol). During our experiments, both a previously dissolved albite surface exhibiting etch pits and a pristine surface lacking dissolution features were dissolved simultaneously within a hydrothermal, flow-through reactor. Experimental results document an up to 2 orders of magnitude difference in dissolution rate between the differently pretreated surfaces, which are dominated by different dissolution mechanisms. The rate difference, which persists over a range of solution saturation state, indicates that the dissolution mechanisms obey different Gibbs free energy dependencies. We propose that this difference in rates is the direct consequence of a kinetic change in dissolution mechanism with deviation from equilibrium conditions. The existence of this kinetic “switch” indicates that a single, continuous function describing the relationship between dissolution rate and Gibbs free energy may be insufficient. Finally, we discuss some of the potential consequences of our findings on albite’s weathering rates with a particular focus on the sample’s history.     

Fluorite dissolution at acidic pH: In situ AFM and ex situ VSI experiments and Monte Carlo simulations

Dissolution of the fluorite (1 1 1) cleavage surface was investigated by means of in situ atomic force microscopy (AFM) and ex situ vertical scanning interferometry (VSI) experiments at pH range 1-3 in HCl solutions. Surface retreat was quantified at different pH values, yielding dissolution rates that were used to derive an empirical rate law for fluorite dissolution:

     

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Solubility and near-equilibrium dissolution rates of quartz in dilute NaCl solutions at 398-473 K under alkaline conditions

The dissolution-precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH)₃ solutions with pH 8.2-9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol⁻¹ of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol⁻¹ are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q¹ or Q² silicon sites, respectively. The dissolution mechanism varies with ΔG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q¹ sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q¹ reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q² rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q³ sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.