Patricia M. Dove

Kinetics of calcite growth: Surface processes and relationships to macroscopic rate laws

This study links classical crystal growth theory with observations of microscopic surface processes to quantify the dependence of calcite growth on supersaturation, σ, and show relationships to the same dependencies often approximated by affinity based expressions. In situ Atomic Force Microscopy was used to quantify calcite growth rates and observe transitions in growth processes on {104} faces in characterized solutions with variable σ. When σ < 0.8, growth occurs by step flow at surface defects, including screw dislocations. As σ exceeds 0.8, two-dimensional surface nucleation becomes increasingly important. The single sourced, single spirals that are produced at lower σ were examined to measure rates of step flow and the slopes of growth hillocks. These data were used to obtain the surface-normal growth rate, Rm, by the pure spiral mechanism. The dependence of overall growth rate upon dislocation source structure was analyzed using the fundamentals of crystal growth theory. The resulting surface process-based rate expressions for spiral growth show the relationships between Rm and the distribution and structures of dislocation sources. These theoretical relations are upheld by the process-based experimental rate data reported in this study. The analysis further shows that the dependence of growth rate on dislocation source structures is essential for properly representing growth. This is because most growth sources exhibit complex structures with multiple dislocations. The expressions resulting from this analysis were compared to affinity-based rate equations to show where popular affinity-based rate laws hold or break down. Results of this study demonstrate that the widely used second order chemical affinity-based rate laws are physically meaningful only under special conditions. The exponent in affinity-based expressions is dependent upon the supersaturation range used to fit data. An apparent second order dependence is achieved when solution supersaturations are very near equilibrium and growth occurs only by simple, single sourced dislocation spirals. These findings indicate the need to apply caution when deducing growth mechanisms and rate laws from temporal changes in bulk solution chemistry. Observations of various types of surface defects that give rise to step formation suggest that popular ‘rate laws’ are sample-dependent composites of rate contributions from each dislocation structure.

The dissolution kinetics of amorphous silica into sodium chloride solutions: effects of temperature and ionic strength

The kinetics of amorphous silica, SiO2 (am), dissolution was quantified in deionized water and NaCl solutions. By using two sources of pure SiO2 glass (fused purified quartz and pyrolyzed SiCl4), rates were measured at 40°C to 250°C by applying three types of reactor systems to assess kinetic behavior over the full temperature range. Dissolution rates of the two materials are similar within experimental error. Absolute rates of amorphous silica dissolution in deionized water exhibit an experimental activation energy, Ea,xp, of 81.9 ± 3.0 and 76.4 ± 6.6 kJ/mol for the fused quartz and pyrolyzed silica, respectively. These values are similar to estimates for quartz within experimental errors. Absolute dissolution rates of SiO2 (am) in deionized water are ∼10× faster compared to quartz. Amorphous silica dissolution rates are significantly enhanced with the introduction of NaCl to near-neutral pH solutions such that 0.05 molal sodium ion enhances rates by 21× compared to deionized water. The new kinetic data are combined with previous measurements of SiO2(am) dissolution rates in ‘pure’ water to evaluate the temperature dependence of dissolution. The comprehensive data set spans 25°C to 250°C and yields the Arrhenius expression log k+ = 0.82191 − 3892.3/T(K) to give an apparent activation energy for dissolution of 74.5 ± 1.4 kJ/mol. These findings step toward the larger goal of understanding silica polymorph reactivity in the complex fluid compositions of natural systems.

The influence of the alkaline earth cations, magnesium, calcium, and barium on the dissolution kinetics of quartz

This study quantifies the influence of the four major dissolved solutes found in terrestrial and marine environments on the dissolution kinetics of quartz. Dissolution rates are dependent upon the concentration and identity of alkali and alkaline earth cations in near-neutral pH solutions. We determine the effect of alkaline earth cations upon quartz reactivity by measuring dissolution rates in 0.0001–0.2 molal solutions of MgCl2, CaCl2, BaCl2, LiCl, KCI, and NaCl at near-neutral pH and 200°C. The results fit a first order rate law, where dissolution rates are slowest in pure water and increase with the introduction of salts in the order: Mgt+ < Ca2+ ≈ Li+ ≈ Na+ ≈ K+ < Ba2+. The trend is consistent with previously reported lower temperature measurements of dissolution rate for amorphous silica and sources of quartz. This suggests cation-specific effects hold for multiple silica polymorphs over temperatures of 25–200°C. Combining evidence from the literature and the kinetic data presented in this study, we propose a simple model: The dissolution rate of quartz. Alkali and alkaline earth cations indirectly enhance dissolution by] modifying rates of solvent motion, exchange, or orientation at the mineral-solution interface. Rate enhancement is proportional to the concentration of ions at the surface and their solvation character. The resulting model predicts (1) the dissolution rate of quartz for a suite of cations with weak surface interactions, (2) the influence of ion concentration, (3) that the catalyzing effect of salts diminish as the concentration of silicic acid in solution increases (decreasing reaction affinity) by increasing the amount of H4SiO4 at the interface, (4) that electrolytes have little rate-enhancing effect on the dissolution kinetics of silicate minerals, and (5) that dissolution rates of other oxide materials containing constituents with sluggish solvent interactions are also enhanced by introduction of alkali or alkali cations. Our findings reiterate the need to understand relationships between near-surface solute/solvent properties and mineral reactivity.

Reversed calcite morphologies induced by microscopic growth kinetics: Insight into biomineralization

Abstract This experimental investigation of calcite growth quantifies relationships between solution supersaturation and the rates of step advancement. Using in situ fluid cell atomic force microscopy (AFM), we show that the movement of monomolecular steps comprising growth hillocks on {10 1 4} faces during the growth of this anisotropic material is specific to crystallographic direction. By quantifying the sensitivity of step growth kinetics to supersaturation, we can produce spiral hillocks with unique geometries. These forms are caused by a complex dependence of step migration rates, vs+ and vs−, upon small differences in solution chemistry as they grow normal to the conventional fast ([ 4 41]+ and [48 1 ]+) and slow ([ 4 41]− and [48 1 ]−) crystallographic directions. As solute activity, a, decreases, vs+ and vs− converge and growth hillocks express a pseudo-isotropic form. At still lower supersaturations where a approaches its equilibrium value, ae, an inversion in the rates of step advancement produces hillocks with unusual reversed geometries. Comparisons of the kinetic data with classical theoretical models suggest that the observed behavior may be due to minute impurities that impact the kinetics of growth through blocking and incorporation mechanisms. These findings demonstrate the control of crystallographic structure on the local-scale kinetics of growth to stabilize the formation of unusual hillock morphologies at the near-equilibrium conditions found in natural environments.

Surface site-specific interactions of aspartate with calcite during dissolution; implications for biomineralization

Abstract Calcite occurs widely as a mineral component in the exoskeletons and tissues of marine and freshwater invertebrates. Matrix macromolecules involved in regulating the biological growth of calcite in these organisms are known to share a carboxylic-rich character that arises from an abundance of the acidic amino acids aspartate (Asp) and glutamate (Glu). This study determines the interactions of Asp with calcite {101̄4} faces during dissolution using in situ fluid-cell atomic force microscopy (AFM) and macroscopic ex situ optical methods. In control experiments, etch-pit morphologies produced_by dissolution in simple undersaturated solutions reflect the inherent symmetry of the {101̄4} faces with a rhombus form. With the introduction of Asp. surface site reactivities are modified to yield isosceles triangular etch pits and hillocks. With continued exposure to Asp-bearing solutions, these triangular pits coalesce and the surface evolves into a network of interconnected tetrahedral etch hillocks. The component tetrahedral “sides”have Miller-Bravais indices of (0001), (1̄101), and (01̄11). These faces intersect the (101̄4) face in the [01̄0], [451̄], and [4̅11] directions to compose the tlnee edges of the triangular etch pits. Structural and stereochemical contraints suggest that the (1̄101) and (01̄11) faces in the hillock are a combination of corresponding faces from the {1102} and {1̄100} crystallographic forms. Results of this dissolution study are consistent with previous growth experiments showing that Asp causes preferential development of the {0001} and possibly the {1̄100} forms of calcite. These observations support mechanisms proposing that the new forms are stabilized by the molecular recognition of Asp functional groups for specific surface sites. Because Asp stabilizes identical faces during growth and dissolution, we suggest that dissolution studies offer an alternative means of determining the crystal forms that develop during biomineralizing processes and a more direct means of identifying those surface sites involved. We demonstrate that the stability of crystallographic directions expressed by step edges is controlled by the relative reactivities of surface sites. Our findings yield new insights into surface structure controls on mineral reactivity.

Microbially Catalyzed Dissolution of Iron and Aluminum Oxyhydroxide Mineral Surface Coatings

Abstract This experimental study investigated the processes by which microbes interact with oxyhydroxide mineral surface coatings using an approach designed to better represent the conditions of natural subsurface environments. The interactions of Shewanella putrefaciens, a facultative anaerobe capable of dissimilatory iron reduction, with coatings of Fe3+ and Al3+ oxyhydroxides on natural quartz and silica glass surfaces were examined. Using synthetic groundwater solutions having- compositions that simulated a typical aquifer, bacteria were seeded onto mineral surfaces (and coatings) and incubated in parallel with abiotic controls for up to 96 h under aerobic and anaerobic conditions. Microbial-mineral surface interactions were determined using the direct observational technique, Fluid Tapping Mode™ Atomic Force Microscopy (TMAFM) in combination with measurements of ferrous iron concentrations and pH of the incubating solutions. Observations of live bacteria-surface interactions exposed to aerobic conditions showed localized pitting on Fe3+ oxyhydroxide coated quartz surfaces within 72 h of incubation. These pits corresponded directly to sites of bacterial surface adhesion and the extent of pitting was accompanied by the accumulation of ferrous iron to low but steady-state concentrations. Localized pitting was not observed on any Al3+ oxyhydroxide coated surfaces. In contrast, iron coated surfaces exposed to bacteria under anaerobic conditions revealed progressive, nonlocalized Fe loss over 96 h. This correlated with a temporal increase in ferrous iron concentrations in the bacteria-exposed solutions compared to the abiotic controls. Aqueous chemical measurements combined with the Fluid TMAFM observations indicate biologically-catalyzed iron reduction under both aerobic and anaerobic incubation. The pitting mechanism observed under aerobic conditions is proposed to result from a redox reaction at the bacteria-iron interface followed by the reoxidation of Fe2+ onto the surface. The evidence suggests that bacteria under anaerobic conditions maximize rates of dissimilatory reduction by remaining passively mobile on the surface. The weaker bacterial adhesion under anaerobic conditions enhances opportunities for bacteria-iron mineral surface contact. These findings may improve our understanding of relationships between the redox environment and bacterial mobility in the subsurface.

Crystal chemical controls on the dissolution kinetics of the isostructural sulfates: Celestite, anglesite, and barite

The isostructural sulfates, barite (BaSO4), celestite (SrSO4), and anglesite (PbSO4), share the crystallographic space group Pnma. With their crystal structure and bulk stoichiometry invariant, we investigate controls on the kinetics of sulfate mineral dissolution. Dissolution rates were measured in hydrothermal mixed flow reactors with solutions of pH 2 to 10 and at 25–140°C. Data were analyzed using a first order rate law. In general, dissolution rates follow the order celestite > anglesite > barite. Although the chemistry of lead differs from strontium and barium, anglesite has intermediate reactivity. All minerals exhibit a similar pH dependence of dissolution where rates decline with increasing pH for the range 2 to 5 and are approximately independent of pH over the range of 5 to 9. Above pH 9, anglesite dissolution rate increases sharply with increasing pH. An analysis of the temperature dependence of dissolution at near-neutral pH shows that the reactivity trend is controlled by differences in the preexponential component of the rate constant. Thus, reaction frequency, not energetics, primarily determines relative reactivity and suggests steric and/or solvation controls on reaction rate. Dissolution rates at near-neutral pH correlate inversely with both ionic radius and average bond length of the structural divalent atom. This leads us to consider the nature of the aqueous metal complexes that are released to solution upon hydrolysis. We find a positive correlation between solvation number and dissolution rate. This correlation extends to describe reported rates of anhydrite (CaSO4) dissolution. Combining evidence, we suggest that rates of isostructural sulfate mineral dissolution are limited by the relative solvation affinity of the divalent metal atoms for near-surface water. Hydrolysis of the sulfate group is probably not rate-limiting since anionic hydration is extremely fast and approximates rates of aqueous diffusion. Dissolution rates are limited by the mineral component having the lowest solvation affinity. This model predicts dissolution rates of sulfate minerals within a solid-solution compositional series and may describe other minerals with strong anisodesmic character. Our study reiterates the role of near-surface solvent properties in controlling mineral reactivity in aqueous solutions.

Compatible real-time rates of mineral dissolution by Atomic Force Microscopy (AFM)

Abstract The Fluid Cell attachment to the Atomic Force Microscope can be used to emulate batch and flow-through reactors to observe the progression of mineral-water interaction processes on individual surfaces. Recent applications of this method include the in-situ measurement of mineral dissolution or growth rates by comparing time-sequenced images. Because AFM images are collected as lines of information by the physical rastering of a lever over a mineral surface, there are inherent limitations to the range of reaction rates compatible with in-situ AFM methods. This investigation examines the AFM-compatible range of monolayer formation or removal rates on individual mineral surfaces. We estimate that AFM can be used to observe dissolution or growth processes occurring at rates in the range of 10 −10 − 10 −6 mol m −2 s −1 . This calculated estimate of the reaction rate range was compared with estimates of dissolution rates from time-sequenced in-situ observations of dissolution on specific cleavage faces of calcite, barite and celestite. Our AFM observations show a dissolution rate trend which follow the order calcite (at near-zero P CO 2 ) > celestite > barite in deionized water at 30°C. These observations of relative differences in dissolution rates are consistent with studies of bulk dissolution rates using geochemical reactors. Yet, the absolute rates estimated from time-sequenced images are considerably slower than bulk reaction rates in all cases. This is probably because our experiments: (1) use relatively smooth areas which are less reactive than roughened areas with large step, defect or pit densities; and (2) cannot simultaneously observe the reactivity of other, possibly more reactive, mineral surfaces. Our findings demonstrate the sensitivity of Fluid Cell AFM as a probe of small differences in the reactivity of individual steps and surfaces. They also suggest that AFM cannot be used to observe reactions on individual surfaces which have dissolution rates outside of the estimated compatible rate boundaries of 10 −10 to 10 −6 mol m −2 s −1 . Mineral surfaces with reactivities outside of this span of rates may be adjusted into the AFM-compatible range by controlling chemical affinity through saturation state or solution pH. These observations offer guidelines for designing new in-situ AFM studies of mineral-water interactions.

Terrestrial Sequestration of CO2 – An Assessment of Research Needs

Publisher Summary This chapter provides a brief review of major characteristics of reservoir structures and lithologies serving as a guide to reservoir selection for CO 2 disposal. The chapter focuses on existing experience and uncertainties in reservoir characterization and response to CO 2 injection and long-term containment of sequestration sites. Special issues germane to CO 2 disposal arise in the assessment of depleted reservoirs, whose properties are known to have changed during single or repeated pore-pressure drawdown and fluid redistribution. Oil and gas reservoirs and aquifers share some common geometric elements. Generally, both are tabular bodies in which the fluid flow is constrained by upper and lower less-permeable lithologies. Primary aspects of CO 2 sequestration in geologic formations include the geohydrologic characterization, injection behavior, and long-term containment of supercritical CO 2 for storage in aquifers and reservoirs. The efficiency of a CO 2 enhanced oil-recovery flood depends strongly on the equilibrium phase behavior of mixtures of CO 2 with the oil.