Lurdes Fernández-Díaz

The Role of Magnesium in the Crystallization of Calcite and Aragonite in a Porous Medium

ABSTRACT Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite) as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a porous transport medium (a column of silica gel). Under our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaCO3 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the supersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgCO3 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evol tion is related to the supersaturation and interface roughness during crystal growth.

Fluid supersaturation and crystallization in porous media

The relationship between the supersaturation at the point of crystallization and the rate at which supersaturation increases has been studied from nucleation experiments on barite BaSO 4 , strontianite SrCO 3 , witherite BaCO 3 and gypsum CaSO 4 .2H 2 O. The crystallization experiments have been carried out by the counter-diifusion of cations and anions through a column of porous silica gel transport medium. Nucleation is suppressed in a finely-porous medium resulting in very high values of supersaturation before crystallization from the solution begins. This threshold supersaturation for nucleation depends on the solubility of the salt, the porosity of the medium and the supersaturation rate. Nucleation inhibitors were used to extend the range of supersaturation attainable. In all cases the experimental data fits the general expression: rate of change of supersaturation ∝ (threshold supersaturation) m . These results are compared to previous work from the field of chemical engineering on the relationship between supersaturation, volume and cooling rate in aqueous salt solutions. These experiments have important implications to supersaturation in natural fluids and subsequent crystallization in relation to geological problems including crystallization in low temperature sedimentary environments and fluid-rock ratios in hydrothermal mineral deposits.

Nucleation, growth, and zoning phenomena in crystallizing (Ba,Sr)CO3, Ba(SO4,CrO4), (Ba,Sr)SO4, and (Cd,Ca)CO3 solid solutions from aqueous solutions

Crystals of (Ba,Sr)CO3, Ba(SO4,CrO4), (Ba,Sr)SO4, and (Cd,Ca)CO3 solid solutions were obtained by counterdiffusion of reactants through a column of porous silica hydrogel. For each system, a set of experiments, starting with mother solutions of different concentrations, was carried out. The composition of the solids was analyzed by electron microprobe. The method allows the establishment of the influence of the supersaturation on the distribution of material between solid and aqueous phases. With this aim, equilibrium calculations and effective composition of the nucleating crystals are compared. Two extreme nucleation behaviours have been observed. The solid solutions (Ba,Sr)SO4 and (Cd,Ca) CO3, with endmember solubility products differing by several orders of magnitude, tend to nucleate from aqueous solutions in a bimodal way. In these systems there is a strong preferential partitioning and only a very narrow range of aqueous-phase compositions can coexist in equilibrium with intermediate solid solutions. At high supersaturations, the range of aqueous solutions from which intermediate solid solutions can nucleate expands, but the bimodal effect remains. At the other extreme, the solid solutions with close endmember solubility products, like (Ba,Sr)CO3 and Ba(SO4,CrO4), tend to nucleate in an evenly distributed way. For these solid solutions, a wide range of fluid compositions is in equilibrium with intermediate solid solutions. When nucleation occurs at high supersaturation, the substituting ions incorporate into the nuclei nearly in the same stoichiometric proportion as in the aqueous phase, and the partition coefficients reach values approaching unity. Finally, the growth process has been studied by monitoring the compositional evolution of the crystals. There is a close relationship between the bimodal trend of a solid solution and the appearance of high gradient compositional zoning. The solid solutions (Ba,Sr)SO4 and (Cd,Ca)CO3 develop sharp compositional zoning when the fluid composition passes through a certain range during the growth process. On the contrary, when the solid solutions (Sr,Ba) CO3 and Ba (SO4, CrO4) grow at high supersaturations, the solids have nearly the same stoichiomerry as the aqueous phase, and the crystals tend to be homogeneous or, at most, to undergo a very gradual compositional evolution.

The effect of barium on calcite {101¯4} surfaces during growth

In situ atomic force microscopy (AFM) experiments have provided information about the effect of Ba21 on crystal growth of calcite {101¯4} surfaces. The microtopographic features observed have been interpreted by considering both the structural control that the calcite surfaces exert on the incorporation of divalent cations and the supersaturation state of the solution used. Pinning of the calcite growth steps occurs at low Ba concentrations, suggesting specific sites for Ba incorporation. When the Ba content of the solution is increased the advancement of monomolecular steps is observed. Although [4¯41]1 and [481¯]1 steps advance showing characteristic jagged edges, the parallel steps (i.e., [4¯41]2 and [481¯]2) remain practically immobile. This fact can be explained by considering the nonsymmetrically related distribution of large and small sites along the calcite steps and the easier incorporation of barium on the former. The measured increase in the height of the newly grown steps is also consistent with such preferential incorporation of Ba in certain positions. A further increase in the Ba concentration of the solutions leads to the formation of bidimensional nuclei on the calcite {101¯4} surfaces. The nature of these nuclei is discussed taking into account the supersaturation of the solution with respect to two possible structures that can accommodate Ba: the calcite-type structure and the aragonite-type structure.

Metastability in diffusing-reacting systems

Abstract Nucleation in non-homogeneous diffusing-reacting systems occurs under a changing supersaturation. Under these conditions, the rate of creation of supersaturation determines the value of the supersaturation threshold at the nucleation time, i.e., the actual maximum metastability level. Measurements of metastable zone-widths and their dependence. Finally, this relation is checked for solutions doped with additives and metastability measurements in U-tube gel systems are proposed to compare inhibitor effectiveness.

Metastable phenomena on calcite {101̄4} surfaces growing from Sr2+–Ca2+–CO32− aqueous solutions

In situ atomic force microscopy (AFM) experiments, scanning electron microscopy (SEM) imaging and composition analysis, and X-ray diffraction have provided information about the growth, dissolution and transformation processes promoted by Sr2 + –Ca2 + –CO3 2 aqueous solutions in contact with calcite {101¯4} surfaces. Experiments have shown a wide variety of surface phenomena, such as the influence of the Sr-bearing newly-formed surface on the subsequent growth (template effect), the growth and subsequent dissolution of surfaces and the nucleation of secondary three-dimensional nuclei on calcite surfaces. These phenomena reveal the metastability of the crystallisation system and are a consequence of the interplay between thermodynamics (the relative stability of the two calcite and aragonite structure solid solutions that can be formed), supersaturation of the aqueous solution with respect to the two possible solid solutions, and the crystallographic control of the surfaces on cation incorporation.

Supersaturation functions in binary solid solution–aqueous solution systems

In this paper, we present a brief review of the thermodynamic equilibrium of binary solid solution–aqueous solution (SS-AS) systems and derive an expression hat allows us to evaluate the supersaturation or undersaturation of a given aqueous solution with respect to the whole range of solid compositions: the δ(x) function. Such an expression is based on the two conditions that define the SS-AS thermodynamic equilibrium. The derivation of the new supersaturation function, δ(x), was made by considering in detail the compositional relationships between solid and aqueous phases. To represent the new formulation on Lippmann diagrams, we have defined a new thermodynamic concept: the “actual activity.” In addition, we show how our supersaturation function behaves for both ideal and subregular solid solutions. The behaviour and applicability of both the δ(x) function and a previous supersaturation function, β(x), defined by Prieto et al. (1993), is discussed.

In situ atomic force microscope observations of a dissolution– crystallisation reaction:The phosgenite– cerussite transformation

The dissolution–reprecipitation reaction of phosgenite (Pb2Cl2CO3) to cerussite (PbCO3) has been observed in situ in a fluid cell of an atomic force microscope (AFM). The (001) face of phosgenite, in contact with static carbonated aqueous solutions, rapidly begins to dissolve. AFM observations show that dissolution occurs by generation and spread of square-shaped etch pits with sides parallel to 〈110〉 directions. The dissolution of the 〈110〉 steps is isotropic and the etch pits therefore remain square shaped during the dissolution process, as dictated by the existence of a fourfold axis perpendicular to the phosgenite (001) face. Two types of 〈110〉 etch pits were found: short-lived shallow pits, of one unit cell depth (8.8 A), and deep pits, which rapidly reach depths between 10 and 60 nm. A few minutes after the dissolution begins, only the deep pits remain and subsequent dissolution of the phosgenite (001) surface proceeds by increasing their width and depth. The increase of Pb2+ and CO32− concentration in the aqueous solution as a consequence of the dissolution sharply increases the supersaturation for PbCO3. As a result, after a certain incubation time, cerussite crystals nucleate on the phosgenite (001) surface and a coupled process of dissolution–crystallisation starts. Cerussite crystals, which grow by a spiral-growth mechanism, distort the concentration field around them. As a consequence, phosgenite dissolution is accelerated in the proximity of such growing cerussite individuals and both the formation of new deep etch pits and the development of irregular dissolution fronts are observed. Further phosgenite dissolution leads to an increase of cerussite nucleation and growth rates, in such a way that this dissolution–crystallisation phenomenon can be considered as an autocatalytic process.

Spatial and evolutionary aspects of nucleation in diffusing-reacting systems

Nucleation behaviour in diffusing-reacting systems involves spatial phenomena that are explainable by means of spatial considerations. Particularly, precipitate locations may be justified from the profiles of different physico-chemical parameters at the nucleation time. However, this instantaneous observation is not enough to explain other aspects (nucleation density, metastability level, etc.) that are in relation with the system evolution. Evolutionary variables must be introduced to account for experiments in systems of this kind. In this paper a parameter, the supersaturation rate, is quantified from experimental data of supersaturation evolution. The study is applied to growth of barium and strontium carbonates in a U-tube gel system. Supersaturation rates have an important bearing on the supersaturation level at the nucleation time and consequently on the nucleation density.

The carbonatation of gypsum: Pathways and pseudomorph formation

Abstract In this paper, we present an experimental study of the interaction between gypsum (010) surfaces and aqueous solutions of Na2CO3 with different concentrations. This interaction leads to the carbonatation (i.e., the transformation into carbonate minerals) of gypsum crystals, which under ambient conditions shows the characteristics of a mineral replacement and leads to the formation of pseudomorphs consisting of an aggregate of calcite crystals. Carbonatation progress was monitored by scanning electron microscopy (SEM) and glancing incidence X-ray diffraction (GIXRD). The carbonatation advances from outside to inside the gypsum crystal and occurs through a sequence of reactions, which involves the dissolution of gypsum and the simultaneous crystallization of different polymorphs of CaCO3 [amorphous calcium carbonate (ACC), vaterite, aragonite, and calcite], as well as several solvent-mediated transformations between these polymorphs. The sequence in which CaCO3 phases form is interpreted taking into consideration nucleation kinetics and the qualitative evolution of several chemical parameters in the aqueous solution. The textural characteristics of the transformed regions are described. The degree of faithfulness of the pseudomorphs obtained is related to the kinetics of the carbonatation process, which in turn depends on the initial concentration of carbonate in the aqueous solutions. Finally, changes in the rate at which the transformation front advances are discussed on the basis of both textural and physicochemical considerations.