Carlos M. Pina

Molecular-scale mechanisms of crystal growth in barite

Models of crystal growth have been defined by comparing macroscopic growth kinetics with theoretical predictions for various growth mechanisms,. The classic Burton–Cabrera–Frank (BCF) theory predicts that spiral growth at screw dislocations will dominate near equilibrium. Although this has often been observed,, such growth is sometimes inhibited,, which has been assumed to be due to the presence of impurities. At higher supersaturations, growth is commonly modelled by two-dimensional nucleation on the pre-existing surface according to the ‘birth and spread’ model. In general, the morphology of a growing crystal is determined by the rate of growth of different crystallographic faces, and periodic-bond-chain (PBC) theory, relates this morphology to the existence of chains of strongly bonded ions in the structure. Here we report tests of such models for the growth of barite crystals, using a combination of in situ observations of growth mechanisms at molecular resolution with the atomic force microscope, and computer simulations of the surface attachment of growth units. We observe strongly anisotropic growth of two-dimensional nuclei with morphologies controlled by the underlying crystal structure, as well as structure-induced self-inhibition of spiral growth. Our results reveal the limitations of both the BCF and PBC theories in providing a general description of crystal growth.

The composition of solid solutions crystallising from aqueous solutions: the influence of supersaturation and growth mechanisms

Abstract In this paper we present a new approach to the problem of the crystallisation in solid solution–aqueous solution (SS–AS) systems, in which two main controlling factors have been considered: (i) the supersaturation state of the multicomponent solution in contact with the growing crystal and (ii) the growth mechanisms, operating at a molecular scale on the various faces of the crystal. Supersaturation has been evaluated as a function of the solid solution (the β function) and the transitional supersaturation between spiral growth and two dimensional nucleation mechanisms has been considered as a linear function of the solid composition (the β * line). By superimposing β functions and β * line on a supersaturation–solid composition diagram, we can define compositional regions growing according to different growth mechanisms. In order to test our model, a number of in situ Atomic Force Microscope (AFM) experiments have been conducted in the Ba 2+ –Sr 2+ –SO 4 2− –H 2 O system, using barite (001) as the substrate. The general growth behaviour observed is consistent with the predictions given for a number of initial aqueous solution compositions. Microprobe analysis shows that the new (001) layers grown under conditions where the maximum supersaturation corresponded to intermediate compositions of the (Ba,Sr)SO 4 solid are very Sr-rich. A qualitative explanation for such a compositional shift is given on the basis of nucleation rate calculations in the Ba 2+ –Sr 2+ –SO 4 2− –H 2 O system. Finally we discuss the effect of the substrate on the formation and distribution of two-dimensional (Ba,Sr)SO 4 nuclei on a barite (001) surface.

Molecular-scale surface processes during the growth of calcite in the presence of manganese

This paper deals with the growth behaviour of the Mn-Ca-CO3-H2O solid solution-aqueous solution system on calcite {104} surfaces. This system represents a model example, which allows us to study the effect of a number of controlling factors on the crystallisation: (1) the supersaturation function, β(x), and nucleation rate function, J(x), for the Mn-Ca-CO3-H2O system, (2) the relationship of such functions to the molecular scale growth mechanisms operating on growing surfaces, and (3) the surface structure of the calcite {104} faces. In situ atomic force microscopy (AFM) growth experiments revealed a wide variety of surface phenomena, such as the transition between growth mechanisms, anisotropic changes in the step rates, and the influence of the Mn-bearing newly formed surface on subsequent growth (step stoppage followed by the formation of two-dimensional nuclei and the reproduction of the original calcite {104} surface microtopography). These phenomena result from the interplay between the controlling parameters and are explained in those terms.

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.

Microtopography of the barite (0 0 1) face during growth:: AFM observations and PBC theory

Under moderate supersaturation conditions, crystal growth on the barite (0 0 1) surface takes place by the development of two-dimensional nucleation simultaneously with the advancement of molecular-height cleavage steps on the surface. The most frequent growth steps have a height of a half-unit cell, as has been predicted by periodic bond chain (PBC) theory, and they are parallel to the S1 2 0T PBC directions. Along opposite directions the velocity of S1 2 0T growth steps is strongly anisotropic. Moreover, the directions of fast growth alternate for successive elementary growth layers. The anisotropy of the growth rates can be explained by taking into account the crystallographic features and orientation of the complete PBC within each (0 0 2) elementary growth slice. On the other hand, the alternation of the fast growth direction for S1 2 0T steps in successive d002 growth layers is related to the existence of a 21 screw axis perpendicular to the (0 0 1) surface. Two-dimensional nucleation on the barite (0 0 1) surface is characterized by the development of islands with a circular sector shape and half-unit cell in height. The two-dimensional islands nucleated on the initial surface show the same orientation. As growth proceeds, islands coalesce and a homogeneous layer with a thickness of 3.5 A is formed. Nucleation on this new surface produces islands oriented in the opposite sense to those in the previous layer. Goniometric measurements and X-ray di¤raction experiments conÞrm that the straight edges of the islands are parallel to the [1 2 0] and [1 2 0] crystallographically equivalent directions. The third side of each island is curved, rough and tangent to [010]. Both the morphology and development of two-dimensional nuclei on the barite (001) face clearly indicate that the growth process is structurally controlled. The asymmetry of [120], [120] and [010] PBCs and their crystallographic features can be considered as responsible for the geometry and spread of the circular sector islands formed on each elementary (002) growth layer.

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.

The kinetics of nucleation of solid solutions from aqueous solutions: A new model for calculating non-equilibrium distribution coefficients

The nucleation kinetics of binary solid solutions, with general formula BxC1−xA, crystallising from aqueous solution can be described using a generalised expression for the nucleation rate: the function, J(x), in which supersaturation, interfacial free energy and other parameters of the classical nucleation rate equation are considered as functions of the solid composition. As an example, we studied the behaviour of such J(x) functions for the case of the (Ba,Sr)SO4 and (Ba,Sr)CO3 solid solutions. J(x) functions are very sensitive to slight changes in the composition of the aqueous solution, which result in strong modifications of the nucleation kinetics. The implications of the relationship between supersaturation and nucleation rate functions for the general nucleation behaviour in solid solution-aqueous solution (SS-AS) systems are discussed. Finally, we present a method for constructing non-equilibrium Roozeboom diagrams based on the nucleation kinetics in SS-AS systems. Our Roozeboom diagrams calculated for different departures from equilibrium conditions are consistent with previous experimental work and they can be used to predict actual distribution coefficients.

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.

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.