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Abstract: The crystallisation behaviour in the SrCl2–Na2SO4–H2O system at room temperature is strongly modified by the presence of dissolved silicon. Homogeneous nucleation experiments show that silicon inhibits the formation of celestite while promoting the precipitation of SrSO4·0.5H2O and an amorphous phase. Interfacial free energies for celestite have been calculated for increasing silicon concentrations from measurements of induction times for homogeneous nucleation. The slight increase in the interfacial free energies confirms that dissolved silicon is an inhibitor of celestite nucleation. In addition, dissolved silicon has striking morphological effects. Celestite grown in the presence of silicon typically shows rounded and peanut-like morphologies formed by numerous disoriented crystals. The anomalous celestite morphologies and the increase in both induction times and interfacial free energies reveal a complex interaction between silicic acids and celestite surfaces. In situ atomic force microscopy (AFM) was used to study in detail the effect of dissolved silicon on the growth of celestite (001) faces. AFM observations show that the presence of silicon increases the growth velocity of a first monolayer on the celestite (001) face. However, once such a first monolayer is formed, no further multilayer growth is observed. Height and friction AFM images show clear differences in contrast between the first monolayer and the celestite (001) substrate, revealing differences in composition and/or structure. High-resolution AFM images of the first monolayer show patterns consistent with the celestite (001) surface lattice, indicating that a limited amount of silicon can be incorporated into the celestite structure. Therefore, both the observed inhibition of nucleation and growth of celestite and the changes in crystal morphologies can be partially related to the formation of self-limiting Si-bearing celestite monolayers. [Copyright &y& Elsevier]

Abstract: In this paper we present high resolution atomic force microscopy (AFM) images of dolomite (104) cleavage surfaces immersed in pure water. These images show a rectangular lattice with surface unit cell dimensions in general agreement with those derived from the dolomite bulk structure. Furthermore, the two-dimensional fast Fourier transform (2D-FFT) plots of the high resolution images exhibit a pattern of periodicities consistent with both the alternate orientation of the carbonate groups and the positions for calcium and magnesium atoms on the dolomite (104) surface. However, the Mg2+ and Ca2+ sublattices could not be resolved. Finally, the images in both the real and the Fourier space do not reveal any clear evidence of reconstruction of the dolomite (104) surfaces. [Copyright &y& Elsevier]

Abstract: In situ atomic force microscopy (AFM) was used to study the molecular-scale reactivity of anhydrite (100), (010), and (001) faces exposed to water and CaSO4 aqueous solutions at room temperature. In pure water, dissolution occurs by step retreat and etch pit nucleation and growth. Both the kinetics of the step retreat and the shape of the etch pits are surface-specific and crystallographically controlled. In CaSO4 aqueous solutions with concentrations ranging from 0.030mol/l to 0.075mol/l, the growth kinetics on anhydrite (100) and (010) faces was studied. Growth is also strongly controlled by crystallographic constraints and occurs exclusively from pre-existing step edges by highly anisotropic spreading of monolayers (∼3.5Å in height). The AFM observations demonstrate that monolayer growth can occur on anhydrite (010) and (100) faces even from slightly supersaturated solutions. In addition, the comparison of the step kinetics on anhydrite faces shows that the mechanisms of step dissolution and growth are essentially the same, with the direction of migration of crystal building units being reversed at the anhydrite saturation point. Moreover, the analysis of both high resolution AFM images and lateral force microscopy (LFM) images confirms that the newly-formed monolayers are anhydrite growing in structural continuity with the original (100) and (010) surfaces. However, the formation of the first monolayers is metastable and two-dimensional nucleation and further multilayer growth of anhydrite are strongly inhibited even at high supersaturations. [Copyright &y& Elsevier]

Abstract: In situ atomic force microscopy (AFM) was used to study the growth behaviour of anglesite (PbSO4) monolayers on the celestite (001) face. Growth was promoted by exposing the celestite cleavage surfaces to aqueous solutions that were supersaturated with respect to anglesite. The solution supersaturation, βang , was varied from 1.05 to 3.09 (where βang = a(Pb2 + )· a(SO4 2−)/Ksp,ang ). In this range of supersaturation, two single anglesite monolayers (∼3.5Å in height each) from pre-existent celestite steps were grown. However, for solution supersaturation βang <1.89±0.06, subsequent multilayer growth is strongly inhibited. AFM observations indicate that the inhibition of a continuous layer-by-layer growth of anglesite on the celestite (001) face is due to the in-plane strain generated by the slight difference between the anglesite and celestite lattice parameters (i.e. the linear misfits are lower than 1.1%). The minimum supersaturation required to overcome the energy barrier for multilayer growth gave an estimate of the in-plane strain energy: 11.4±0.6mJ/m2 . Once this energy barrier is overcome, a multilayer Frank–Van Der Merwe epitaxial growth was observed. [Copyright &y& Elsevier]

Abstract: In this paper, we present in situ atomic force microscopy (AFM) observations of the interaction between celestite (SrSO4) (001) surfaces and Na2CO3 aqueous solutions. The observations indicate that the interaction is characterized by a rapid alteration (carbonatation) and dissolution of the original surface, shortly followed by the formation of a new phase. EDX analyses indicate that the new phase is strontianite (SrCO3). Its crystallization involves the formation and spreading of islands of about 2.75nm in height, which chiefly occurs on the step edges of the dissolving celestite substrate. The thickness of the islands remains almost constant during their spreading, which occurs mainly parallel to the celestite [010] direction. As a result of the progressive coalescence of the islands, a fairly homogeneous epitaxial layer forms on the celestite (001) face. At the initial stages, the formation of islands on the celestite (001) faces enhances dissolution, indicating the existence of a coupling between dissolution and crystallization reactions. Our measurements on series of AFM images provided quantitative information about coupled dissolution-growth rates on a nanoscale. The effect of the coupled reactions on the celestite (001) surface on a microscopic scale was also studied by scanning electron microscopy (SEM). [Copyright &y& Elsevier]

Abstract: The growth of barite {001} and {210} faces from aqueous solutions moderately supersaturated with respect to barite (β barite ≈12 for experiments on {001} surfaces and β barite ≈7 for experiments on {210} surfaces) and bearing different concentrations of carbonate has been studied in situ using an atomic force microscope (AFM). Nanoscopic observations show that, above a certain carbonate concentration threshold in the aqueous solution, the advancement of monolayers (∼3.5Å in height) on barite {001} and {210} surfaces is strongly inhibited. However, inhibition never affects the growth of the first monolayer, whose growth rate increases in the presence of carbonate. In contrast, the second monolayer growth rate decreases as the concentration of carbonate in the solution increases. For high carbonate concentrations in the solution, growth stops after the formation of the first monolayer. While on barite {001} faces, the formation of a second monolayer does not occur for carbonate concentrations higher than 0.2mM, on barite {210} faces the complete inhibition of the second monolayer is observed for carbonate concentrations higher than 0.05mM. Once growth on {001} or {210} faces is completely inhibited, i.e. such surfaces are in the “dead zone”, growth can be recovered by increasing supersaturation. In order to study the recovery behaviour of barite {001} and {210} faces from the “dead zone”, an additional series of AFM experiments have been conducted. In these experiments, carbonate-free aqueous solutions with increasing supersaturations with respect to barite were passed over {001} and {210} surfaces previously “poisoned” with carbonate. Our experimental results show that the recovery of growth on barite {001} faces requires an important increase of the solution supersaturation. In contrast, the recovery of barite {210} surface growth does not require any supersaturation increase, but spontaneously occurs in a few minutes. Our observations of inhibition and growth recovery on barite surfaces at a nano-scale are discussed and compared with the descriptions given by the classical crystal growth inhibition models. [Copyright &y& Elsevier]

Abstract: In situ AFM experiments have been conducted in order to obtain information about kinetics of celestite epitaxial growth on barite. Growth has been promoted by passing aqueous solutions supersaturated with respect to celestite over freshly cleaved barite (001) surfaces. Solution supersaturation, β celestite, was varied from 1 to 45.7 . At supersaturations below 10 neither two-dimensional nucleation neither step advancement are observed on barite (001) surfaces. However, once the two-dimensional nucleation barrier is overcome (β celestite >10), nuclei preferentially form on cleavage steps parallel to [100], [110] and [120] directions and more scarcely on terraces. The subsequent growth of two-dimensional nuclei leads to the development of celestite “islands”. Their morphology is defined by (001) face and {210} and {100} forms and can be explained on the basis of PBC theory. The coalescence of such islands results in the formation of a homogeneous SrSO4 layer. Growth rates along [001] direction have been measured for the whole supersaturation range. The growth rate equation for “Birth and Spread” crystal growth mechanism has been successfully fitted to our experimental data. The fitting process has provided basic growth parameters in a good agreement with theoretical ones. Both the high transitional supersaturation required for two-dimensional nucleation and the high interfacial energy value obtained from the fitting of the “Birth and Spread” equation indicate low affinity of SrSO4 growth units for barite (001) faces. This is consistent with the relative high mismatch between celestite and barite structure. The behaviour of the epitaxial growth described in this work can help to interpret the oscillatory zoning frequently occurring in both natural and synthetic crystals of the Ba x Sr1−x SO4 solid solution. [Copyright &y& Elsevier]

The effect of ‘foreign’ ions on the growth of pure crystals from aqueous solution is an important topic addressed by both crystal growth, mineral and surface sciences. However, many aspects of the role played by those ions remain unclear. Our atomic force microscope (AFM) observations of the propagation of monomolecular steps of crystals in solid solution–aqueous solution (SS–AS) systems show that step velocities are determined by the surface structure of the underlying layer, so that each new growth layer exerts a decisive influence on subsequent layers. Moreover, we show that our observations cannot be completely explained by present crystal growth models based on either the pinning of elementary step motion by impurities or changes in free energy by incorporation of different cations into the lattice. A ‘surface strain relaxation’ model is proposed to explain the experimental observations. Our results, apart from providing an alternative explanation for the development of “dead zones”, can shed light on poorly understood phenomena such as the development of compositional zoning in crystals. [Copyright &y& Elsevier]

Abstract: In situ atomic force microscopy (AFM) observations of the interaction between calcite surfaces and Cd-bearing aqueous solutions have been carried out, by maintaining the solutions static in the AFM fluid cell. The interaction involves the dissolution of the original surface and the simultaneous epitaxial growth of multilayer three-dimensional islands ∼2.75nm in height of Cd-rich members of the Cd x Ca1− x CO3 solid solution. Dissolution occurs by the retreating of steps and the formation and growth of etch pits. Both the etch pits and the multilayer islands are elongated along the direction. Such an unusual elongation direction is interpreted as a kinetic effect controlled by both the structural characteristics of the calcite surface and the structure and elastic properties of the overgrowth. Using aqueous solutions highly concentrated in cadmium, the formation of ∼2.75nm thick islands occurs after the initial growth of a solid solution monolayer ∼0.3nm thick, which finally dissolves as the islands grow. This effect seems to be a result of the coherent strain energy accumulated in the substrate–monolayer interface. Significantly, the dissolution rate of the calcite surface decreases when the concentration of Cd2+ in the aqueous solution increases, suggesting that, together with co-precipitation, Cd adsorption plays an important role in the interaction process. During co-precipitation, the progressive coalescence of the Cd-rich islands results in formation of a nanometric epitaxial layer on the calcite surface. This layer armours the substrate from further dissolution and determines the end of the process at a “partial” pseudo-equilibrium endpoint. [Copyright &y& Elsevier]

Inthis article, we present a study of the epitaxial growth of nitratine(NaNO3) on calcite (CaCO3) from ethanolic solutions.By using sodium nitrate saturated solutions in ethanol we were ableto observe the initial stages of nitratine crystallization on calcite(10.4) cleaved surfaces with atomic force microscopy (AFM). Althoughthe oriented epitaxial growth of nitratine crystallites on calciteis independent from the solvent used (water, ethanol, or mixturesof them), the use of ethanolic solutions saturated with respect tonitratine is preferable for surface imaging in the AFM fluid cell.Additional nanotribology AFM experiments allowed us to measure shearstrengths necessary to remove crystallites of nitratine from calcite(10.4) faces. On the basis of both AFM observations and measured shearstrengths, the mechanism of epitaxial growth of nitratine on calcite(10.4) is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Crystal growth has been promoted in the fluid cell of an Atomic Force Microscope (AFM) by passing Ba–Sr–SO4 aqueous solutions over barite (001) cleavage surfaces. Steps advance in structural continuity with the original barite (001) surfaces and two-dimensional nucleation occurs preferentially on the newly-formed terraces. The terraces are, on average, 7.5% lower than pure barite terraces. Since the ionic radius of Sr2+ is smaller that the ionic radius of Ba2+, the reduction of terrace height is consistent with an extensive incorporation of Sr2+ into the barite structure. Therefore, it can be considered that the newly-formed terraces have compositions corresponding to terms of the Ba x Sr1−x SO4 solid solution. A non-linear dependence of step rate on [SrSO4] concentration in the solution (and therefore on supersaturation) has been found. The growth behaviour has been discussed by considering both the physicochemical properties of the Ba x Sr1−x SO4 solid solution–aqueous solution (SS–AS) system and a kinetic-based step growth model. [Copyright &y& Elsevier]

• Zabuyelite (Li 2 CO 3) overgrowth was promoted on dolomite from aqueous solutions. • Overgrowths on dolomite formed using both supersaturated and undersaturated solutions. • Overgrowth friction coefficient is one order of magnitude lower than dolomite. Cleaved dolomite (CaMg(CO 3) 2) crystals were immersed in supersaturated, saturated, and undersaturated solutions with respect to zabuyelite (Li 2 CO 3), and the growth process was characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). In all cases, overgrowths formed by 2D-nucleation could be clearly distinguished in the lateral force images recorded by AFM. With highly supersaturated solutions, AFM imaging was hindered by the formation of 3D crystal aggregates. When saturated solutions were used, the overgrowths reproduce the topography of the substrates, indicating that the lattice of the overgrown phase undergoes a compressive strain. Furthermore, we performed a nanotribological characterization by sliding a nanotip over both the 2D-overgrowths and dolomite substrates. Thus, friction coefficients, µ, could be easily quantified. As a result, a µ = 0.08 on the 2D-overgrowths was measured, which is one order of magnitude lower than the value of µ = 0.89 on the dolomite substrate. This friction reduction in the sliding of a nanotip on the overgrowth as compared to the friction observed on the dolomite substrate, together with the fact that atomic-scale stick-slip is observed on dolomite but not on the 2D overgrowths, point towards an anomalously weak interaction of Li 2 CO 3 overgrowths with the AFM probe. [ABSTRACT FROM AUTHOR]