Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid-liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.

Upon slow crystallization from silica-containing solutions or gels at elevated pH, alkaline-earth carbonates spontaneously self-assemble into remarkable nanocrystalline ultrastructures. These so-called silica biomorphs exhibit curved morphologies beyond crystallographic symmetry and ordered textures reminiscent of the hierarchical design found in many biominerals. The formation of these fascinating materials is thought to be driven by a dynamic coupling of the components' speciations in solution, which causes concerted autocatalytic mineralization of silica-stabilized nanocrystals over hours. In the present work, we have studied the precipitation kinetics of this unique system by determining growth rates of individual aggregates using video microscopy, and correlated the results with time-dependent data on the concentration of metal ions and pH acquired online during crystallization. In this manner, insight to the evolution of chemical conditions during growth was gained. It is shown that crystallization proceeds linearly with time and is essentially reaction controlled, which fits well in the proposed morphogenetic scenario, and thus, indirectly supports it. Measurements of the silica concentration in solution, combined with analyses of crystal aggregates isolated at distinct stages of morphogenesis, further demonstrate that the fraction of silica coprecipitated with carbonate during active growth is rather small. We discuss our findings with respect to the role of silica in the formation of biomorphs, and moreover, prove that the external silica skins that occasionally sheath the aggregates--previously supposed to be involved in the growth mechanism--originate from secondary precipitation after growth is already terminated.

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particle-based reaction channels, [ 3 , 4 ] which may involve amorphous [5][6][7][8][9] or liquid precursors. [10][11][12][13][14] Calcium carbonate is an often chosen model system in this context due to its broad importance as an abundant mineral of biological [ 15 ] as well as geological origin, [ 16 ] related implications for the natural CO 2 cycle, [ 17 ] its use in many industrial applications, [ 18 ] and the fact that it is one of the major sources of water hardness and thus responsible for scaling and incrustation. [ 19 ] Recent work revealed that prior to nucleation of solid CaCO 3 , neutral ion clusters are formed as solutes even in undersaturated solutions. [20][21][22] As opposed to classical theories, these so-called prenucleation clusters exist in thermodynamic equilibrium with the dissolved ions [ 20 ] and were fi rst imaged by Pouget et al. with the aid of cryogenic transmission electron microscopy (cryo-TEM). [ 23 ] Further, computer simulations and re-evaluations of experimental data suggested that prenucleation clusters are in fact highly dynamic, liquidand chain-like polymers of calcium carbonate ion pairs. [ 24 ] This structural form rationalizes the thermodynamic stability of the clusters on the basis of strong hydration in combination with a distinct entropic contribution originating from a high degree of disorder. Simulations further indicated that a transition of such polymeric species towards bulk-like structures requires a Colloidal Stabilization of Calcium Carbonate Prenucleation Clusters with SilicaCalcium carbonate precipitation proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which fi nally transform to crystals. Although the existence of stable clusters in solution prior to nucleation has been demonstrated, the molecular mechanisms by which they precipitate are still obscure. Here, direct insight into the processes that drive the transformation of individual clusters into amorphous nanoparticles is provided by progressive colloidal stabilization of different transient states in silica-containing environments. Nucleation of calcium carbonate in the presence of silica can only take place via cluster aggregation at low pH values. At higher pH, prenucleation clusters become colloidally stabilized and cannot aggregate. Nucleation through structural reorganization within the clusters is not observed under these conditions, indicating that this pathway is blocked by kinetic and/or thermodynamic means. The degree of stabilization against nucleation is found to be suffi cient to allow for a dramatic enrichment of solutions with prenucleation clusters and enable their isolation into the dry state. This approach renders direct analyses of the clusters by conventional techniques possible and is thus likely to facilitate deeper insight into the chemistry and structure of these elusive species in the future.clusters with silica can afford relatively concentrated cluster solutions, thus facilitating their characterizati...

Biomineralization can afford crystal frameworks of great diversity and utmost complexity, frequently featuring hierarchical structures and morphologies beyond any crystallographic restrictions. The formation of such architectures is usually directed by organic molecules or matrices, which modify crystallization in a deliberate manner. Their influence often leads to sinuous forms, which, by intuition, suggest the presence of life and distinguish these minerals from their inanimate, mostly euhedral counterparts. However, such a strict distinction does not hold. In fact, smooth curvature and higher-order structuring can occur also in purely inorganic environments: simply by precipitating alkaline earth carbonates in silica-containing media, aggregates of highly oriented carbonate nanocrystals can be obtained that display striking [a] Physical

Organic additives are well known to influence the nucleation and growth of minerals. A combination of experimental and theoretical methods has been used to probe how three simple additives, containing varying numbers of carboxylate groups, influence the early stages of the growth of calcium carbonate. Computationally, the free energy landscape has been examined for each additive binding to Ca 2+ , the calcium carbonate ion pair, the surface of an amorphous calcium carbonate nanoparticle, and the basal plane of calcite. The different influence of the three organic ligands on the early stages of growth of calcium carbonate observed experimentally can be rationalised in terms of the degree of association of each anion with the species present prior to, and immediately after nucleation.

The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalised by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions.