Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO 3 ) is often used as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO 3 nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the socalled nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid-liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO 3 in dilute aqueous solutions. We propose that a dense liquid phase (containing 4-7 H 2 O per CaCO 3 unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO 3 in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca 2+ + z CO 3 2− → z CaCO 3 . The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.calcium carbonate | nucleation | crystal growth | cryo-electron microscopy | molecular simulation I n the process of forming a solid phase from a supersaturated solution, nucleation is the key step governing the timescale of the transition. Controlling nucleation is an essential aspect in many crystallization processes, where distinct crystal polymorphism, size, morphology, and other characteristics are required. It is, therefore, important to obtain a fundamental understanding of nucleation mechanisms.More than 150 years ago, a basic theoretical framework, classical nucleation theory (CNT) (1, 2), was developed to describe such nucleation events. CNT describes the formation of nuclei from the dynamic and stochastic association of monomeric units (e.g., ions, atoms, or molecules) that overcome a free-energy barrier at a critical nucleus size and grow out to a mature bulk phase. Calcium carbonate (CaCO 3 ) is a frequently used model system to study nucleation (3-5); however, despite the many years of effort, there are still phenomena associated with CaCO 3 crystal formation where the applicability of classical nucleation concepts have been questioned (6). These include certain microstructures and habits of biominerals formed by organisms (7), or geological mineral deposits with unusual mineralogical and textural patterns (8).Three anhydrous crystalline polymorphs of CaCO 3 are observed in nature: vaterite, aragonite, and calcite in order of increasing thermodynamic stability. In many cases, the precipitation of CaCO 3 from solution is described as a multistep process, with amorphous phases first pr...

The results of molecular dynamics simulations of hydrated amorphous calcium carbonate (CaCO 3 ·nH 2 O: ACC) are presented. ACC properties were investigated on atomistic, supramolecular, and thermodynamic levels. The clustering of water occluded in the ionic ACC framework was found to be well described by percolation theory, and with a percolation transition for water through ACC at a hydration level, n, of ca. 0.8. Percolation in ACC systems is quantitatively similar to site percolation on a simple cubic lattice where the percolation threshold is observed at p c = 0.312. Predominantly fourfold tetrahedral molecular coordination of water molecules in the bulk liquid state is changed to sixfold connectivity in ACC. Kinetic stability of ACC is enhanced by dehydration and reaches maximal values when the water content is below the percolation threshold. The computed free energy shows a region of thermodynamic stability of hydrated ACC (1 < n < 6) with respect to calcite and pure water. This region is bounded by two crystallohydrates, monohydrocalcite (n = 1) and ikaite (n = 6), that have lower free energies than ACC. During dehydration at n < 1 the thermodynamic stability of ACC decreases, which favors the processes of nucleation and crystallization. On the other hand, water mobility within ACC also decreases during dehydration, thus making dehydration more difficult. So, the stability of hydrated ACC is controlled by a balance of two opposing factors: kinetics and thermodynamics.

Graphite and related sp2 carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of...