Thematic Articles

Mineral nucleation and growth can produce remarkable structures in nature. Unique examples include the colossal gypsum crystals from Naica (Mexico), the stalactites/stalagmites in Zhijin Cave (China), and the colorful hydrothermal structures of Dallol (Ethiopia). These formations exemplify the beauty and complexity that can emerge from rather simple mineral nucleation and growth processes. Beyond that, they reflect specific conditions, including near-equilibrium states, extreme salinities, or exceptional slow growth rates. As these conditions are nearly impossible to replicate in a laboratory setting, these natural systems offer unique insights into geochemical processes.

Mineral crystallization is central to myriad natural processes from the formation of snowflakes to stalagmites, but the molecular-scale mechanisms are often far more complex than models reflect. Feedbacks between the hydro-, bio-, and geo-spheres drive complex crystallization processes that challenge our ability to observe and quantify them, motivating an expansion of crystallization theories. In this article, we discuss how the driving forces and timescales of nucleation are influenced by factors ranging from simple geometric confinement to distinct interfacial solution structures involving solvent organization, electrical double layers, and surface charging effects. Taken together, these ubiquitous natural phenomena can preserve metastable intermediates, drive precipitation of undersaturated phases, and modulate crystallization in time and space.

Once nucleation is established, mineral growth is the process by which crystals increase in size, either through the addition of individual ions (monomers) or the attachment of more complex species that range from oligomers to nanoparticles. The relative contribution of these two mechanisms, which may occur separately or simultaneously, varies with fluid properties such as supersaturation as well as crystallographic characteristics wherein nonclassical mechanisms involving particle attachment are often more prevalent at early stages of crystallization and classical growth by monomers is dominant at later stages. However, there is no general rule for the type of crystal growth dominating in any given scenario as the interaction of aqueous fluid properties, together with kinetic and thermodynamic factors, will determine the pathway for growth. Ultimately the growth pathway(s) of minerals determines properties such as crystal habit and defect density. The environments where mineralization occurs are as diverse as the materials themselves and require state-of-the-art techniques to probe the details of their formation. Here, we review the current understanding of pathways by which mineral growth occurs in geological, biological, and synthetic processes.

Mineral formation from ions in aqueous solutions begins with complex initial stages, where amorphous and liquid-like precursors play pivotal roles before crystalline growth occurs. Both classical and non-classical nucleation and growth theories, introduced in previous chapters, offer explanations, each with their own strengths and limitations, for the complex intermediate phases observed in experimental research. Analytical techniques play a critical role in detecting and characterizing precursor phases, offering valuable insights into nucleation and growth mechanisms across various temporal and spatial scales. Molecular dynamics and modelling provide in-depth perspectives on these phases, allowing for a closer examination of their nucleation and growth mechanisms at the molecular level, and revealing the intricate processes that govern their behaviour.

Minerals are indispensable components of our daily lives, sought after for their importance in natural and industrial processes, as well as their aesthetic appeal. There have long been established theories on mineral formation mechanisms, but many questions remain unanswered, and evidence suggests that our traditional view of crystallization is too simplistic. In recent decades, there has been a renaissance in this field, with new studies shedding light on the underlying physical processes. This introductory chapter aims to provide readers with a concise overview of the intricate world of mineral crystallization and its relevance in various research fields, including biomineralization, geochemistry, and industrial applications.