Thematic Articles

New approaches are allowing computer simulations to be compared quantitatively with experimental results, and they are also raising new questions about reactivity at mineral–water interfaces. Molecular simulations not only help us to understand experimental observations, they can also be used to test hypotheses about the properties of geochemical systems. These new approaches include rigorous calibration of simulation models against thermodynamic properties and atomic structure. They also encompass rare event theory methods that allow simulation of slow, complex mineral surface reactions. Here, we give an overview of how these techniques have been applied to simulate mineral–water interface structure, growth/ dissolution mechanisms, and cluster formation.

The survival of important pieces of our architectural and sculptural heritage is challenged by irreparable damage due to crystallization of soluble salts. Mineral precipitation is also a problem in many industrial processes, leading to costly scale formation. Most of the mechanisms that control these crystallization reactions can be modified or slowed down by using specific additives. Recent advances in elucidating the mechanisms of mineral nucleation and growth and molecular-level mineral–additive interactions have led to the development of novel treatments for the prevention of mineral scale and salt damage.

Foreign ions can be incorporated into minerals during mineral growth and mineral–water interactions, resulting in solid phases with substitutional impurities in their structure. These “cocrystallization” processes control the mobility of minor elements in the environment and can be exploited as a remediation strategy to remove toxic metals from polluted waters and in the design of engineered barriers for the retention of metals, radionuclides, and other inorganic wastes generated by industry. The effectiveness of such remediation tools relies on thermodynamic and mechanistic factors that operate at different scales in space and time.

Humans have always been fascinated by natural crystals, with their wonderfully perfect shapes, colors, and sizes. The inspiration drawn from their delicate morphologies and sometimes incredible sizes has motivated an enthusiastic pursuit of knowledge to understand the formation of these natural wonders. A promising picture is emerging that is painted by brushes from two schools of thought. One treats crystallization as the successive attachment of individual ions or molecules, and the other as an aggregation of nanosized clusters, either in an ordered fashion or in an initially random arrangement followed by self-reorganization into a crystalline structure. The earlier model, the classical theory, is derived from equilibrium thermodynamics and the atomic structures of crystal surfaces. In contrast, the new, nonclassical model is observation-based, thanks to the advent of highresolution imaging techniques. Together, they represent our current state of knowledge as we work towards unraveling the secrets of crystallization.

Many areas of science and industry require a fundamental understanding of crystal dissolution. Examples are diverse and include chemical weathering, pharmaceutical delivery, and the response of marine carbonates to CO2 increases. Understanding these processes ultimately demands knowledge of reaction dynamics. Techniques allowing high-resolution observations of dissolving crystals have greatly improved our understanding of reaction kinetics at a variety of scales. Atomic force microscopy and vertical scanning interferometry can reveal reaction mechanisms and permit tests of working hypotheses. However, understanding the substantial complexity and heterogeneous distribution of surface reactivity cannot be made by simple observation alone but requires advances in fundamental theory. Model simulations of molecular-scale processes provide the critical link between nanoscale surface observations of crystal dissolution and the phenomenological result at scales of environmental importance.

The reactions that occur at the mineral–water interface are central to all geochemical processes. They affect a wide range of important Earth processes, all of which involve geochemical element cycling. Examples include weathering and soil formation, nutrient availability, biomineralization, acid mine drainage, the fate of contaminants, nuclear waste disposal, and minor element incorporation and partitioning during mineral growth. Each of these processes, and its reaction rates, is ultimately controlled by reactions that occur at mineral surfaces. Through the development of advanced analytical methods, direct observations of mineral reactions at the nanoscale have enabled exciting new possibilities for clarifying the mechanisms governing mineral–fluid reactions.