The Mineral-Water Interface

The typical satellite view of Earth’s surface reveals ubiquitous contact between rocks and water (Fig. 3.1). Rocks are composed primarily of minerals, naturally crystallized materials having a periodic structure. The long-range structure of crystals, expressed internally as the periodic lattice, determines their fundamental physical and chemical properties. Water, in addition to supplying the basis for life on Earth, is also its critical solvent. It is the dominant medium through which rocks and minerals “communicate” during chemical precipitation and dissolution reactions. However, at room temperature the mobility of ions via diffusion to and from sites in the solid bulk crystal is extremely limited. Dissolution and precipitation reactions thus usually occur at the mineral-water interface (Fig. 3.2). This interface is the locus of exchange and interaction between the surface atoms of the solid and the overlying aqueous phase. In addition to water molecules, the fluid contains dissolved components: e.g., inorganic salts, hydrogen and hydroxyl ions, gases such as CO₂ and O₂, and organic molecules. These components interact with each other as well as with the mineral surface, yielding a complex distribution of species and functional groups (moieties) that characterize even compositionally “simple” solutions. This fluid-solid interaction alters both the surface layers of the crystal and the boundary layer of the fluid (Fig. 3.3). As used here, the term “boundary layer” applies to that fluid in direct contact with the mineral surface. Although the bulk fluid may be in turbulent motion (e.g., a stirred reactor), intermolecular attractive forces between the mineral surface and the fluid bring the fluid velocity to zero (“no-slip” condition). In classical theory, this constraint reduces advection and turbulent mixing within the boundary layer, whose thickness is a function of the flow characteristics prevailing in the overlying bulk fluid. The demands of reactive fluxes from precipitation or dissolution of the underlying mineral surface must be satisfied by the diffusive flux of components through the boundary layer. Much discussion is often devoted to the question of whether reactions are “controlled” by transport or surface reaction mechanisms (i.e., molecular detachment or attachment); because of the interplay between diffusion and reaction, the more pertinent question is whether the crystal surface is close to thermodynamic equilibrium with the fluid (see e.g., discussion in Lasaga, 1998). It is thus critical to recognize that at this interface neither the crystal nor the fluid is equivalent to its bulk counterpart. This central distinction is the subject of this chapter: the nature of the interfacial contact between a crystalline surface and an aqueous fluid, how this region of the crystal differs in terms of physical and chemical properties and behavior from its surroundings, how these differences are the basic engine for dynamic, scale-dependent interface processes, and how these atomic-scale processes express themselves as macroscopic phenomena.