Abstract Rocks and minerals communicate chemically with their surroundings via reactions at surfaces. The surface of the Earth contains tens of trillions of square kilometers of mineral surface area that act as key chemical agents in geochemical processes at and near the Earth’s surface. Mineral surfaces influence the regional and global cycling of elements through partitioning, dissolution-precipitation, and catalytic reactions that occur in ground-, sea-, surface-, and atmospheric waters. For example, chemical weathering of silicate minerals, via surface-controlled dissolution reactions, is thought to be one mechanism for draw-down of atmospheric CO 2 (Berner et al., 1983). Mountain-building episodes have been implicated in global climate control (Molnar and England, 1990; Raymo and Ruddiman, 1992; Berner, 1994). Global climate may be in part controlled by the rate of production of mineral surface area available for weathering. The study of reactions at mineral surfaces under a range of conditions is thus important for understanding processes from the local to the global scale. Understanding the behavior and influence of surfaces in natural processes is necessary in order to model natural systems at all scales and to predict their responses to perturbations. The structure and reactivity of mineral surfaces have therefore become an important and growing research subject. Because much or most of the surface area in geologic systems is associated with the clay minerals and clay-sized particles, these particles are often important in geochemical processes that depend on the catalytic, adsorbent, or other reactive behavior of surfaces. These minerals, and models/proxies for them and their surfaces