A NEW KINETIC APPROACH TO MODELING WATER-ROCK INTERACTION: THE ROLE OF NUCLEATION, PRECURSORS, AND OSTWALD RIPENING

Abstract

A new approach to water-rock interaction is developed which replaces the assumption of partial equilibrium with a complete calculation of the rates at which minerals form and dissolve. The evolution of reaction-flow systems towards equilibrium with respect to secondary phases is examined in terms of the important processes which generate and modify reactive surface areas of minerals: heterogeneous nucleation, crystal growth, dissolution, and crystal ripening. The widespread occurrence in surficial environments of metastable solid phases and waters supersaturated with the thermodynamically most stable minerals is attributed to the slow rates of reactive surface area generation of the latter. This is primarily because the high mineral-solution interfacial tensions of the stable minerals make it difficult to nucleate them directly. In many cases, a stable mineral circumvents direct nucleation by using as a template for its own growth a more soluble, metastable precursor. Continued growth will ultimately bring the solution composition towards equilibrium with respect to the stable secondary mineral. The common persistence of the precursor and/or intermediate phases halloysite and allophane in weathering profiles indicates that geologically significant periods of time may be required to achieve equilibrium with kaolinite. The reactive flow model presented explicitly calculates reactive surface areas of secondary minerals using crystal size distributions. The model includes rate expressions for heterogeneous nucleation, growth/ dissolution, and Ostwald ripening along with an expression describing the competition of a stable mineral and its precursor for the available growth sites at the precursor's surface. The model is used to simulate the weathering of a fresh granite by infiltrating rain water. The simulations provide a semi-quantitative picture of how a reaction front develops in time and space.