Reaction Mechanisms and Solid-Gas Phase Reactions: Theory and Density Functional Theory Simulations

Geochemistry was dominated by thermodynamic theory for much of its first century of existence (Anderson and Crerar 1993; Nordstrum 2006). Interest in high temperature igneous and metamorphic processes combined with the presumption of long time periods that are available for geochemical reactions justified the assumption of equilibrium thermodynamics. Consequently, geochemists were interested in reactants and products, i.e., examining A, B, and C in reactions like A + B → C without concern for what happens in between as represented by the arrow. In fact, for many geochemical reactions, the reactants and products can be treated as components without direct knowledge of the actual molecular-level structure. For example, in thermodynamic treatments of solutions H4SiO4(aq) is represented as SiO2(aq), even though O=Si=O molecules do not exist in solution. It is now clear that even many higherature processes do not reach equilibrium (Rubie and Brearly 1987; Han et al. 2015), so understanding the kinetics of geochemical processes and all the species involved has been a growing focus since the 1980s (Stumm et al. 1987). A comparison of the two approaches is outlined in Figure 1. © 2018 Mineralogical Society of America.