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We defined geochemistry as the application of chemical knowledge and techniques to solve geologic problems. It is appropriate, then, to begin our study of geochemistry with a review of physical chemistry. Our initial focus will be on thermodynamics. Strictly defined, thermodynamics is the study of energy and its transformations. Chemical reactions and changes of states of matter inevitably involve energy changes. By using thermodynamics to follow the energy, we will find that we can predict the outcome of chemical reactions, and hence the state of matter in the Earth. In principle, at least, we can use thermodynamics to predict at what temperature a rock will melt and the composition of that melt, and we can predict the sequence of minerals that will crystallize to form an igneous rock from the melt. We can predict the new minerals that will form when that igneous rock undergoes metamorphism, and we can predict the minerals and the composition of the solution that forms when that metamorphic rock weathers and the nature of minerals that will ultimately precipitate from that solution. Thus, thermodynamics allows us to understand (in the sense that we defined understanding in Chapter 1) a great variety of geologic processes.

Thermodynamics embodies a macroscopic viewpoint, that is, it concerns itself with the properties of a system, such as temperature, volume, and heat capacity, and it does not concern itself with how these properties are reflected in the internal arrangement of atoms. The microscopic viewpoint, which is concerned with transformations on the atomic and subatomic levels, is the realm of statistical mechanics and quantum mechanics. In our treatment, we will focus mainly on the macroscopic (thermodynamic) viewpoint, but we will occasionally consider the microscopic (statistical mechanical) viewpoint when our understanding can be enhanced by doing so. More detailed treatments of geochemical thermodynamics can be found in Anderson and Crerar (1993), Nordstrom and Munoz (1986), and Fletcher (1993).

In principle, thermodynamics is only usefully applied to systems at equilibrium. If an equilibrium system is perturbed, thermodynamics can predict the new equilibrium state, but cannot predict how, how fast, or indeed whether the equilibrium state will be achieved. (The field of irreversible thermodynamics, which we will not treat in this book, attempts to apply thermodynamics to nonequilibrium states. However, we will see in Chapter 5 that thermodynamics, through the principle of detailed balancing and transition state theory, can help us predict reaction rates.)

Kinetics is the study of rates and mechanisms of reaction. Whereas thermodynamics is concerned with the ultimate equilibrium state and not concerned with the pathway to equilibrium, kinetics concerns itself with the pathway to equilibrium. Very often, equilibrium in the Earth is not achieved, or achieved only very slowly, which naturally limits the usefulness of thermodynamics. Kinetics helps us to understand how equilibrium is achieved and why it is occasionally not achieved. Thus, these two fields are closely related, and together form the basis of much of geochemistry. We will treat kinetics in Chapter 5.