Introduction

What is today called reaction path modeling was introduced to geochemistry by Helgeson (1968). It uses the concept of the progress variable, proposed by deDonder in the 1920s and used extensively by Prigogine and Defay (1965), but, before Helgeson, almost unknown in geochemistry. The mathematical foundations are not difficult, but we omit this aspect here, and try instead to convey a sense of what the modeling achieves and does not achieve. Readers are referred to Anderson and Crerar (1993, §19.3), Helgeson (1979), Helgeson et al. (1970), and Wolery (1992) for discussions of the theory and mathematical formulations of the models. It was introduced in general terms in §2.3.2.

The general aim is to be able to trace what happens during irreversible reactions or processes, such as dissolution or precipitation of minerals, mixing of solutions, or cooling or heating of complex systems. If the reader has absorbed Chapter 3, or any similar material, one will suspect that this cannot be done with thermodynamics, because it can only deal with equilibrium states. Irreversible reactions might begin in a metastable equilibrium state (say a crystal of K-feldspar and a liter of pure water, separated) and end in a stable equilibrium state (the K-feldspar, plus alteration products, such as kaolinite, at equilibrium in a solution containing K, Al, and Si), but in between these states the system must necessarily pass through states of disequilibrium. How can thermodynamics deal with this?