2.13 Kinetics Summary
  • Although chemical equilibrium concepts and data form the foundation of much of petrology, it is important to recognize that igneous and metamorphic rocks approach chemical equilibrium at finite rates and may not have achieved equilibrium.
  • Familiar examples of disequilibrium include the supercooling of water in classroom demonstrations or superheating of water (by depressurization) leading to geyser eruptions. In these and other examples, slow nucleation of the new phase inhibits the reaction needed to reach equilibrium.
  • Surface energy can be used to understand some aspects of nucleation. Because the surface to volume ratio of very small phases is large, a critcal nucleus size is needed for nucleation leading to sustained growth. In some cases, nucleation of a new phase on the surface of another phase (heterogeneous nucleation) can reduce the nucleus size needed for sustained growth.
  • The rates of chemical reactions depend exponentially on temperature. Temperature is a measure of the movement and kinetic energy of atoms. Atom movements are needed to bring atoms together for chemical reactions and their kinetic energy makes the breaking and forming of bonds possible.
  • The exponential dependence of the rates of chemical processes is conveniently described by Arrhenius equations, which give the logarthim of reaction rates as a linear function of inverse temperature.
  • Atom movements in a phase, such as a mineral or magma, can smooth and may eventually eliminate gradients in chemical composition. These atom movements, called chemical diffusion, may be described and quanitified using diffusion coeffients and Fick's Laws.
  • Laboratory measurements of diffusion coefficients may be used to interpret mineral composition gradients in terms of the time needed to form or eliminate those gradients (diffusion chronometry).