2.1 Introduction
Because igneous and metamorphic rock have formed at high temperatures, they commonly consist of mineral assemblages that have approached chemical equilibrium. Petrologists use this aspect of igneous and metamorphic rocks to constrain and understand their formation histories using the results of equilibrium experiments in the laboratory. Much of this website is devoted to explaining and using equilbrium tools such as phase diagrams. However, all igneous and metamorphic rocks are out of equilibrium (metastable) at the temperatures and pressures that permit a geologist to hold and study them. And some rocks contain individual phases, such as glass or sillimanite, that are metastable at low temperatures. Furthemore, many rocks have textures that demonstrate disequilibrum on various scales such as chemically-zoned minerals (e.g. Mn and Ca in Garnet or Fe and Mg in olivine) or reaction bands (e.g. garnet necklaces separating plagioclase and augite). What factors determine whether or not the phases (minerals, magmas, fluids, gases) in a rock will react and achieve chemical equilibrium?
In petrology, the rates of chemical changes (e.g. chemical reactions, chemical diffusion, crystal nucleation and growth) are finite and depend dramatically on temperature. To understand fully the processes that have created and styled igneous and metamorphic rocks, the rates of processes must be considered. Laboratory experiments identify which mineral/phase or assemblage of minerals/phases is stable at a particular temperature and pressure, but kinetic considerations determine the conditions and rates at which the stable assemblage will actually appear. "The rate of any chemical reaction, including the crystallization of igneous rocks, is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium" (Kirkpatrick, 1981, p.321). This chapter is about the kinetics of petrologic processes and some of the things that petrologists and materials scientists study to help them understand rates of chemical changes.
Figure 2.01. Tortoise. Click on the image to see a larger version.
Figure 2.02. "Hare." Click on the image to see a larger version.
