INTRODUCTION

So far, we have dealt only with systems and minerals of fixed composition. Many systems in nature, however, are open; that is, material can be added or subtracted. In addition, most of the common rock-forming minerals belong to solid solution series in which compositions can vary widely (Becker and Prieto, 2006). Magmas and intergranular solutions in metamorphic rocks are other examples of natural materials of variable composition. Changes in composition bring about changes in energy. Thus, when systems strive for equilibrium, compositional adjustments must be made so as to minimize free energies.

Changes in energy resulting from changes in composition are of two types. First, there is the simple addition of material and the energy that it brings with it. For example, when you add 1 gallon of gasoline to your gas tank, you expect a certain amount of work in return, depending on the efficiency of the automobile. Similarly, a surge of new magma into a cooling body of magma introduces a certain amount of heat, which depends on the temperature, heat capacity, and volume of the added magma.

Material added to a system may react with the initial material and produce additional energy. The amount of energy created in this way depends on the nature of the reaction. Consider, for example, two systems, one consisting of oil and the other of sulfuric acid. To each is added an equal volume of water.