So far, we have dealt only with systems and minerals of fixed composition. Most of the common rock-forming minerals, however, belong to solid solution series in which compositions can vary widely. Magmas and intergranular solutions in metamorphic rocks are further examples of natural materials of variable composition. In addition, many natural systems are open; that is, material can be added or subtracted. 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.
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