Читать книгу Earth Materials - John O'Brien - Страница 65

The phase rule (Gibbs 1928) governs the number of phases that can coexist in equilibrium in any system and can be written as:

where

 P represents the number of phases present in a system. Phases are mechanically separable varieties of matter that can be distinguished from other varieties based on their composition, structure and/or state. Phases in igneous systems include minerals of various compositions, and crystal structures, amorphous solids (glass) and fluids such as liquids or gases. All phases are composed of one or more of the components used to define the composition of the system.

 C designates the minimum number of chemical components required to define the phases in the system. These chemical components are usually expressed as proportions of oxides. The most common chemical components in igneous reactions include SiO2, Al2O3, FeO, Fe2O3, MgO, CaO, Na2O, K2O, H2O, and CO2. All phases in the system can be made by combining components in various proportions.

 F refers to the number of degrees of freedom or variance. Variance means the number of independent factors that can vary, such as temperature, pressure, and the composition of each phase, without changing the phases that are in equilibrium with one another. We will use the first phase diagram in the next section to show how the phase rule can be applied to understanding phase diagrams. A discussion of the phase rule and of phase diagrams related to metamorphic processes is presented in Chapter 18.