Thermodynamic Components of Phases and Systems

copyright by Philip A. Candela, 1997.

The thermodynamic components (of a phase or system) represent the minimum set of chemical variables necessary to define the chemical variability of the phase or system. The thermodynamic components must be independently variable (in the phase or system), or the fundamental definition of chemical potential will be violated. That is, MgO is NOT a thermodynamic component of olivine (but Mg2SiO4 is). The presence of olivine in a magma, therefore, does not buffer the activity of MgO in the magma. However, the activity of MgO is buffered, or fixed, if a melt is saturated with an olivine of a given composition, and the activity of SiO2 is fixed (at a given P & T).

Many scientists are under the mistaken impression that a component must somehow "exist" or be capable of existing as a phase; however there is no such requirement.

The chemical variability of a system is defined by the system components e.g., the system methane hydrate (solid), liquid water, and {methane+water vapor} atmosphere has the components H2O and CH4; a system with (Fe,Mg)2SiO4 (olivine), (Fe,Mg)SiO3 (orthopyroxene) and SiO2 (quartz) has three components: any three compositions in the system MgO-SiO2-FeO will do, and we will discuss some of the possibilities below.

The chemical variability of a phase is defined by the phase components. For example, approximately 99% of the compositional variability of the mineral olivine can be described by two components. That is, olivine commonly occurs as a binary phase. The two components represent points in composition space, and any two points define a line. Usually, mineral end members are chosen as the phase components, but any combination of the end members will serve just as well. Rather than choosing the end members Mg2SiO4 and Fe2SiO4, we may choose Mg2SiO4 and ½(Fe2SiO4-Mg2SiO4) to represent olivine in composition space. That latter component results in the quantity FeMg-1