One Component Systems

The H2O phase stability diagram in Figure 01 is a good example of a system with just one chemical component: H2O. For a one-component system, temperature and pressure alone determine what phase is present at equilibrium. There are other one-component systems that are important to petrology. Temperature-pressure stability diagrams for two of these systems, Al2SiO5 and SiO2, are shown here as Figure 02 and Figure 03. Click on either diagram to see a larger version with more information. Notice that for both Al2SiO5 and SiO2, only one phase is stable at a randomly selected temperature and pressure. Two phases are stable along reaction curves. Three phases are stable only at special "triple points" where three reaction curves meet.

Two Component Systems

When chemical equilibrium is considered between two or more phases that have different chemical compositions, more than one chemical component is needed to describe the equilibrium relations. If all of the phases involved have compositions that can be plotted along a single composition axis, only two components are needed. For example, a rock might consist of the two minerals quartz (SiO₂) and albite (NaAlSi₃O₈). Both of these minerals and the mineral jadeite (NaAlSi₂O₆) can be plotted on an axis defined by NaAlSi₂O₆ and SiO₂.

The equlibrium relations of these three minerals have been studied experimentally (Birch and LeCompte, 1960; Newton and Kennedy, 1968; Bell and Roseboom, 1969). Results of those experiments are shown on a temperature-pressure stability diagram (Figure 04). At elevated temperatures and pressures, albite will react (break down) to make jadeite + quartz by the reaction.

1 (NaAlSi₃O₈)^Ab = 1 (NaAlSi₂O₆)^Jd + 1 (SiO₂)^Qz

Click on Figure 04 to see a larger version where it is shown that the minerals in equilibrium for the two-component NaAlSi₂O₆-SiO₂ system depend not only on the T and P, but also on the bulk composition of the rock. For this two-component system two minerals are stable for a randomly selected T and P, and three minerals are stable along the Ab = Jd + Qz reaction curve. Notice that in moving from one component to two components, the likely number of phases present increased from one to two.

Can you predict the likely number of phases for a three-component system?   

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