4.5 A Three Component System

A T-P stability diagram for a three-component (Mg2SiO4, SiO2, Al2O3) system with six mineral phases (Crn, En, Fo, Ky, Prp, Qz, Spr) is shown in Figure 05. Mineral abbreviations can be found on the Mineral Names and Info page. Equilibrium curves for four chemical reactions among these minerals are shown. Click on Figure 4.05 to see a larger version with interactive features.

As is the case for 2-component systems, the equilibrium mineral assemblage depends not only on the T and P, but also on the bulk composition of the rock. The large version of Figure 4.05 shows the T-P diagram on the left and a Mg2SiO4-SiO2-Al2O3 ternary composition diagram on the right. When you mouseover the T-P diagram, dots showing the compositions of stable minerals will appear on the ternary diagram along with equilibrium tie lines. The tie lines divide the Mg2SiO4-SiO2-Al2O3 diagram into smaller triangles. Each triangle identifies three minerals that may be in equilibrium at the selected T and P. Observe that tie lines and small triangles change when your mouse crosses a reaction curve on the T-P diagram.

Select a particular T and P by clicking on the T-P diagram of Figure 4.05, then mouseover the Mg2SiO4-SiO2-Al2O3 diagram to select a bulk composition. The mouse position determines the bulk composition (weight percentages of the three components), which is shown on the upper right. The mineral assemblage at equilibrium for the bulk composition is displayed in the uppper left as weight percentages of the minerals and shown as colored areas proportional to the mineral weight percentages as determined by the ternary lever rule.

Click to select a bulk composition on the Mg2SiO4-SiO2-Al2O3 ternary diagram. Then click the "Change TP" button and mouseover the T-P diagram to see how the mineral assemblage will change with T and P for your selected bulk composition. If you then click on the "Show MAD" button, the T-P diagram will become a Mineral Assemblage Diagram, showing the range of T-P conditions for each mineral assemblage that can occur at equilibrium for the selected bulk composition. Notice that the mineral assemblages change at reaction curves, but that a particular chemical reaction might not change the mineral assemblage for some bulk compositions.

For this 3-component system three minerals are stable for a randomly selected T and P. Four minerals are stable only along the reaction curves. In moving from two components to three components, the likely number of phases present increased from two to three. The relationship between the number of phases and the number of components is a feature of chemical equilibrium known as "The Phase Rule."

Next Page