Figure 7.05. Ternary phase diagram for melting in the system NaAlSi₃O₈-CaMgSi₂O₆-CaAl₂Si₂O₈ at 0.1 MPa pressure. Temperature is in degrees Celcius and composition is in weight percent. Congruent melting temperatures for pure Albite (Ab), pure Diopside (Di), and pure Anorthite (An) are shown. The liquidus (saturation) surfaces for plagioclase (Ab-An solid solution) and diopside are shown by temperature contours. The binary cotectic curve where the two saturation surfaces meet is also shown. Drag the temperature slider on the right to show isothermal sections through the diagram. Once the slider is selected, you may use up and down arrow buttons to have more control over the speed of temperature change.

Click on the "Show Phases" button to see the equilibrium phase assemblages for bulk compositions given by the mouse position as the mouse is moved over the diagram. Click on the diagram to hold a particular bulk composition. To see how the phase assemblage changes for the selected bulk composition (shown by a white circle), click on the "Change T" button and use the temperature slider. To cancel the selected bulk composition, click on the "Animate" button.

With the "Show Phases" button clicked, mouse over the diagram to the composition Ab(20)Di(20)An(60) and click. Then click the "Slide T" button and watch the equilibrium phases and their porportions for this bulk composition change as the temperature is lowered with the slider. At equilibrium, the first mineral to crystallize is plagioclase (with an An(93) composition) at 1423°C. By 1245°C, 57% of the liquid has crystallized and the plagioclase growing has an An(84) composition. At 1244°C, Di begins to crystallize along with Pl. At 1234°C, the last drop of liquid reacts to form Di and Pl. If equilibrium prevails, the resulting rock will contain 20 wt.% Di and 80 wt.% Pl of composition 75 wt.% An and 25 wt.% Ab.

This diagram is based on data from laboratory experiments by Bowen (1915) and by Kushiro (1973).     Return to Previous View