We can use the Di-An diagram to see exactly what the equilibrium phase assemblage should be for any bulk composition in the diopside (CaMgSi2O6) - anorthite (CaAl2Si2O8) system at our choice of temperature. If two phases are present, the lever rule will give their proportions. The large version of the Di-An diagram (Figure 4.01) has a button ("Show Phase %") that will display the equilibrium proportions of the phases as the mouse is moved across the diagram. There is also a "Coordinates" button to show the bulk composition and temperature of the mouse position. To see the power of this diagram, let's use it to follow quantitatively the crystallization of a "basaltic" lava.
The figure below shows snapshots of the Di-An diagram at various temperatures for a bulk composition of 60 weight percent CaAl2Si2O8. Click on the radio buttons to see what the equilibrium assemblage will be at several different temperatures for this bulk composition. Zoom in to see details ("Command +" for Mac, "Windows +" for PC). To view an interactive version of the diagram, click on it.
Figure 4.02a. CaMgSi2O6-CaAl2Si2O8 (Di-An) phase diagram. The equilibrium phase assemblage (Liquid only) indicated for the composition (60% An component) and temperature (1500°C) of the orange circle. Click on the diagram to see a larger version.
Upon cooling from high temperature, this bulk composition is liquid at 1500°C, 1450°C, and 1300°C. When the temperature reaches 1389°C, anorthite begins to crystallize. The proportion of anorthite increases and the proportion of liquid decreases as the temperature falls through 1350°C, 1300°C, and 1275°C. The weight percent CaAl2Si2O8 in the liquid decreases as anorthite crystals grow. When the temperature reaches 1274°C, the liquid composition reaches 42% CaAl2Si2O8 and diopside will begin to crystallize along with anorthite. All the liquid is gone when the temperature reaches 1273°C, so 69% of the solidification happens at 1274°C.
To make sure that you have a good understanding of the Di-An diagram, use the enlarged version of Figure 4.01 and the "Coordinates" and "Show phase" buttons (if needed) to answer the following questions.
For the chemical system CaMgSi2O6-CaAl2Si2O8 (Di-An), what phase or phases will coexist at equilibrium for a chemical composition of 20 weight percent An at 1300°C?
Yes. The point for 20 weight percent An and 1300°C plots in the 'Di+Liquid" field. No. The point for 20 weight percent An and 1300°C plots in the 'Di+Liquid" field. To see what phases will be present at equilibrium, plot the composition and temperature on the Di-An Phase Diagram and read the label on the diagram where the values plot.
Yes. The diopside-saturated liquid at 1300°C would have a composition of 35 weight percent An. No. The diopside-saturated liquid at 1300°C would have a composition of 35 weight percent An. You can find this value by using the "Coordinates" button in the enlarged version of the Di-An Phase Diagram. Look for the composition of the ice-saturation curve at 1300°C.
Press "Enter" after you type in the number.
Yes. At equilibrium all mixtures of diopside and anorthite will begin to melt at 1274°C. The magma that forms is the product of a reaction between the diopside and the anorthite. The reaction consumes diopside and anorthite in the proportions 58 grams of diopside to 42 grams of anorthite.
No. At equilibrium all mixtures of diopside and anorthite will begin to melt at 1274°C. To find the answer to this question, use the the enlarged version of the Di-An Phase Diagram. To determine the temperature of first melt, locate the composition of the rock, then follow that composition up in temperature until it enters a field that contains liquid. In this case, magma is the product of a reaction between the diopside and the anorthite. The reaction consumes diopside and anorthite in the proportions 58 grams of diopside to 42 grams of anorthite.
