Equilibrium and fractional crystallization can be envisioned for 3-component systems on a ternary liquidus diagram. For the CaMgSi2O6-Mg2SiO4-CaAl2Si2O8 system, the interactive tools provided for the large version of the equlibrium diagram shown here as Figure 8.04 make this easy to do.
Figure 04. CaMgSi2O6-Mg2SiO4-CaAl2Si2O8 Diagram. Click on the diagram to view the larger, interactive version.
To track equilibrium crystallization, click on the diagram to open the larger version, then click on the "Show Phase %" button. The cursor should have a black dot beneath it. Move the cursor over the forsterite liquidus surface and click. The black dot should be "dropped" on the liquidus surface, selecting a bulk composition. Click on the "changeT" button and then lower the temperature with the slider. The diagram will change to show isothermal sections that give the equilibrium assemblages for the slider-selected T. Furthermore, the "Phase Proportions" label will show the specific phase assemblage for the chosen bulk composition (the black dot), including the weight percent proportions of the equlibrium phases. For all bulk compositions, except those on the edges of the diagram, the last drop of liquid will crystallize at 1270°C at the ternary eutectic.
Fractional Crystallization
For perfect fractional crystallization, all the crystals are removed as they are formed. Therefore, the bulk composition will change as the liquid crystallizes and it will always be the composition of the liquid. Repeat the steps with Figure 04 that were used to follow equilibrium crystallization. This time, however, pay attenion to the changes in liquid composition and answer the following questions.
Press "Enter" after you make a choice.
Yes. The liquid composition moves directly away from the composition of forsterite. When only one mineral is crystallizing, the liquid composition moves directly away from the composition of that mineral. You can see this for diopside and for anorthite as well on the large version of Figure 04 by placing the black dot in the diopside or anorthite field, clicking on the "changeT" button, and then lowering the temperature with the slider.
No. The liquid composition moves directly away from the composition of forsterite. When only one mineral is crystallizing, the liquid composition moves directly away from the composition of that mineral. You can see this for diopside and for anorthite as well on the large version of Figure 8.04 by placing the black dot in the diopside or anorthite field, clicking on the "changeT" button, and then lowering the temperature with the slider.
Press "Enter" after you make a choice.
Yes. The liquid composition moves along the forsterite-anorthite cotectic curve.
No. The liquid composition moves along the forsterite-anorthite cotectic curve.
