6.6 Questions
Important features of ternary liquidus diagrams can be discovered by exploring the interactive tools provided for Figure 6.07, reproduced above. The following questions will prompt you to do some of that exploration. Press "Enter" after you make your answer selection.
(1a) For a magma with the bulk composition 20% Di(CaMgSi2O6), 15% Fo(Mg2SiO4), 65% An(CaAl2Si2O8), what mineral will be the first to crystallize upon cooling from high temperature?
Yes! The bulk composition plots in the Anorthite (An) liquidus region, so An will be the first mineral to crystallize on cooling. The field labels identify the three liquidus surfaces that cover the ternary diagram. Plot a bulk composition on the ternary diagram and the first mineral to crystallize for that composition can be read from the field label.
No. The bulk composition plots in the Anorthite (An) liquidus region, so An will be the first mineral to crystallize on cooling. To see this result: (1) Click on both the "Coordinates" button and on the "Show Phase %" button; (2) mouse over the diagram and click on the diagram when the Bulk Compsition readout shows the Di 20%, Fo 15%, An 65% composition; (3) Click on the "Change T" button and move the slider to change the temperature until a mineral appears in the Phase Proportions readout. The field labels identify the three liquidus surfaces that cover the ternary diagram. Plot a bulk composition on the ternary diagram and the first mineral to crystallize for that composition can be read from the field label.
(1b) For a magma with the bulk composition 20% Di(CaMgSi2O6), 15% Fo(Mg2SiO4), 65% An(CaAl2Si2O8), at approximately what temperature will crystallization begin at equilibrium?
Yes! The liquidus surface has a temperature of 1360°C above the bulk composition of the magma. Therefore, the magma will begin to crystallize when its temperature cools to 1360°C.
No. The liquidus surface has a temperature of 1360°C above the bulk composition of the magma. Therefore, the magma will begin to crystallize when its temperature cools to 1360°C.
(1c) For a magma with the bulk composition 20% Di(CaMgSi2O6), 15% Fo(Mg2SiO4), 65% An(CaAl2Si2O8), what is the second mineral to crystallization on cooling at equilibrium?
Yes! A magma with this bulk composition will crystallize An first, then Fo. Because An is being removed from the liquid, the liquid composition must move away from An along a line that passes through An and the bulk composition.
No. A magma with this bulk composition will crystallize An first, then Fo. Because An is being removed from the liquid, the liquid composition must move away from An along a line that passes through An and the bulk composition. To see this result: (1) Click on both the "Coordinates" button and on the "Show Phase %" button; (2) mouse over the diagram and click on the diagram when the Bulk Compsition readout shows the Di 20%, Fo 15%, An 65% composition; (3) Click on the "Change T" button and move the slider to change the temperature until a mineral appears in the Phase Proportions readout; (4) Keep moving the temperature slider until a second mineral appears in the Phase Proportions readout.
(1d) For a magma with the bulk composition 20% Di(CaMgSi2O6), 15% Fo(Mg2SiO4), 65% An(CaAl2Si2O8), what is the temperature when the last drop of magma crystallizes?
Yes! The last drop of liquid to crystallize for a magma of any initial bulk composition will occur at the ternary eutectic temperature, 1270°C. The chemical composition of that last drop of liquid will be 50 wt.% Di, 8 wt.% Fo, 42 wt.% An, the ternary eutectic composition.
No. The last drop of liquid to crystallize for a magma of any initial bulk composition will occur at the ternary eutectic temperature, 1270°C. The chemical composition of that last drop of liquid will be 50 wt.% Di, 8 wt.% Fo, 42 wt.% An, the ternary eutectic composition.
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