6.7 More Questions
The (CaSiO3)-(CaAl2Si2O8)-(SiO2) Liquidus Diagram is similar to the Di-Fo-An Liquidus Diagram of Figure 6.06. It has the added complication of the silica polymorphs (tridymite and cristobalite), and there is a field of liquid immiscibility. This diagram has not been animated with a "Show Phase %" button, so you must read the diagram without that extra aid. Press "Enter" after you make your answer selection to the following questions.
(2a) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, what mineral will be the first to crystallize upon cooling from high temperature?
Yes! The bulk composition plots in the Wollastonite (Wo) liquidus region, so Wo will be the first mineral to crystallize on cooling. The field labels identify the four 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 Wollastonite (Wo) liquidus region, so Wo will be the first mineral to crystallize on cooling. The field labels identify the four 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.
(2b) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, what is the second mineral to crystallization on cooling at equilibrium?
Yes! A magma with this bulk composition will crystallize Wo first, then Trd. Because Wo is being removed from the liquid, the liquid composition must move away from Wo along a line that passes through Wo and the bulk composition. That line intersects the Wo-Trd cotectic.
No. A magma with this bulk composition will crystallize Wo first, then Trd. Because Wo is being removed from the liquid, the liquid composition must move away from Wo along a line that passes through Wo and the bulk composition. That line intersects the Wo-Trd cotectic.
(2c) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, at approximately what temperature will tridymite crystallization begin at equilibrium?
Yes! As Wo is crystallizing, the liquid composition must move away from Wo along a line that passes through Wo and the bulk composition. That line intersects the Wo-Trd cotectic at about 1400°C, which is the temperature at which Trd will begin to crystallize along with Wo.
No. As Wo is crystallizing, the liquid composition must move away from Wo along a line that passes through Wo and the bulk composition. That line intersects the Wo-Trd cotectic at about 1400°C, which is the temperature at which Trd will begin to crystallize along with Wo.
(2d) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, what is the approximate proportion of the original magma that has crystallized when the liquid composition reaches the Wo-Trd cotectic?
Yes! About 33% of the magma has crystallized as Wo when the liquid composition reaches the Wo-Trd cotectic. You can use the binary lever rule to calculate the proportions of Wo and Liq from the bulk composition.
No. About 33% of the magma has crystallized as Wo when the liquid composition reaches the Wo-Trd cotectic. You can use the binary lever rule to calculate the proportions of Wo and Liq from the bulk composition. Draw a line from the Wo corner through the bulk composition to the Wo-Trd cotectic. The length of that line from the bulk composition to the Wo-Trd cotectic, divided by the length of the total line from Wo to the Wo-Trd cotectic, will give you the proportion of crystals (all Wo crystals).
(2e) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, 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, 1170°C. The chemical composition of that last drop of liquid will be 32 wt.% Wo, 40 wt.% An, 28 wt.% SiO2, 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, 1170°C. The chemical composition of that last drop of liquid will be 32 wt.% Wo, 40 wt.% An, 28 wt.% SiO2, the ternary eutectic composition.
(2f) For a magma with the bulk composition 74% Wo(CaSiO3), 8% An(CaAl2Si2O8), 18% SiO2, what is the weight percent of tridymite in the rock when the magma has completely crystallized at equilibrium?
Yes! There will be 18 wt.% Trd in the rock. Because the three phases (Wo, An, Trd) have chemical compositions at the corners of the ternary diagram, their proportions in the final rock will be the same as their wt.% proportions in the bulk composition.
No. There will be 18 wt.% Trd in the rock. Because the three phases (Wo, An, Trd) have chemical compositions at the corners of the ternary diagram, their proportions in the final rock will be the same as their wt.% proportions in the bulk composition.
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