Binary (two-component) equilibrium diagrams that show melting relationships for other mineral systems share many features with the Di-An diagram. The figure below shows the Leucite-Silica diagram, a simplified granite system. This diagram summarizes experimental data by Schairer and Bowen (1938, 1955). To view an interactive version of the diagram, click on it.
Figure 4.03. KAl(Si2O6)-SiO2 (Le-Silica) equilibrium phase diagram. Click on the diagram to see a larger, interactive version for more information.
The Leucite-Silica diagram has four liquidus curves, each giving conditions for saturation with one of the four minerals: leucite (Lct), sanidine (Sa), tridymite (Trd), or cristobalite (Crs). The sanidine and tridymite liquidus curves meet at 990°C defining a eutectic point. The eutectic point gives the composition (54.4 weight % SiO2) of the lowest temperature liquid possible in this system. This is also the composition of the first liquid to appear upon heating a mixture of sanidine and tridymite in any proportion.
Figure 4.03 shows that leucite, by itself, melts to a liquid of the same composition at 1686°C, just as ice by itself melts to pure water at 0°C. You can follow leucite melting by itself on the Lct-Silica diagram by clicking on the "Show Phase %" button, putting the cursor on the leucite composition (0 % SiO2), and moving the cursor up temperature. Because both the melting crystal and the liquid formed are the same composition, this is called congruent melting. Cristobalite (Figure 4.03), diopside and anorthite (Figure 4.01) also melt congruently. Sanidine, however, does not melt congruently. Follow sanidine melting on the Lct-Silica diagram by clicking on the "Show Phase %" button, putting the cursor on the sanidine composition (21.6% SiO2), and moving the cursor up temperature.
Yes. Sanidine reacts to form the assemblage Lct + Liquid. Because liquid is one of the products of the reaction, it is a melting reaction. No. Sanidine reacts to form the assemblage Lct + Liquid. Because liquid is one of the products of the reaction, it is a melting reaction.
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Yes. Sandine reacts at 1150°C to form Lct and a liquid with 42.2% SiO2. Because the liquid that forms is not the same chemical composition as the sanidine (21.6% SiO2), sanidine is said to melt incongruently. No. Sandine reacts at 1150°C to form Lct and a liquid with 42.2% SiO2. Because the liquid that forms is not the same chemical composition as the sanidine (21.6% SiO2, sanidine is said to melt incongruently.
