Figure 4.06. Al-O-Si composition diagram. To see the aluminosilicate minerals andalusite (And), kyanite (Ky), and sillimanite (Sil) plotted in terms of atom percent Al, O, and Si, select the appropriate button. Because all three minerals have the same chemical composition, all plot at the same point. Reactions among these minerals that do not involve other minerals follow the thermodynamic rules of a 1-component system. According to the mineralogical phase rule, for randomly chosen pressures and temperatures only one of the three minerals should be stable.

The minerals quartz (Qz) and corundum (Crn) also can be plotted on the Al-O-Si composition diagram. All 5 minerals plot along a single line, meeting the critera for a 2-component system.

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