Figure 4.07. Al2O3-SiO2 diagram.
The diagram on the left shows the temperature-pressure (T-P) stability of of the aluminosilicate minerals andalusite (Al2SiO5), kyanite (Al2SiO5), and sillimanite (Al2SiO5), as was shown in Figure 4.02 for the 1-component system (Al2SiO5). Use the slider on the right to add Al2O3 or SiO2 to this system, making a 2-component system. Changing the bulk composition (shown in the red box) with the slider adds either corundum (Crn) or quartz (Qz) to the stable aluminosilicate mineral as the system changes from a 1-component system to a 2-component system.
Note that adding Crn or Qz does not affect the reactions among Ky, And, and Sil. Although the system has two components, permitting two minerals at a random temperature and pressure, those two minerals cannot be two aluminosilicate minerals.
The composition axis is in oxygen units, moles of oxygen from component SiO2 relative to the total number of moles of oxygen in the bulk composition. Oxygen percent is a good proxy for modal percent because most of the volume of silicate minerals is taken up by oxygen atoms. This diagram is constructed from the thermodynamic model dataset SPaC (version 2015) developed originally by Spear and Cheney (1989).
