Figure 01. Aluminosilicate stability diagram.
This diagram shows the stabilities of the three aluminosilicate polymorphs kyanite (Ky), sillimanite (Sil), and andalusite (And). The diagram and data are derived from the thermodynamic model dataset SPaC (version 2015) developed originally by Spear and Cheney (1989), which uses the Pattison (1992) triple point. As you mouseover the P-T diagram, Gibbs energies (G) for the each of the phases is shown on the right numerically and with slider positions (in kJ/mol based on formation from the elements at 298.15 K and 1 bar). Gibbs energy values vary more strongly with temperature, than with pressure. Notice that the Gibbs energy differences between the phases are small relative to the sizes of the numbers. The slider scale is fixed with a maximum of -2650 kJ/mol and a minimum of -2750 kJ/mol. To see an expanded scale showing Gibbs energies relative to a fixed position for G(Sil), click the "Fixed G(Sil)" button.
