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ssss1 Phase diagram for silica depicting the temperature–pressure stability fields for the major polymorphs and the liquid phase.

Source: Adapted from Wenk and Bulakh (2004). © John Wiley & Sons.

stishovitecoesitetridymitecristobaliteinvariant

In other situations depicted in ssss1, two phases coexist under the conditions marked by phase stability boundary lines rather than points. As a result, the phase rule (P = C + 2 − F) yields 2 = 3 − F, so that F must be 1. For example, under the conditions at point Y (900 °C, 9.2 GPa), both coesite and stishovite can coexist. If the temperature increases the pressure must also increase, and vice versa, in order for the system to remain on the phase stability boundary line where these two phases coexist. There is only one independent variable or 1 degree of freedom. The temperature and pressure cannot be changed independently. In a similar vein, two phases, one solid and one liquid, can coexist anywhere on the melting curve that separates the liquid and a single solid stability fields.