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However, for any point within a phase stability field (e.g., point Z) only one phase is stable (e.g., low quartz). The phase rule (P = C + 2 − F) yields 1 = 1 + 2 − F, so that F must be 2. This means that the temperature and the pressure can change independently without changing the phase composition of the system. For point Z, the temperature and pressure can increase or decrease in many different ways without changing the phase that is stable, as long as they remain within the stability field. There are two independent variables and 2 degrees of freedom. All points to the right of the melting curve in the liquid field represent the stability conditions for a single phase, liquid silica.

One can also use this diagram to understand the sequence of mineral transformations that might occur as Earth materials rich in silica experience different environmental conditions. From a liquid silica system cooling at a pressure of 0.3 GPa cristobalite will begin to crystallize at ~1650°. As the system continues to cool, it will reach the cristobalite/tridymite phase boundary (~1460 °C), where cristobalite will be transformed into tridymite. Ideally, the system will continue to cool until it reaches the tridymite/high quartz phase boundary. Here it will be transformed into high quartz, then cool through the high quartz field until it reaches the low quartz/high quartz phase boundary, where it will be converted to low quartz and continue to cool. Two phases will coexist only at phase boundaries during phase transformations that take finite amounts of time to complete (ssss1).