How do changes in temperature and pressure form metamorphic minerals?

Metamorphic minerals form through the process of recrystallization, where existing rocks undergo chemical and structural changes in response to specific pressure and temperature conditions. These conditions dictate which mineral assemblages are stable, as governed by the laws of thermodynamics.12

The influence of these variables generally follows these principles:
* Pressure: Increasing pressure typically favors the formation of denser mineral structures. For example, at high pressures, the mineral calcite may be replaced by its denser polymorph, aragonite.1
* Temperature: Increasing temperature generally favors the development of anhydrous and less dense mineral phases.1

The specific minerals produced are also heavily controlled by the original chemical composition of the rock (the protolith). As rocks are buried or uplifted, changes in temperature and pressure trigger chemical reactions that alter the mineralogy. While prograde metamorphism occurs during burial and heating, retrograde metamorphism occurs during uplift and cooling; however, the latter is often incomplete because volatile components like water are typically removed during the initial heating phase, preventing the reversal of reactions.1324