This diagram shows shale being metamorphosed.
Chlorite and muscovite form at relatively low temperatures. Garnet forms at higher temperatures and pressures. Sillimanite indicates the highest level of temperatures and pressures.
Shale is metamorphosed to slate and then to phyllite. Schist is next to form, then gneiss when high-grade metamorphism is reached. Beyond 800°C, the rock may completely deform by melting.
Another version showing kyanite.
