metamorphism, which is the process by which rocks are altered in form, texture, or composition by heat, pressure, and/or chemical reactions. In the case of the formation of phyllite, the original slate rock was subjected to high temperatures and pressures, which caused the clay minerals within it to recrystallize and form mica. This process fundamentally altered the rock's mineral composition and texture, resulting in the formation of a new type of rock. Metamorphism is a common geological process that can occur in a variety of settings, from deep within the Earth's crust to the surface.