Here's why:
* Slate forms from low-grade metamorphism of shale. The intense pressure during metamorphism causes the clay minerals in shale to recrystallize into very fine-grained mica minerals, giving slate its characteristic fine, smooth texture. The crystals are so small that they are often only visible under a microscope.
Other foliated metamorphic rocks, like phyllite, schist, and gneiss, have larger crystals due to higher metamorphic grades.
