Phyllite was originally a fine-grained sedimentary rock such as shale or mudstone, composed mainly of clay minerals in a semi-random orientation. The rock was then buried and subjected to enough directed pressure to move the clay mineral grains towards parallel alignment, and enough heat and chemical activity to begin transforming the clay mineral grains into chlorite or a mica mineral. At that point, it was the metamorphic rock known as slate.

Continued heat and chemical activity completed the clay-to-mica transformation and caused the mica grains to enlarge. Additional directed pressure brought the mica grains into a strong parallel alignment. The result is the rock known as “phyllite”.