Here's why:

* Slate is a metamorphic rock formed from shale. It has a fine-grained texture and is characterized by its ability to split into thin, flat sheets.

* Phyllite is formed when slate undergoes further metamorphism. The increased heat and pressure cause the clay minerals in slate to recrystallize, creating a more pronounced schistosity (the parallel alignment of minerals). This results in a slightly coarser texture and a more lustrous sheen than slate.

So, the transformation goes like this:

Shale -> Slate -> Phyllite

As metamorphism continues, phyllite can eventually transform into schist, another metamorphic rock with a more pronounced foliation.