Phyllite is a metamorphic rock form intermediate between slate and schist. It consists of platy minerals that are larger than those in slate, but still too small to be clearly discernable to the naked eye. Similar in appearance to slate, it is distinguished from slate by a glossy sheen compared to the dull appearance of slate. It usually exhibits cleavage, but not with the regularity of slate. Its composition is mainly very fine crystals of muscovite or chlorite.
Lutgens and Tarbuck