Whereas slate has a dull surface because its metamorphic minerals are extremely fine grained, phyllite has a sheen from tiny grains of sericitic mica, graphite, chlorite and similar minerals, because with further heat and pressure, the reflective grains grow more abundant and join each other. And whereas slate usually breaks in very flat sheets, phyllite tends to have a corrugated cleavage.
This rock has nearly all of its original sedimentary structure erased, although some of its clay minerals persist. Further metamorphism converts all of the clays into large grains of mica, along with quartz and feldspar. At that point, phyllite becomes schist.