Fine-grained metamorphic rock formed by the recrystallization of fine-grained, parent sedimentary rocks, such as mudstones or shales. Phyllite has a marked tendency to split into sheets or slabs; it may have a sheen on its surfaces due to tiny plates of micas. Its grain size is larger than that of slate but smaller than that of schist.
Learn more about phyllite with a free trial on Britannica.com.
The protolith (or parent rock) for a phyllite is a slate. Its constituent platy minerals are larger than those in slate but are not visible with the naked eye. Phyllites are said to have a "phyllitic texture" and are usually classified as having a low grade in the regional metamorphic facies.
Phyllite has a good fissility (a tendency to split into sheets) and will form under low grade metamorphic conditions. Phyllites are usually black or gray. The foliation is commonly crinkled or wavy in appearance.