Phyllite

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Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradiation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the surfaces of cleavage (or schistosity).

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 on the regional metamorphic Barrovian sequence.

Phyllite has a 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.