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 gradation 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). Phyllite is formed from the continued metamorphism of slate.
The protolith (or parent rock) for a phyllite is a shale or pelite. 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 to gray or light greenish gray in color. The foliation is commonly crinkled or wavy in appearance.
Phyllite is commonly found in the Dalradian metasediments of northwest Arran.
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