phyllite

phyllite

[fil-ahyt]

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.

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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 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.

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