Phyllite

Phyllite is a fine-grained metamorphic rock whose aligned microscopic mica minerals give it a silky sheen and wavy foliation. It forms at a higher metamorphic grade than slate but lower than schist.

Mica: Mica is a group of sheet silicate minerals that cleave into thin, flexible sheets. Fine mica grains aligned in phyllite produce its characteristic sheen and foliation.
Regional metamorphism: Regional metamorphism alters large rock volumes through elevated pressure and temperature, commonly during mountain building. Phyllite commonly develops where burial and tectonic compression transform shale-rich rocks.
Slate: Slate is a very fine-grained foliated metamorphic rock that splits along flat slaty cleavage. Phyllite has larger mica grains, a silky sheen, and commonly wavier surfaces than slate.
Dimension stone: Dimension stone is natural rock quarried and cut to specified shapes or sizes for construction and decoration. Phyllite’s texture and cleavage can make it suitable for selected facing and decorative stone uses.
Foliation: Foliation is planar fabric in metamorphic rock, produced by aligned minerals or compositional layering. Phyllite’s wavy cleavage is a foliation shaped by directed pressure.
Pressure solution: Pressure solution dissolves minerals at highly stressed grain contacts and redeposits material in lower-stress spaces. It can help flatten grains and strengthen the planar fabric in deforming phyllite.
Schist: Schist is a medium- to coarse-grained foliated metamorphic rock with visible aligned platy minerals. Schist records a higher grade and coarser mica growth than phyllite.
Roofing slate: Roofing slate is slate split into thin, durable tiles for covering roofs. Phyllite may split into sheets, but its durability and weathering vary and are less predictable.
Metamorphic grade: Metamorphic grade describes the intensity of heat and pressure experienced by a rock. Grade places phyllite between slate and schist in a common progression.
Recrystallization (geology): Recrystallization is the growth or reorganization of mineral grains in response to changing physical conditions. Fine mica and other minerals recrystallize as phyllite develops from slate-like material.

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