phyllite

Phyllite is a fine-grained foliated metamorphic rock that forms from slate or mudstone under low to intermediate-grade regional metamorphism. It lies between slate and schist in the metamorphic sequence and commonly derives from shale or mudstone.

The mineral assemblage is dominated by microscopic plate-like grains of muscovite or sericite, with chlorite, quartz, and often trace biotite or feldspar. This microstructure gives a characteristic silky or glassy sheen on freshly broken surfaces, more pronounced than in slate.

Phyllite displays a developed, usually wavering foliation called phyllitic cleavage. The cleavage is fissile and can be detected by a knife test, but it is typically less crisp and more indented than slate’s cleavage. The rock’s grain size is finer than 0.5 mm, finer than most schists, contributing to its smooth, slightly shimmering appearance when weathered.

Formation and occurrence: phyllite forms through regional metamorphism of mudstone or shale at relatively low to medium temperatures and pressures, resulting in recrystallization and realignment of platy minerals. It is common in metamorphic belts associated with convergent plate tectonics.

Uses and distinguishing features: phyllite is less durable and less widely used than schist, though it is sometimes employed as decorative stone or in building applications. It is distinguished from slate by its sheen and waviness of foliation; it is finer-grained and less clearly fissile than slate, yet coarser than some slates.