phyllitic

Phyllitic is a texture or character describing metamorphic rocks that display a fine, micaceous, foliated appearance with a silky to pearly sheen on cleavage surfaces. It is most closely associated with phyllite, a rock that forms at low- to medium-grade regional metamorphism, typically from shales or mudstones.

Phyllitic rocks show a well-developed foliation, but the cleavage is generally faint or wrinkled compared with slate. The surfaces often have a glossy luster due to very fine flakes of mica, such as muscovite or sericite, oriented in parallel layers. The mineral assemblage is typically quartz, mica, and minor chlorite or other alteration products, with grain sizes finer than those of schist.

Formation and relationship to other rocks: Phyllite develops when shale or mudstone undergoes metamorphism at relatively low to intermediate grades, causing the clay minerals to recrystallize into tiny mica flakes and producing a micaceous, reflective texture. If metamorphism continues and grains coarsen, phyllite can evolve into schist, which exhibits larger mineral grains and more pronounced schistosity. The progression slate → phyllite → schist reflects increasing metamorphic grade.

Occurrence: Phyllitic textures are widespread in regions of regional metamorphism, including ancient orogenic belts and sedimentary basins that experienced tectonic heating and pressure. The term “phyllitic” is descriptive, used to characterize texture rather than to denote a strict, standalone rock category.

See also: phyllite, slate, schist.