Home » Rock Directory

Phyllite

A green, gray, or red metamorphic rock, similar to slate but often having a wavy surface and a distinctive micaceous luster.

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.

Phyllite has a fissility (a tendency to split into sheets) and will form under low grade metamorphic conditions. Phyllites are usually black or dark gray. The foliation is commonly crinkled or wavy in appearance. Phyllites are said to have a "phyllitic texture" and are usually classified as having a low grade on the Barrovian Sequence.They are regional metamorphic.

Phyllite is an intermediate-grade, foliated metamorphic rock type that resembles its sedimentary parent rock, shale, and its lower-grade metamorphic counterpart, slate. Like slate, phyllite can be distinguished from shale by its foliation, called slaty cleavage, and its brittleness, or fissility. Both slate and phyllite are generally dark-colored; their most common color is dark gray-blue, but dark red and green varieties also exist. Unlike slate, phyllite has a characteristic glossy sheen, its foliation is usually slightly contorted, and it rarely retains traces of the original sedimentary bedding. Phyllite also lacks the large, visible mica crystals and high-grade index minerals diagnostic of schist, its higher-grade metamorphic cousin.

Siltstone and phyllite are composed of smaller particles than sandstone and signify that the deposition occurred in a quieter environment. This is consistent with the shallow water sedimentation of the Harpers Formation, the silt-sized particles being carried by water to collect offshore. The even smaller particles of mudstone and shale (shale is fissile, it breaks into sheets whereas mudstone is non-fissile) are the basis for phyllite. Shale is metamorphosed into slate which has a slight sheen due to small grains of mica and graphite. Phyllite is metamorphosed shale, the reflective particles having fused such that a glittering surface persists. Phyllite is from the Greek word physis meaning leaf; the cleavage surface that occurs when phyllite is fractured resembles a leaf.

Phyllite has a greater degree of metamorphism than slate but less than schist. It is associated with regional metamorphism do to mountain building. Phyllite has larger crystals than slate. This gives it a greater degree of light reflection or sheen. The sheen is used to distinguish phyllite from slate.

[ Close ]