Phyllite is a fine-grained, foliated metamorphic rock. The name “phyllite” is derived from the Latin, meaning “leaf-stone”. The name probably refers to its usual green color and its ability to be flaked into thin sheets. Phyllite has a marked fissility, meaning that the rock is easily split into clean slabs along the parallel alignments of platy minerals that are formed during metamorphism.
Phyllite is one of four types of rock that are the result of the Barrovian metamorphism of its protolith, or parent rock, shale. Shale is a fine-grained sedimentary rock made up of silt and clay particles. When shale is subjected to the tectonic pressures of mountain-building events, it is metamorphosed into slate, phyllite, schist and gneiss. Therefore, slate is the first-formed metamorphic rock from shale and further pressures will metamorphose the slate into phyllite. The four metamorphic rocks to be formed from shale all appear very different. The differences between them are due to the varying degrees of heat and pressure undergone by the protolith.
Phyllite is primarily composed of quartz, sericite mica, and chlorite. Because of the relatively low pressures exerted upon its parent rock, some of the original mineralogy and sedimentary nature of the rock may be maintained after it has metamorphosed. Phyllite crystals are larger than those of slate, but smaller than those of schist. Minute crystals of graphite and mica give phyllite a distinctive sheen that may be golden in color.
The most common colors for phyllite are green, gray or red. It has an appearance that is very similar to slate, but its foliation often creates a crinkly or wavy surface. Phyllite displays a micaceous luster which further distinguishes it from slate.
• Where Phyllite is found
Phyllite forms in areas of regional metamorphism where where beds of sedimentary rocks have been subjected to moderate heat and compression by the colliding of continental plates and mountain-building events. Both slate and phyllite form in sedimentary basins that are deeply buried, or in accretionary wedges above subduction zones. It is found all over the world from the Appalachians in North America to the Scottish Highlands and the Alps in Europe.
Because of its lustrous sheen, phyllite is often used as a decorative stone in countertops. It may also be used in building facades and for decorative crushed stone.