Phyllite is a foliated metamorphic rock. Due to low-grade pressure, heat, and chemical activity, the rock is composed of flake-shaped (small, flat, and thin) mica minerals in parallel alignment.
Phyllite doesn’t have many commercial uses but is found in many countries. However, its slabs are used in landscaping, paving, sidewalks, and cemetery markers.
Phyllite mainly consists of tiny grains of mica minerals like muscovite or sericite. Most specimens contain fine-grained quartz and feldspar in abundance. The mineral grains in the rock cannot be observed with the naked eye. You will need a hand lens or microscope to view these minerals. The mineral grains are small because the rock is formed due to low-grade metamorphism.
Some phyllite specimens have shown crystals of metamorphic minerals like cordierite, garnet, andalusite, biotite, and staurolite.
Some phyllite varieties contain a large percentage of mica, which gives it a black color.
Phyllite forms when sedimentary rocks are buried and altered (changed) by regional metamorphism. Heat and pressure convert one rock to another type of rock.
The formation process begins with fine-grained sedimentary rocks like shale or mudstone. These rocks are rich in clay minerals with a semi-random orientation. The regional metamorphism pressure and heat move the clay minerals to a parallel alignment or recrystallization. The heat and pressure transform the clay mineral grains into chlorite or mica minerals. The rock at this point is known as slate.
The ongoing heat and pressure on the slate complete the clay-to-mica transformation and forces mica grains to enlarge. The more the pressure, the more parallel alignment becomes. The final product is phyllite with parallel alignment.
Where is Phyllite Found?
Phyllite is found in accretionary wedges above subduction zones. It is a common rock that has huge reserves worldwide. The biggest reserves are in North America, Europe, and Asia in the Appalachian mountains.
Phyllite is formed as a result of low-grade metamorphism. It isn’t a hard and durable rock. Hence, it doesn’t have many industrial uses because it is not strong enough to serve as a crushed stone and aggregate.
Phyllite slabs are sometimes used for landscaping and paving projects.
Phyllite is studied to obtain information about the geologic conditions of the area. It can tell about the upper limit of heat and pressure in that area.