Home Metamorphic Rocks Foliated Metamorphic Rocks Phyllite

Phyllite

Phyllite is a foliated metamorphic rock that has been low pressure and heat. Phyllite formation from slate that is further metamorphosed so that very fine grained mica mineral achives. It composed of mainly flake-shaped mica minerals. This mica minerals is strong parallel alignment, so easly to split into sheets or slabs. Also alignment of mica grains gives a reflective sheen on phyllite. Generally Phyllite is gray, black or greenish color and frequently weathers  out of a tan or brown.

Name origin: The term of phyllite comes from the Greek phyllon, meaning “leaf”.

Colour: Black to gray or light greenish gray in color, Shiny Gray

Protolith or Parent Rock: Parent rock for phyllite is  shale or pelite, or slate which in turn came from a shale protolith.

Metamorphic Type: Regional

Metamorphic Grade: Low Grade (Higher than Slate)

Metamorphic Environment: Low grade regional metamorphism along a convergent plate boundary

Hardness: 1-2 on the Mohs Hardness, Also fissility (a tendency to split into sheets)

Grain size: Very fine grained white mica achieves a preferred orientation

Group: Metamorphic Rock

Texture: Foliated, Fine-grained, Well-developed schistosity

Degree Of Metamorphism: Between Slate And Schist.

Minerals: Graphite, muscovite, sericite, or chlorite, or the translucent fine-grained white mica, quartz, Plagioclase

Dominant Minerals: Mica Minerals

Classification of Phyllite

Metamorphic rocks are classified according to their texture. Phyllite has fine-grained mica flakes in a preferred orientation, whereas slate has extremely fine clay flakes that achieve a preferred orientation, and schist has large flakes in a preferred orientation. Among foliated metamorphic rocks, it represents a gradation in the degree of metamorphism between slate and schist.

Phyllite Chemical Composition

Phyllite is mainly composed of fine grains of mica minerals that are muscovite, or sericite. Also fine grained feldspar and quartz are frequently major in phyllite. On the other hand Crystals of other metamorphic minerals such as andalusite, biotite, cordierite, garnet, and staurolite might also form within phyllite. Their crystals often grow large enough to be seen and identified with the unaided eye. These larger crystals are known as porphyroblasts. When organic-rich shale is the initial protolith of phyllite, the organic materials are often transformed into graphite. Many phyllites contain enough mica to give them a black color and a submetallic luster.

Formation of the Phyllite Rock

The heating and compression of clay-rich, bed sedimentary rocks called shale creates a series of rock types of increasing metamorphic grade: slate, filite, schist and gneiss. During the metamorphism of the shales and sometimes during the volcanic ash layers, the metamorphism transforms platy clay minerals into small mica layers. As the heat and compression density, called metamorphic quality increases, the mica sheets align themselves with the direction of stress and grow. The crystals of sheet-silicate minerals such as chlorite, biotite and muscovite in the filite are large enough to give the rock its distinctive satin luminance and fluffy cleavage, but are not as visible as invisible to the eye. The amount of heat and pressure required to convert the shale into the filament is usually sufficient to remove any original sedimentary layer. Additional metamorphism converts filaments from schist; All the original clay and small mica crystals are transformed into large mica crystals, the remaining organic substances are destroyed and the high-grade metamorphic index minerals such as garnet and staurolite grow in the micro matrix.

Where is Phyllite Located

Phyllite is very abundant metamorphic rock in the word. It forms It forms when sedimentary rocks are buried and mildly altered by the heat and directed pressure of regional metamorphism. These are almost always convergent plate boundary environments involving continental lithosphere.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.

Uses of The Rock

Phyllite is lustrous sheen rock. So it is often used as a decorative stone in countertops.

Phyllite may be used as decorative aggregates, floor tiles, and other interior home decorations or used as exterior building or facing stone, and garden decorations.

Also, it may be used in building facades and for decorative crushed stone.

Slabs of phyllite are occasionally trimmed and used as landscape, paving or sidewalk stone.

Facts About The Rock

  • Phyllite is associated with regional metamorphism due to mountain building.
  • Continued metamorphism converts clay minerals into large grains of mica, along with quartz and feldspar. At that point, phyllite becomes schist.
  • Phyllite is often found as black to gray, or light greenish gray in color. It has a crinkled or wavy appearance as its foliation.
  • Phyllite is a durable and soft rock.
  • Other uses may include cemetery markers, commemorative tablets, creative artwork, and writing slates.
  • It is scaled between 1-2 on the Mohs Hardness scale and has a specific gravity or 2.72 – 2.73.
  • Phyllite has a resistance to heat, pressure, and water.

References