Metamorphic rocks are formed when existing rocks are transformed by intense heat and pressure. Their texture describes the size, shape, and arrangement of the mineral grains. Foliation is a particular characteristic often found in these rocks, providing a structural record of the forces that acted upon the rock during its formation. Understanding this texture is fundamental to classifying and interpreting the geological history of many rocks.
Defining Foliated Texture
Foliation describes the repetitive layering or planar alignment of mineral grains within a metamorphic rock. Derived from the Latin word folium (“leaf”), this texture gives the rock a sheet-like or banded structure. This alignment is most noticeable in rocks containing platy or elongated minerals, such as mica, chlorite, and hornblende. The parallel orientation of these minerals creates a distinctive planar fabric, allowing the rock to split more easily along these planes. The presence of foliation distinguishes a rock from non-foliated metamorphic rocks.
The Processes That Create Foliation
Foliation requires the application of differential stress combined with heat. Differential stress is pressure that is not equal in all directions, unlike uniform confining pressure. This uneven pressure, common in regions of mountain building or tectonic plate collision, forces the rock’s internal components to reorganize. Minerals, especially those that are flat or elongated, physically rotate so their long axes lie perpendicular to the direction of maximum stress.
Heat and pressure also cause minerals to recrystallize and grow in new orientations. This favors the growth of new platy minerals, such as mica, with their flat surfaces oriented parallel to one another. The resulting planar fabric develops at right angles to the strongest compressional force, minimizing the energy required to accommodate the stress. This process sometimes involves metamorphic differentiation, where minerals of different compositions separate into alternating layers.
Common Types of Foliation and Appearance
Foliation varies based on the intensity of metamorphism (metamorphic grade) and the resulting grain size.
Slaty Cleavage
At the lowest metamorphic grades, slaty cleavage forms. This is characterized by a very fine-grained texture where mineral grains are microscopic. This texture allows the rock to break into thin, flat sheets, created by the alignment of microscopic clay and mica particles.
Schistosity
As the metamorphic grade increases, platy minerals grow larger, leading to schistosity. This texture is defined by a coarser, visibly wavy layering where individual mica flakes are easily seen. The rock surface often has a distinct, glittering sheen due to the reflection of light off these aligned crystals.
Gneissic Banding
At the highest metamorphic grades, gneissic banding develops, which is the coarsest type of foliation. Here, minerals have segregated into alternating light-colored bands of felsic minerals (like quartz and feldspar) and dark-colored bands of mafic minerals (such as biotite and hornblende).
Examples of Foliated Rocks
The types of foliation correspond directly to specific rock names, reflecting the progressive nature of metamorphism.
Slate is the finest-grained foliated rock, exhibiting slaty cleavage, and forms from the low-grade metamorphism of shale. It is used for durable roofing tiles and flagstones.
Phyllite represents an intermediate step between slate and schist. Its mica crystals are slightly larger than those in slate, producing a noticeable satin-like sheen on the foliation surfaces.
Schist is a medium-grade rock defined by schistosity, often containing abundant visible flakes of mica and sometimes large accessory crystals.
Gneiss is a high-grade rock displaying gneissic banding, representing the most intense transformation before the rock begins to melt.