Foliation (geology)

Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss.

Examples include the bands in gneiss (gneissic banding), a preferred orientation of planar large mica flakes in schist (schistosity), the preferred orientation of small mica flakes in phyllite (with its planes having a silky sheen, called phylitic luster – the Greek word, phyllon, also means "leaf"), the extremely fine grained preferred orientation of clay flakes in slate (called "slaty cleavage"), and the layers of flattened, smeared, pancake-like clasts in metaconglomerate.

The growth of platy minerals, typically of the mica group, is usually a result of prograde metamorphic reactions during deformation.

Typical examples of metamorphic rocks include porphyroblastic schists where large, oblate minerals form an alignment either due to growth or rotation in the groundmass.

Igneous rocks can become foliated by alignment of cumulate crystals during convection in large magma chambers, especially ultramafic intrusions, and typically plagioclase laths.

Granite may form foliation due to frictional drag on viscous magma by the wall rocks.

Lavas may preserve a flow foliation, or even compressed eutaxitic texture, typically in highly viscous felsic agglomerate, welded tuff and pyroclastic surge deposits.

Thus, they are not always 'planar' in the strictest sense and may violate the rule of being perpendicular to the regional stress field, due to local influences.

When describing a foliation it is useful to note Following such a methodology allows eventual correlations in style, metamorphic grade, and intensity throughout a region, relationship to faults, shears, structures and mineral assemblages.