Foliation

Foliation is a planar arrangement of minerals or structural features in metamorphic rock, commonly produced by pressure, deformation, or mineral growth.

Differential stress: Stress whose magnitude varies with direction, causing rocks to deform unevenly. It compresses or stretches rocks preferentially, helping orient minerals into foliation.
Metamorphic rock: Rock altered from pre-existing rock by heat, pressure, or chemically active fluids without complete melting. Foliation is a fabric found chiefly in metamorphic rocks.
Slate: A fine-grained metamorphic rock that splits readily along slaty cleavage. Its planar cleavage is a characteristic fine-grained foliation.
Bedding (geology): Layering in sedimentary rock formed by successive episodes of deposition. Bedding may resemble foliation, but it originates during sedimentation rather than metamorphic fabric development.
Metamorphic recrystallization: The growth and rearrangement of mineral grains during metamorphism without melting the rock. Recrystallizing grains can grow with preferred orientations that define foliation.
Phyllosilicate: A silicate mineral built from sheets of linked tetrahedra and commonly split along those sheets. Platy phyllosilicates such as mica readily align to produce foliation.
Schist: A medium- to coarse-grained metamorphic rock with conspicuous aligned platy minerals. Its visible mica-rich foliation gives schist its distinctive texture.
Compositional banding: Alternating layers of contrasting mineral composition within a rock. Banding can be a foliation, but compositional layering is not itself proof of mineral alignment.
Pressure solution: Dissolution of minerals at stressed grain contacts and precipitation in lower-stress spaces. It can flatten grains and redistribute material along foliation planes.
Preferred orientation: A nonrandom alignment of crystal or particle orientations within a material. Aligned mineral grains are the microscopic basis of many foliated fabrics.

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