Foliation

foliation

Planar arrangement of structural or textural features in any rock type, but particularly that resulting from the alignment of constituent mineral grains of a metamorphic rock along straight or wavy planes. Foliation commonly occurs parallel to original bedding, but it may not be obviously related to any other structural direction. Foliation is exhibited most prominently by sheety minerals, such as mica or chlorite.

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foliation

1. Botany

a. the process of producing leaves

b. the state of being in leaf

c. the arrangement of leaves in a leaf bud; vernation

2. Architect

a. ornamentation consisting of foliage

b. ornamentation consisting of cusps and foils

3. any decoration with foliage

4. Geology the arrangement of the constituents of a rock in leaflike layers, as in schists

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foliation [‚fō·lē′ā·shən]

(botany)

The process of developing into a leaf.

The state of being in leaf.

(geology)

A laminated structure formed by segregation of different minerals into layers that are parallel to the schistosity.

(metallurgy)

Beating metal into thin sheets.

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foliation

1. The cusps or foils with which the divisions of a Gothic window are ornamented.

2. Leaf-like decoration.

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Foliation 

the ability of rocks to split relatively easily parallel to a certain plane when struck. This mechanical anisotropy of rocks is caused by dynamic metamorphism in which the platy and rodlike grains of micas, hornblende, chlorites, and other minerals that make up a rock take on an identical orientation as a result of recrystallization or rotation. In irregularly shaped grains of minerals, orientation of the optical axes of the grains and other crystallographic characteristics may be observed. Petrotectonic analysis is used to study the rules of orientation and the rules for decoding the motions that cause the orientation.

Foliation may be regional, manifesting itself over large areas, or local, related to shifts along tectonic displacements. It frequently occurs during rock folding, with the foliation usually subparallel to the axial planes of the folds. Foliation also forms in cleavage: the crystals are flattened in a plane perpendicular to the axis of compression and the rock acquires a plane-parallel oriented internal structure. Some geologists use the term “cleavage” as a synonym for “foliation.”

REFERENCES

Pek, A. V. Treshchinnaia tektonika i strukturnyi analiz. Moscow-Leningrad, 1939.
Fairbairn, H. W. Strukturnaia petrologiia deformirovannykh gornykh porod. Moscow, 1949. (Translated from English.)

A. V. PEK