Cleavage (geology)

Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism.

Generally, these structures are formed in fine grained rocks composed of minerals affected by pressure solution.

They state that cleavage is a type of secondary foliation in fine grained rocks characterized by planar fabric elements that form in a preferred orientation.

The presence of fabric elements such as preferred orientation of platy or elongate minerals, compositional layering, grain size variations, etc.

Continuous or penetrative cleavage describes fine grained rocks consisting of platy minerals evenly distributed in a preferred orientation.

Spaced cleavages can be categorized based on whether the grains inside the microlithons are randomly oriented or contain microfolds from a previous foliation fabric.

The development of cleavage foliation involves a combination of various mechanisms dependent on the rocks composition, tectonic processes, and metamorphic conditions.

[1] During ductile deformation, mineral grains with a high aspect ratio are likely to rotate so that their mean orientation is in the same direction as the XY plane of finite strain.

Dynamic recrystallization occurs when a rock undergoes metamorphic conditions and reequilibrium of a minerals chemical composition.

If the heat is too intense, foliation will be weakened due to the nucleation and growth of new randomly oriented crystals and the rock will become a hornfels.

This is thought to be because the folding is controlled by buckling of the stronger sandstone beds with the weaker mudstones deforming to fill the intervening gaps.

[1] In geotechnical engineering a cleavage plane forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.)

Different ways in which a cleavage can develop in a sedimentary rock . A: original sedimentary rock; B: pencil cleavage ; C: diagenetic foliation (parallel to bedding ); D: slaty cleavage.
Schematic drawing of continuous and spaced cleavage
Metamorphosed shale depicting slaty cleavage. Note the grains of mica, quartz, and ilmenite aligned with a preferred orientation.
A thin section depicting spaced cleavage. The cleavage domains are darker biotite grains, and the microlithons between consist of mostly muscovite and quartz. The grains in the microlithons are starting to align in a preferred orientation. A new foliation overprinted an old, showing the beginning signs of a crenulation cleavage.
Petrographic thin section showing Crenulation Cleavage within the late Middle Ordovician (Llandeilian Stage) Hendre Shales Formation (Drefach Group) in a roadside exposure [Grid Ref: SN 2883 2053] along the B4299 Meidrim road heading south towards St Clears, Carmarthenshire, Wales. A first (Caledonoid) and second (Armorican) cleavage are identified. Field of view = 1 cm diam.
Anticline in sandstone shale sequences with axial planar cleavage, refraction of cleavage visible on right-hand limb