Cleavage and Its Types
Folded sedimentary and metamorphic rocks often display a fundamental internal grain known as cleavage. The presence of cleavage in a rock permits the rock to be split into thin plates and slabs. The term “cleavage” may be defined as the tendency of a mineral to break more easily with smooth surface along plane of weak bonding as determined by the structure of its crystal lattice. Hence cleavage is the property which is related to the atomic arrangement within the mineral.
Types of Cleavage
Where domainal structure (i.e the distinction between cleavage domains and microlithons) can be seen with the unaided eye, the cleavage can be described as Disjunctive cleavage, meaning interrupted. Where the domainal character of a cleaved rock is too fine to be resolved without the aid of a petrographic or an electron microscope, the cleavage is described as Continuous cleavage.
(1) Continuous cleavage.
The main types of continuous cleavage are slaty cleavage, phyllitic structure and schistosity. All three are generally associated with strongly folded and distorted metasedimentary and metavolcanic rocks.
(a) Slaty cleavage is typically associated with very fine-grained «0.5 mm) pelitic (shaley) rocks metamorphosed to low grade. Where slaty cleavage is well developed, it imparts to rocks an exquisite splitting property. Indeed, the presence of slaty cleavage allows a rock to be cleaved into perfectly tabular, thin plates or sheets. Roofing slates and old-fashioned slate blackboards owe their existence and usefulness to slaty cleavage.
(b) Phyllitic structure is intermediate in grain size and overall character between slaty cleavage and schistosity. In outcrop, phyllites display a soft, pearly, satiny luster. They glisten in the sun, but lack the distinct individual mica grains seen in schists. Phyllites exhibit the capacity to split neatly but not perfectly.
(c) Schistosity rocks with schistosity are typically medium-grained (1-10 mm), containing flakes of mica that are visible in hand specimen. The grain size, which is larger than that of slates, mostly reflects greater recrystallization accompanying metamorphism. The most obvious outcrop characteristic of schistosity is the parallel, planar alignment of micas, including muscovite, biotite, chlorite, and sericite. Schists seldom split cleanly and evenly when struck with a hammer. Instead, they break off in the form of discoidal to crudely tabular hand specimens or slabs.
(2) Disjunctive cleavage
There are two main types of disjunctive cleavage:
(a) Crenulation cleavage is very distinctive in that it cuts a host rock that possesses a preexisting continuous cleavage, especially phyllitic structure or schistosity. In rocks that contain crenulation cleavage, a preexisting continuous cleavage is typically “crenulated” into microfolds. Two kinds of crenulation cleavage are recognized. One is discrete and the other is zonal. Discrete crenulation cleavage is a disjunctive cleavage in which very narrow cleavage domains sharply truncate the continuous cleavage of the microlithons, almost like tiny faults. Zonal crenulation cleavage, on the other hand is marked by wider cleavage domains that coincide with tight, appressed limbs of microfolds in the preexisting continuous cleavage preserved within microlithons. Whether discrete or zonal, cleavage domains in rocks possessing crenulation cleavage are closely spaced, generally between 0.1 mm and 1 cm. Discrete crenulation cleavage tends to form in slate. Zonal crenulation cleavage tends to form in schist and phyllite.
(b) Spaced cleavage is a second type of disjunctive cleavage. It consists of an array of parallel to anastomosing, stylolitic to smooth, fracture like partings that are often occupied by clayey and carbonaceous matter. Spaced cleavage is typically found in folded but unmetamorphosed sedimentary rocks, especially impure limestone and marl, and in some impure sandstones as well. Spacing of the partings (i.e., the cleavage domains) typically ranges from 1 to 10 cm, and thus the microlithons are quite thick compared with all other cleavages. Thickness of the partings often is on the order of 0.02-1 mm, although they may be as thick as 1 cm or more.