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Each layer can be as thin as a sheet of paper, or over a meter in thickness. [1] The word comes from the Latin folium , meaning "leaf", and refers to the sheet-like planar structure. [ 1 ] It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from ...
Grains of platy minerals, such as clay minerals, are rotated to form parallel layers perpendicular to the direction of compaction, which begin to impart cleavage to the rock. Slaty cleavage is fully developed as the clay minerals begin to be converted to chlorite and mica. Organic carbon in the rock is converted to graphite. [11]
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. Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. [1]
In geology, texture or rock microstructure [1] refers to the relationship between the materials of which a rock is composed. [2] The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye ...
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Typically, over half the mineral grains in a schist show a preferred orientation. Schists make up one of the three divisions of metamorphic rock by texture, with the other two divisions being gneiss, which has poorly developed schistosity and thicker layering, and granofels, which has no discernible schistosity. [4] [8]
These show less well-developed schistosity. [24] Granofels, which show no obvious foliation [23] or schistosity. [24] A hornfels is a granofels that is known to result from contact metamorphism. A slate is a fine-grained metamorphic rock that easily splits into thin plates but shows no obvious compositional layering. The term is used only when ...
In traditional English and North American usage, a gneiss is a coarse-grained metamorphic rock showing compositional banding (gneissic banding) but poorly developed schistosity and indistinct cleavage. In other words, it is a metamorphic rock composed of mineral grains easily seen with the unaided eye, which form obvious compositional layers ...