The planar or layered characteristics of metamorphic rocks that are evidence of the pressures and/or temperatures to which the rock was exposed. These can be structural such as cleavage, textural such as mineral grain flattening or elongation, or compositional such as mineral segregation banding. In geology, the term foliation refers to the property, possessed by some crystalline rocks, of dividing into plates or slabs, which is due to the cleavage structure of one of the constituents, as mica or hornblende. It may sometimes include slaty structure or cleavage, though the latter is usually independent of any mineral constituent, and transverse to the bedding, it having been produced by pressure. Planar fabric element that occurs penetratively on a mesoscopic scale in a rock. Primary foliation includes bedding and igneous layering; secondary foliations are formed by deformation induced processes. Joints are not normally considered as foliations since they are not penetrative on a mesoscopic scale. A laminated structure formed by segregation of different minerals into layers that are parallel to the schistosity.
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