Phyllitic Cleavage forms in strongly aligned micas and clays and produce silky, sometimes wavy foliation.
Phyllitic cleavage develops in clay rich rocks subjected to greenschist facies metamorphism. Smectites, illites etc. are transformed to muscovite and chlorite. As temperatures increase and rocks reach low to middle greenschist facies metamorphic grades, clays react to form fine-grained muscovite and chlorite, which are courser grained than the claysIf these reactions occur in an anisotropic stress field, the new minerals will have a strong preferred orientation and form a phyllite with a foliation called a phyllitic cleavage. At higher metamorphic grades, past the middle greenschist facies, minerals react to form coarser grained muscovite and biotite. If these reactions occur in an anisotropic stress field, the new minerals will have a strong preferred orientation and form a schist with a foliation called a schistosity. Schists may also contain larger minerals as porphyroblast that grew during the metamorphism or porphyroclasts that are remnants of primary minerals.
Phyllite is a fine-grained metamorphic rock formed by the low grade metamorphism of fine-grained, sedimentary rocks, such as mudstones or shales. Phyllite has a marked fissility (a tendency to split into sheets or slabs) due to the parallel alignment of platy minerals; it may have a silky sheen on its surfaces due to tiny plates of micas. Its grain size is larger than that of slate but smaller than that of schist.
Phyllite is formed by relatively low-grade metamorphic conditions in the lower part of the greenschist facies. Parent rocks may be only partially metamorphosed so that the original mineralogy and sedimentary bedding are partially preserved. Depending upon the direction of the stresses applied during metamorphism, phyllite sheets may parallel or crosscut the original bedding; in some rocks, two stages of deformation, called precrystalline and postcrystalline deformations, can be distinguished on the basis of two orientations of definable surfaces in the rock. Precrystalline surfaces have slaty cleavage, or flow cleavage, whereas postcrystalline surfaces have fracture, or strain-slip cleavage. Such terms can be used only when the type of deformation and its relation to time can be determined.