CLEAVAGE, SCHISTOSE

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.