PRESSURE & TEMPERATURE
Given sufficient time, new minerals and mineral assemblages form that are stable in the new PT conditions. With time the crystals grow larger.
DIFFERENTIAL STRESS (PRESSURE)
Minerals recrystalize so as to develop new orientated texture (fabric) - in particular platy minerals (e.g., micas) grow so as to be oriented perpendicular to the stress - this develops a FOLIATION. The first stage, when metamorphism is low grade the crystals are small and a SLATY CLEAVAGE (=SLATE) is developed. As metamorphism increases further the crystals get larger and SCHISTOSITY (=SCHIST) is developed.