A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix.Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than the matrix of the rock[citation needed].Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings.However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to the Earth's surface during metamorphism and deformation has long been the subject of debate.[1] Later research, however, led to an alternative formation model in which a porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame.
