In the series, each mineral tends to develop idioblastic surfaces against any mineral that occurs lower in the series. This series can, in a rather general way, enable us to determine the origin of a given rock. For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase. Porphyroblasts are often riddled with inclusions of other minerals that were enveloped during growth of the porphyroblast. These are said to have a poikioblastic texture .
Most metamorphic textures involve foliation.
Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate , if it has a phyllitic foliation, it is termed a phyllite , if it has a shistose foliation, it is termed a schist . A rock that shows a banded texture without a distinct foliation is termed a gneiss . A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite , if the grain size is large and individual minerals can be easily distinguished with a hand lens. Protolith Protolith refers to the original rock, prior to metamorphism.