The major differences between foliated and nonfoliated metamorphic rocks are in the areas of texture, appearance and the type of pressure applied during recrystallization. The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated.
Both foliated and nonfoliated rocks begin their lives as either sedimentary, igneous or another metamorphic rock. Due to geological changes, a metamorphic rock of the foliated or nonfoliated type will be created. These geological changes can be due to heat and temperature from the recrystallizing rock being found deep within the earth's crust. "Friction and distortion" caused by the movement of the tectonic plates under the earth's surface or the introduction of hot magma to the parent rock.
Type of Pressure
If the pressure applied to the recrystallizing rock is unequal, then a foliated rock will form. The force on the reforming rock must be strong and "one directional." However, if the pressure applied to the recrystallizing rock is equal all over, a nonfoliated rock will be created.
A foliated metamorphic rock will have banded minerals. The mineral flakes will appear to be parallel to the rock and will look layered. When a foliated rock breaks, a thin rock fragment will result.
A nonfoliated rock will have almost the opposite texture. The minerals will appear to be randomly oriented without obvious banding and have a granular appearance. Unlike a foliated rock, there will be no layers and they will not flake apart into thin layers when broken.
Foliated rocks are most often formed from mudstones and contain "fine-grained" or "platy" minerals that are usually too small to see with the naked eye; although some can be seen without aid. Examples of foliated rocks are slate, phyllite and schist.
Nonfoliated rocks contain more coarse grained minerals and generally have a random shape. Because of this, these rocks are very granular in appearance. Examples of nonfoliated rocks are quartzite, marble and anthracite coal.