Textural and structural features are often determined by the changes in pressure and temperature. We briefly covered these aspects in the previous article, but let us cover them again to make sure the concept is understood. Texture is classified by how the parent rock is changed when stress is applied to it. When the stress is applied evenly, resulting in confining pressure, the parent rock often undergoes recrystallization but often no structural changes are seen. These types of metamorphic rocks are referred to as non-foliated rocks. When the stresses are unequal, called differential stress, the parent rock will undergo recrystallization and re-alignment of the minerals into an orientation parallel to the direction of stress. These types of metamorphic rocks are said to have a planar structure and are referred to as foliated. Knowing and identifying the differences between non-foliated and foliated rocks is the first step in classifying metamorphic rocks.
Among the foliated textures, metamorphic rocks are further classified based on how the minerals within the parent rock are affected by the changes in temperature and pressure. In general terms, increases in temperature and pressure result in different, more complex textures, and different metamorphic rocks. The diagram at the right demonstrates this for a fine-grained mudrock. At the lowest amount of metamorphism, the minerals are aligned along a preferred direction in response to the applied stresses. The minerals within the parent rock often are not recrystallized and remain microscopic. The resulting rock can be easily split along cleavage planes and is called a slate. As metamorphism continues, the minerals remain in the same orientation, and certain minerals like micas and chlorites increase in grain size. This results in a fine-grained rock that is similar to slate but often develops a silky sheen on cleavage surfaces and is called a phyllite. As metamorphism continues the minerals become large enough that we can see the crystals without the aid of a hand lens. Some platy minerals, like mica, develop a strong preferred orientation and is now called a schist. Schists often develop porphyroblasts, crystals of one or two minerals present in the rock that are distinctly larger than the average crystals in the rock. As more metamorphism occurs the minerals in the rock begin to separate out into medium to coarse bands of different mineralogy and texture, often with separation of the lighter colored minerals from the darker colored ones. The rock is now called a gneiss.
