Metamorphic Rock Types

Metamorphic rocks are classified by texture and by mineral composition.

Foliated metamorphic rocks. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. Slates are generally fine‐grained, dark‐colored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under low‐grade temperature and pressure conditions. Phyllites are slightly more metamorphosed than slates and contain mica crystals that impart a glossy sheen. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. Some varieties of schist are mica, garnet‐mica, biotite, kyanite, and talc schist. A schistose rock composed of the mineral serpentine is called a serpentinite.

Migmatites form when temperatures are hot enough to partially melt the rock. The magma is sweated out, or injected, as layers between foliation planes in the rock.

An example of the categories a shale would pass through as temperatures and pressures increase (from low grade to high grade) is as follows: shale/slate/phyllite/mica schist/gneiss/migmatite.

Nonfoliated metamorphic rocks. If a rock is not foliated, its name is derived from its chemical composition. A quartz‐rich rock such a sandstone, for example, is called a quartzite when it has been metamorphosed. A metamorphosed limestone is called a marble. When rocks (especially shales and basalts) are affected by contact metamorphism, they often develop a texture called hornfels. A hornfels rock is characterized by evenly distributed, very fine‐grained mica crystals that give it a more massive, equigranular appearance.