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Schist

Any of various medium-grained to coarse-grained metamorphic rocks composed of laminated, often flaky parallel layers of chiefly micaceous minerals.

Medium- to coarse-grained, mica-bearing metamorphic rock with well-developed foliation (layered structure) termed schistosity. Schist is derived primarily from fine-grained, mica-bearing rocks such as shales and slates. The schistosity is formed by rotation, recrystallization, and new growth of mica; it is deformational in origin. The planar to wavy foliation is defined by the strong preferred orientation of platy minerals, primarily muscovite, biotite, and chlorite. The relatively large grain size of these minerals (up to centimeters) produces the characteristic strong reflection when light shines on the rock.

Schist is a medium-grade metamorphic rock, so it has experienced more heat and pressure than both slate and phyllite. The main change from phyllite to schist is that foliation is much more distinct due to the recrystallization of mica and chlorite mineral crystals. These larger crystals tend to reflect light very well, so schist usually has a high luster than phyllite and slate. Samples of schist often contain some larger, unusual mineral crystals such as garnet and tourmaline, which are generally referred to as porphyroblasts. Such minerals can form as heat and pressure transform several different minerals into a new and distinct mineral, a process referred to as neomorphism.

Schists are named by the assemblage of minerals that is most characteristic in the field; for example, a garnet-biotite schist contains porphyroblasts of garnet and a schistosity dominated by biotite. Schists can provide important information on the relationship between metamorphism and deformation.

Schist contains more mica than gneiss does, and since mica is a flaky mineral it is hard to work with. Schist would be more difficult to polish than gneiss, and would tend to stand up less well to weathering if placed, say, on the outside of a building. But, still, you might be able to find examples of schist used as a polished ornamental stone.

Another use for schist is that it is a source of the semi-precious gemstone called garnet. Garnet is a mineral that is nearly always found only in schists, and there are many schists throughout the world in which large crystals of garnet can be found. In the United Kingdom, for example, schists from Scotland are well known for their beautiful garnets.

Material Notes:
Metamorphic rock. It has a finer mica banding than gneiss.

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