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Formation

Schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions (nonhydrostatic stress). Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt). The schistosity develops perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers. While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations. At the microscopic level, schistosity is divided into internal schistosity, in which inclusions within porphyroblasts take a preferred orientation, and external schistosity, which is the orientation of grains in the surrounding medium-grained rock.

The composition of the rock must permit formation of abundant platy minerals. For example, the clay minerals in mudstone are metamorphosed to mica, producing a mica schist. Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called slate, which with further metamorphism becomes fine-grained phyllite. Further recrystallization produces medium-grained mica schist. If the metamorphism proceeds further, the mica schist experiences dehydration reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss.

Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of ultramafic igneous rocks, as is talc schist. Talc schist also forms from metamorphosis of talc-bearing carbonate rocks formed by hydrothermal alteration. Graphite schist is uncommon but can form from metamorphosis of sedimentary beds containing abundant organic carbon. This may be of algal origin. Graphite schist is known to have experienced greenschist facies metamorphism, for example in the northern Andes.

Metamorphosis of felsic volcanic rock, such as tuff, can produce quartz- muscovite schist.