Schist is a foliated metamorphic rock composed of plate-shaped mineral grains. The mineral grains are large enough to be seen with the human eye because the rock has been exposed to regional metamorphism.
Schists are very interesting rocks. It is not necessary for specific minerals to be present in a rock to be termed schist. However, it must have some platy metamorphic minerals in proper alignment to exhibit distinct foliation. This specific type of orientation allows the rock to be broken easily into slabs along the grain direction. This breakage process is known as schistosity.
Some rare schist specimens have platy minerals that do not come from shale and mudstone clay minerals. The platy minerals in these specimens can be talc, graphite, or hornblende from carbonaceous or basaltic sources.
Apart from these minerals, the schist also contains quartz and feldspars.
Different varieties of schist are named as per their color. The color comes due to eye-visible minerals of different colors. You will see rocks labeled muscovite schist, biotite schist, and chlorite schist. Some other names used to describe schist are garnet schist, staurolite schist, hornblende schist, graphite schist, and kyanite schist.
Sometimes you may also find a schist named in three words combination. For example, garnet graphite schist; this system is based on mineral concentration found in the rock. The more dominant mineral takes the second spot in the name. In garnet graphite schist, graphite has a larger concentration.
Schist forms as a result of metamorphism. It has been exposed to moderate level heat and pressure. The rock usually forms from sedimentary rocks like shale or mudstones. These rocks are known as parent rocks or protoliths.
Schist forms at convergent plate boundaries. The metamorphic heat, pressure, and chemical activity convert the clay minerals in protoliths (shale or mudstone) into platy (plate-like) mica minerals such as biotite, chlorite, and muscovite. The pressure transforms the clay minerals from a random orientation to a parallel alignment. The new orientation is perpendicular to the direction of the applied force. This converts the rock into a low-grade metamorphic rock known as slate, which has a dull luster and can be split into thin sheets along parallel mineral alignments.
Additional metamorphic pressure forces the mica grains to grow perpendicular to the compressive force. The growth and further alignment give a silky luster to the rock, and the rock is known as phyllite. More pressure makes the platy minerals big enough to be seen with the naked eye, and the rock is known as a schist. If more heat and pressure continue, the rock gets even harder and forms a granular rock known as gneiss.
Schist is a valuable rock because it hosts many precious and high-price gemstones like garnet, kyanite, andalusite, sphene, sapphire, garnet, iolite, chrysoberyl, and ruby.
Schist is not used in the construction industry or as an aggregate. The abundant mica grains and their schistositygives it lows physical strength.