SRI Petrography ID: 37
Schist is a crystalline metamorphic rock, mostly composed of more than 50% tabular and elongated minerals with grainsize coarse enough to be visible to the unaided eye. Schists have a developed tendency to split into layers.
Schists have a foliated or plated structure (schistosity). Schists are distinguished from other foliated rocks (e.g. slates and gneisses) by the size of their mineral crystals:
The crystals are larger than those of slates (macroscopically not visible) but smaller than those of gneisses (coarse grained, opposed to finely foliated schists). The protolith of schists can be igneous (e.g. basalt, volcanic tuff) or sedimentary (clay, mud).
Schists are classified and named on basis of their mineralogy:
The most common minerals of schist are mica; mica-schist contains quartz and mica (biotite or muscovite) as main minerals, schist is often named after additional minerals, e.g.
- hornblende schist
- talc schist
- chlorite schist
- graphite schist
- garnet schist
- glaucophane schist
green, grey, dark brown (Biotite schist), silver (Muscovite schist), also other colours
depending on the schist-type. Main minerals: quartz, mica (muscovite, chlorite, biotite), feldspar, talc, graphite, hornblende, garnet.
Schists are abundant in all Precambrian (Archean and Proterozoic) rocks
Schists are coarser grained than slates (the minerals of slates cannot be seen by the naked eye), but finer grained than gneiss.
In contrast to the folia of slates, the folia of schists are irregular and rough-surfaced.