Calc-schists are usually argillaceous limestones in which a large development of biotite or phlogopite has occasioned foliation.
The commercially valuable micas of Canada and Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence.
The principal members of the group are muscovite, biotite, phlogopite and lepidolite.
Muscovite and biotite are commonly found in siliceous rocks, whilst phlogopite is characteristic of calcareous rocks.
Muscovite and phlogopite are practically the only species used commercially, the former being the more common.