Calc-schists are usually argillaceous limestones in which a large development of biotite or phlogopite has occasioned foliation.
The principal members of the group are muscovite, biotite, phlogopite and lepidolite.
The angle between the optic axes varies from 70 0 -50° in muscovite and lepidolite to Io - o° in biotite and phlogopite; the latter are thus frequently practically uniaxial.
To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlogopite, lepidomelane and most biotites.
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Muscovite and biotite are commonly found in siliceous rocks, whilst phlogopite is characteristic of calcareous rocks.
The commercially valuable micas of Canada and Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence.
Muscovite and phlogopite are practically the only species used commercially, the former being the more common.
Phlogopite is rarely found as colourless transparent sheets and is therefore almost exclusively used for electrical purposes.
(H,K)2(Mg,Fe)2(A1,Fe)2(S104) 3 Phlogopite..