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.

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.

At Moriah and Port Henry, in Essex county, is a stone known as ophlite marble, a mixture of serpentine, dolomite and calcite interspersed with small flecks of phlogopite.

Other minerals present include diopside, pargasite, spinel, phlogopite, apatite, graphite, pyrrhotite and zircon (Davidson 1943 ).

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.

Minute acicular inclusions, probably of rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of "asterism" in some micas, particularly phlogopite: a candle-flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star.

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.

At Moriah and Port Henry, in Essex county, is a stone known as ophlite marble, a mixture of serpentine, dolomite and calcite interspersed with small flecks of phlogopite.