1911 Encyclopædia Britannica/Phlogopite

by Leonard James Spencer

PHLOGOPITE, a mineral belonging to the group of micas (q.v.). It is a magnesium mica, differing from biotite in containing only a little iron; the chemical formula is [H,K,(MgF)]₃Mg₃Al(SiO₄)₃. It crystallizes in the monoclinic system, but the crystals are roughly developed. There is a perfect cleavage parallel to the basal plane, the cleavage flakes are not quite so elastic as those of muscovite. Sometimes it is quite colourless and transparent, but usually of a characteristic yellowish-brown colour. and often with a silvery lustre on the cleavage surfaces, hence the trade name “silver amber mica” for some varieties. The name phlogopite is from Gr. φλογωπός (fiery-looking), the mineral being sometimes brownish-red and coppery in appearance. The hardness is 21/2–3, and the specific gravity 2·78–2·85. The optic axial plane is parallel to the plane of symmetry and the axial angle 0°–10°. Phlogopite occurs chiefly as scales and plates embedded in crystalline limestones of the Archean formation. The mica mined in Canada and Ceylon is mainly phlogopite, and is largely used as an insulator for electrical purposes. In Canada it occurs with apatite in pyroxene rocks which are intrusive in Laurentian gneisses and crystalline limestones, the principal mining district being in Ottawa county in Quebec and near Burgess in Lanark county, Ontario. In Ceylon, the mineral forms irregular veins, rarely exceeding one or two feet in width, traversing granulite, especially near the contact of this rock with crystalline limestone.  (L. J. S.)