- The micas are characterized by a very easy cleavage in a single direction and by the high degree of flexibility, elasticity and toughness of the extremely thin cleavage flakes.
In their optical characters the micas exhibit considerable variations.
Dark coloured micas are strongly pleochroic. Accurate determinations of the optical orientation, as well as the symmetry of the etching figures on the cleavage planes, seem to suggest that the micas, except muscovite, may be anorthic rather than monoclinic in crystallization.
The micas are bad conductors of heat and electricity, and it is on these properties that many of their technical applications depend.
He became deaf after the percussion from the loud crash.
The micas are extremely complex and variable in composition.
It will be seen that they may be divided into two groups - alkali-micas (potash-mica, &c.) and ferromagnesian micas - which correspond roughly with the division into light and dark micas.
The commercially valuable micas of Canada and Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence.
Micas and other platy minerals (such as chlorite), which naturally grow most rapidly on their edges, would show this tendency best, and such minerals usually form a large part of the best slates; but even Sketch (by Du Noyer) of a block of variegated slate from Devil's Glen, Co.
The alkalis are very interesting; often they form 5 or io% of the whole rock; they indicate abundance of white micas or of undecomposed particles of felspar.