X.—Optical Peculiarities in Phlogopite

Extract

It frequently happens that micas exhibit small basal cleavage rifts of roughly circular shape, and these may be so numerous as to render opaque or feebly translucent a plate which would be otherwise quite transparent. The so-called silver-amber mica of the mica trade, a variety of phlogopite which is highly prized for its low conductivity, apparently owes its silvery sheen to the presence of these cleavage rifts, which overlap each other to a considerable extent, and reflect the light copiously. The presence of these basal cleavage rifts in silvery phlogopite is rendered quite obvious when a plate is examined under the microscope in ordinary light, by the interference rings seen round the borders of the rifts. Such a plate of silvery phlogopite exhibits a peculiar phenomenon in consequence of the presence of these rifts.