On the origin of reverse pleochroism' of a phlogopite

The visible spectra of phlogopite with a reversely pleochroic scheme consist essentially of three bands in the 450-550 mmu region superimposed on the wing of a strongly pleochroic band centered in the ultraviolet region. The spectra have their greatest intensity when polarized light vibrates normal to the cleavage plane, the reverse of micas with normal pleochroic schemes. By comparing the phlogopite spectra with those of an iron orthoclase and FeCl 4 (super -) , materials which are known to contain tetrahedrally-coordinated Fe (super 3+) , it has been established that the three-band system in the visible region is also due to this ion. The background absorption is considered to be due to an O (super 2-) Fe (super 3+) charge-transfer band which is most intense when the vibration direction is coincident with the O (super 2-) (unbridged)--Fe bond perpendicular to (001) in the iron-bearing tetrahedra.