Cation-diffusion-induced Characteristic Beam Damage in Transmission Electron Microscope Images of Micas *

Transmission and analytical electron microscopy have been used to characterize the numerous lenticular fissures which were observed in electron micrographs of paragonite. Observations with a very weak electron beam revealed that there were no fissures in the original paragonite and that the degree of damage is a function of exposure to the beam. AEM analysis revealed that there is significant loss of Na in the beam-damaged paragonite. The fissures may be caused by collapse of the paragonite structure to a pyrophyllite-like structure, which has a smaller basal spacing due to loss of interlayer Na. Data from a survey of TEM observations of other phyllosilicates which show similar features imply that the fissures are directly associated with the interlayer cation Na and that beam damage effects are related to the interlayer cations in micas.