During the last five years transmission electron microscopy (TEM) has added numerous important new data to mineralogy and has considerably changed its outlook. This is partly due to the fact that metallurgists and crystal physicists having solved most of the structural and crystallographic problems in metals have begun to show a widening interest in the much more complicated structures of minerals, and partly to recent progress in experimental techniques, mainly the availability of ion-thinning devices. While electron microscopists have become increasingly interested in minerals (judging from...