Interferometric studies of faces of beryl, quartz, and topaz crystals

Abstract

A very simple microscopic technique has been developed for the observation of uni-molecular steps. This technique utilises the diffraction effect at the edge of such steps, and steps of 4.6 A and 7.9 A have been observed on beryl crystals. This effect is discussed and a theoretical account given of methods of microscopic examination likely to result in the greatest sensitivity. It is concluded that steps of 1 A , or less, should be visible when reflexion phase contrast equipment is used. A review is given of past work on crystal growth in which it is shown that only the dislocation theory of growth is capable of satisfactorily explaining much of the experimental data. The microscopic diffraction technique and multiple beam interferometric techniques are applied to the study of the topography of faces of beryl and quartz crystals. As the microscopic technique enables one to observe dislocations directly, experimental proof is given that these crystals have grown by a dislocation mechanism, and that the dislocation theory of growth must therefore be generally applicable. This theory is shown to be correct in its many detailed predictions and quantitative information is obtained about the conditions in which some of these crystals have grown. The activity of dislocation groups, the cross-linking of layers, the "bunching" of layers, and "rod-formation" on beryl, are discussed. The treatment by the dislocation theory of dissolution as the simple reverse of the growth process is shown to be correct. Information is obtained on the density of dislocations and they are shown to occur frequently on transition surfaces and limited slip-zones. Direct visual proof is given that the dislocation theory of slip is correct. The topography of a cleavage face of topaz is briefly discussed.