Interferometric studies on the growth features of quartz crystals

Abstract

Multiple-beam Interferometry has been applied to the study of the surface topography of natural and synthetic quartz crystals. It is shown that the principal habit faces (major rhombohedron R. minor rhombohedron r. and prism m) consist of a number or 'growth pyramids', which are produced by the spreading of multimolecular growth sheets In succession from several initiating centres. A description if given of the properties of these sheets (height, edge structure etc.) on the R faces, and of the vicinal faces constituting the sides of the growth pyramids. There are two respects in which the vicinal faces depart from the condition of being strictly plane. - They possess a cylindrical curvature with a radius of several metres and their inclination to the close-packed plane increases towards the summit of the growth pyramid. From the Interpretation of the results the mechanism of growth of the R faces is determined, and estimates are obtained of the mean distance of diffusion of adsorbed molecules and of the size of the theoretical two-dimensional nucleus. The general features of the minor rhombohedron faces are similar to those of the R faces. Differences arise due to the greater irregularity of the growth fronts of the r faces, and this is probably connected with their greater rate ofspreading. The topography of natural prism faces is briefly described, and the origin of the atriations on these faces. Two types of synthetic crystal have been studied; seeds with a thin layer (order of one wavelength) of synthetic growth, and crystals with a om. or more of synthetic growth. The observations on these crystals indicate that growth initially takes place from a large number of growth nuclei of the order of but that as growth proceeds, most of these nuclei becomes Inactive and ultimately growth takes place from relatively few dominant centres. Evidence is given for the existence of screw dislocations in quarts; and from the observations on natural and synthetic crystals it is concluded that fresh sheets are Initiated by screw dislocations, by the re-entrant angles associated with twinned faces and, possibly, by foreign particles.