Optical and electron optical studies on natural cubic diamonds

Abstract

A detailed experimental study has been made of the surface topography of sixty cubic diamonds from the Congo, by means of optical microscopy, electron microscopy, scanning electron microscopy and cathodoluminescence. Ultra violet absorption studies indicate that all sixty cubic crystals are type I.A comprehensive review is given, of the general properties of diamond, its crystal growth, and previous work on the topography of cube faces. A detailed description of the general characteristics of tetragons is given, and a classification of tetragons according to their type, as point bottomed and flat bottomed, is given for the first time. The mechanism of the formation of tetragons is also discussed in chapter VIII. The arrangement of some small tetragons over a considerable area in the form of rows running parallel to the 100 direction has also been studied. It is suggested that such arrangements of pits are produced along grain boundaries on the surface of the crystal by dissolution in nature. The roughness of the cubic surfaces of natural diamond is largely attributed to the frequent appearance of tetragons, which are very deep and sometimes have complex structures, in addition to the existence of different facets of (111) and (110) on the surface. Of the sixty cubic crystals examined, eight showed different crystallographic facets on their surfaces, in addition to the cubic faces, such as octahedral facets, characterized by trigons. These are observed in different areas on the surface, juxtaposed on the cubic faces with tetragons. Some of the octahedral facets are oriented in opposite directions and show adjacent trigons which sometimes appear as parallelogram shaped features. Another striking feature is the appearance of trigons on the octahedral facet of the side wall of some tetragons. This phenomenon supports the theory that the cubic crystal grows by layers on the (111) planes. The occurrence of trigons also at the truncated. octahedral corners of some cubic crystals has been studied. The presence of many octahedral facets, either at the corners of some crystals or at different places on the surface, suggests that this type of cubic crystal grew by layer deposition on the (111) planes. The square shaped pits produced by laboratory etching in KNO on the natural and polished surfaces of cubic diamond have also been studied. It is found that the etch pits are distributed at random or they are crowded together on some areas on the surface, which are the sites of dislocations. Aspects of the internal structure of diamond, as revealed by etching cut surfaces were also studied. The etch patterns showed areas of heavy and light etching which accord with the pattern obtained by X-ray topography and are explained as a combination of the normal facets and the non-faceted hummocky surfaces whose mean orientation was roughly 100.