The etching of diamond, II. Cleavage, dodecahedron and cube faces

Abstract

The etching of cleavage (111), dodecahedron (110), and cube (100) faces of three different diamonds is studied optically using high-resolution microscopy, multiple-beam interferometry and the light-profile microscope. Etching in the temperature range 500 to 700 degrees C is obtained with fused potassium nitrate. In accordance with crystallographic expectation the etch-pits observed are triangular for (111), boat-shaped for (110) and square for (100). The evolution, distribution and shapes of the pits are discussed. Some depths are evaluated by interferometry and by the light-profile. A new feature which has emerged in each case is the appearance of a strongly marked rectilinear pattern proving each of the three diamonds to have a laminated structure. The (111) face is crossed with lines parallel to (111) edges, the (110) face is filled by a beautifully regular set of concentric hexagons with the sides parallel to (111) edges, and the cube face reveals an equally striking set of concentric rectangles, filling most of the face. These patterns consist of straight, slightly raised, ridges and are regions less susceptible to etch attack than the surrounding areas. Their profiles are demonstrated optically. The pattern offers clear evidence that in each crystal, growth has proceeded by layer formation leading to a strongly laminated fine structure. The etch-patterns reveal for the first time a sectional history of the growth of the diamond. The observations can be accounted for by postulating increased resistance to etching at the onset (or end) of each layer. The proposed growth by sheet layers is in conformity with the failure to find growth spirals from the examination of some hundreds of octahedron diamond faces.