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Topaz cleavages have been etched in potassium hydroxide at various temperatures. Three distinctive types of distribution of etch-pits, for example (a) individual isolated pits, (b) random distribution of micro-pits and (c) rectilinear etch patterns, have been observed. The structure of the individual isolated pits has been investigated in detail; they are of two types, (a) point-bottomed and (b) curl-bottomed. The structures of the pits persist on continued etching. From the asymmetry of the point bottoms of the point-bottomed pits, it is established that these pits nucleate at linear dislocation lines inclined to the cleavage face at an angle of about 25 ° . It is conjectured that curlbottomed pits nucleate at screw dislocations. By etching matched cleavage faces, correspondence in regard to number, shape, size, position and structure of the pits has been established. Correspondence exists even in the etch patterns on the opposite sides of thin flakes of topaz. The rectilinear etch patterns show a considerable amount of displacement when crossing large cleavage steps. The rectilinear traces are traces of planes inclined at an angle of about 45 ° with the cleavage face. It is, therefore, conjectured that they may be the traces of (111) planes which make an angle of about 45 ° with the (001) cleavage face. The correspondence in the rectilinear etch patterns on the two sides of a thin flake shows that these planes go right through the body of the crystal. The implications are discussed.

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(011) cleavages of single crystals of calcium tungstate, grown by the flux-evaporation method in this laboratory, have been subjected to thermal etching between temperatures of 1110 and 1430 °C in a vacuum chamber at a pressure of 10−5 Torr. The etch figures thus obtained successfully reveal the emergence sites of dislocations in the crystals. An excellent correlation between thermal and chemical etch pittings has also been obtained on mirror cleavages. Quantitative measurements made on the etch rate and weight loss with temperature show that the process of the removal of ledges from the cleavage surface has an activation character. The activation energy for evaporation at dislocation sites is less than that responsible for overall evaporation from the exposed surface. The implications are discussed.