16 BELL SYSTEM TECHNICAL JOURNAL 



without determining the handedness and electrical sense of the stone (if 

 twinning is negligible). As will be seen later, a similar situation prevails 

 when analyzing etched X-cut sections for cutting into plates. 



5.3 Nature of Etch-Pits 



When crystal quartz is etched by contact with hydrofluoric acid (or other 

 etching agents) the surface of the quartz is eaten away in such a manner as 



Fig. 5.3 — Examples of OPTICAL twinning alone. Optical twins are commonly small 

 and interlayered, and hence may not be separated and used individually. 



to leave microscopic etch- pits (or hills). These etch-pits are formed of 

 minute facets which are definitely related to the cr\^stal structure. The form 

 of these pits and the orientation of the facets may be used to determine the 

 orientation of the crystal structure at the etched surface being examined. 



The general appearance of four types of etch-pits is shown in the photo- 

 micrographs of Fig. 5.5. These are the pits that are deyeloped on ground 

 surfaces which are approximately parallel to the well known X-, Y-, and Z- 

 cut surfaces of right hand quartz, by the action of hydrofluoric acid. It is 



