ETCH TECHNIQUE 47 



named figures to give the sections simulated natural faces. Figures 5.2 

 and 5.3 are of particular use in learning to distinguish between electrical and 

 optical twinning when examining etched surfaces by reflection. Note that 

 electrical twins are usually large and separated by irregular boundaries, 

 Fig. 5.2. Optical twins are usually separated by straight-line boundaries 



Fig 5 26— Since this section exhibited no bright regions (except flaws/) in view B (i.e. 

 no ri-^ht quartz of electrical sense B) it was not reproduced in view B. The c region is an 

 electrical twin of the adjacent d region, while a is an optical twin of d. It is uncommon 

 for a minor optical twin to be as large as a. 



parallel to natural faces, thus forming triangular, parallelogram, and straight 

 line insets. Fig. 5.3. Optical twins (except for the one major, surrounding 

 twin) are usually very small and often interlayered (with the major twin). 

 Large interlayered regions are entirely unusable and hence are cut away at 

 the earliest possible stage to save the labor of processing worthless material. 

 Small optical twins and small electrical twins (or remnants of electrical 



