44 BELL SYSTEM TECHNICAL JOURNAL 



conditions of electrical sense and handedness. The two large, bright re- 

 gions a and b (appearing in views A and B respectively) are both right quartz 

 but of opposite electrical-sense. Hence the surface is mainly of electrically 

 twinned right quartz. The small dark regions within the borders of a 

 (view A) are bright in view D. Hence these small, triangular and line re- 

 gions are left quartz and of opposite electrical sense to the large region a 

 containing them. They are then optical twins of the large a region. Simi- 

 larly the dark regions of b (view B) are found from view C to be optical 

 twins of the b region. (Flaws labeled/ are cracks, chips, etc.) If the whole 

 section were cut up to make AT plates, for example, and at the proper angu- 

 lar sense according to the a portion of the section, then those plates coming 

 from the b region|,would be of wrong angular sense. Those crossing a 

 boundary between. the a and b regions would be of both senses, i.e., elec- 

 trically twinned. Those few plates which contained some left quartz would 

 be optically twinned. To make the most economical use of this section it 

 should be separated, by cutting along a line approximating the a to & bound- 

 ary, so that each half of the section may be cut at the correct sense of orien- 

 tation. Even when so cut, some of the plates will contain optical twinning 

 and remnants of electrical twinning. This section is typical of much of the 

 raw quartz that must be used for manufacturing piezoelectric plates. 



Figure 5.24 shows a section which is mainh' of left quartz as exhibited by 

 the large bright c and d regions of views C and D. The large c region is 

 optically- twinned to a small extent by the line regions b of view B. One of 

 the d regions is badly optically twinned by the small striated a regions, as 

 seen in A. Such a section would be very uneconomical to process, since 

 separating the larger electrical twins is not feasible. If processed at all, it 

 should probably be entirely cut according to the handedness and sense of 

 the large c portion, the wrong-sensed regions and twinning being cut away 

 at a later stage (after inspection of the slabs in the twinoriascope, for ex- 

 ample). It might be noted that only the optical twinning could have been 

 observed in the initial polarized-light, raw quartz inspection, where such a 

 stone would be passed as moderately good. 



Fig. 5.25 shows an unusual section that is mainly composed of left quartz, 

 regions c and d. The right quartz regions shown in view B are of both 

 opposite-handedness and electrical-sense to the c region inclosing or bordering 

 them. This is the common and expected conditions. The unusual condition 

 is exhibited by the regions c and a, where twins of opposite-handedness 

 but same electrical-sense have a common boundary. Since this boundary 

 could be detected by optical means, the a and c regions might be de- 

 scribed as optical- twins, of an "uncommon variety". However, by con- 

 vention optical twinning has long been used to denote twinning exhibiting 

 both opposite-handedness and opposite-electrical-sense (crystallographically. 



