342 BELL SYSTEM TECHNICAL JOURNAL 



pecially when badly twinned or full of inclusions. Some of these cracks may 

 be due to rough handling, but others are due to growth conditions, or result 

 from temperature changes after growth. 



Though large cracks are readily detected by means to be described, it 

 might be noted that when cracks are sufficiently fine (small separation 

 compared to light wave-lengths) they will no longer be seen. Thus every 

 visible crack may be considered to extend beyond its visible range, and some 

 actual cracks will not be visible at all. Because of such cracks and other 

 defects in quartz, special care should be taken in handling uncut and partially 

 cut stones to prevent their subjection to mechanical or thermal shocks. On 

 the other hand, small, flawless, finished plates will stand considerable shock. 

 It is even common practice to solder to finished plates (after metallizing). 



4.6 Inclusions (Bubbles, Needles, Phantoms, Veils, Etc.) 



The remaining abnormalities of raw quartz (used for piezoelectric ele- 

 ments) may be classed as inclusions. Among these are inclusions of solid, 

 liquid, and gaseous material. The size of individual inclusions may vary 

 from submicroscopic, to those easily visible with the naked eye. The in- 

 clusions may be isolated, or arranged in lines, or planes, or curved surfaces. 

 In many cases the arrangement forms of inclusions (bubbles, needles, phan- 

 toms, veils) have been used to describe inclusions, with little regard to the 

 nature or size of the individual inclusions. This is because inclusions which 

 are too small to analyze individually, are still visible when grouped by hun- 

 dreds in lines or surfaces. 



When inclusions are sufficiently small and closely grouped they give a 

 BLUISH cast (Tyndall effect) to the group. Thus the bluish cast is recog- 

 nized as indicating fineness of grouped inclusions. When the individual 

 inclusions are larger, the group appears white. With still larger inclusions 

 one may actually see separate, individual inclusions, looking like minute 

 bubbles. Thus, describing the group as blue, white or bubble textured is of 

 considerable importance, when analyzing or estimating the usability of 

 quartz with grouped inclusions. 



BUBBLE INCLUSIONS look Uke small bubbles (i.e., small spherecidal 

 cavities) in the quartz. When bubbles appear individually, or randomly 

 scattered, they are referred to as just bubbles. When bubbles occur in 

 organized groups the group is referred to as a bubble phantom, bubble veil, 

 etc. Smaller bubbles appear only as light reflection points and their shape 

 is not seen. 



In general such bubbles may be filled with gas, liquid, solid, or any com- 

 bination of phases. They may be of the same nature as rare, large cavities 

 in which one can easily see a liquid moving about. Analysis of the contents 

 of such cavities has indicated the presence of CO2, water, salt solutions, and 



