12 BELL SYSTEM TECHNICAL JOURNAL 



of quartz are: (1) by optical efifects (birefringence and rotator}^ power), (2) 

 by X-ray reflections from atomic planes, and (3) by the use of etch pits which 

 are developed when the quartz surface is etched in fluorine compounds. 

 Other methods are or may be used in rather special cases. For example, 

 in finished plates of known orientation types, the electrical axis direction is 

 distinguished from other directions by electrical polarity tests (on tension 

 or compression), or a plate known to be one of several types may be tested 

 in an electric circuit for activity, frequency and temperature-coefi&cient, to 

 determine which type it is. The selective fracture characteristics of quartz 

 offer another method of determining orientation. Microscopic fractures re- 

 sulting from grinding a quartz surface ma}' be used for determining orienta- 

 tion. Thus unetched, ground, Z-cut surfaces of quartz give a hexagonal 

 figure, when examined by pinhole illumination, which may be used to de- 

 termine the approximate orientation (but not sense) of the electric axes.- 



By optical methods (see Chapter II) it is possible to determine the orienta- 

 tion of a quartz body relative to only one direction of the structure, the optic 

 or Z axis. Thus optical methods are hmited to determinmg the angle be- 

 tween the optic axis and a line or surface of the body (but not the rotation of 

 that line or surface about the optic axis). Twinning of the "optical" vari- 

 ety may be detected optically, even when located internally, but the deter- 

 mination of its location in depth is approximate. 



By X-ray methods (see Chapter III) it is possible to determine the struc- 

 ture orientation of a quartz body exactly and completely. However, this 

 method is limited in application by the complexity of analysis, except when 

 the approximate orientation is already known. Though twinning can be 

 detected on the surface of the body, it is not generally feasible to explore the 

 surface to locate twinning boundaries. Further, though positive or negative 

 sense of angular orientation is obtainable by X-rays, this part of the complete 

 determination is not reliable unless the specimen examined is known to be 

 free of twinning, or unless the twinning boundary locations are known. 

 Thus X-ray determinations of orientation are generally limited to deter- 

 mining exact orientations in quartz bodies of approximately known orienta- 

 tion (which includes the case in which only one axis is approximately known). 



The etch method of determining orientation is commonly used in con- 

 junction with the optical and X-ray methods to give the information that 

 those methods do not give. The etch method, as most commonly and prac- 

 tically applied, does not give exact orientation angles, nor is it applied to 

 specimens of entirely unknown orientation. However, when a surface of 

 approximately known orientation is etched, it is possible to determine ap- 

 proximately the complete orientation (including sense) of the specimen, and 

 further to detect at this surface both electrical and optical twinning and to 



2 See Fig. 5.20, and further explanation at the end of Sec. 5.53. 



