KAIV QLAKTZ, ITS DEFECTS AND INSPECTION 341 



What has been said of smoky quartz applies equally well to CITRINE 

 (JiUARTZ except that here the coloration is straw colored (yellow to brown). 

 Citrine quartz is even more uncommon, commercially, than smoky quartz. 



4.4 Twinning 



Electrical and optical twinning are among the most common defects of 

 crystal quartz. There are few stones without one or both present. Either 

 type is usually difficult or impossible to detect from the exterior form of the 

 crystal. Presence of either type in a finished piezoelectric element inter- 

 feres with its perfect operation. Twinning is an abnormality of growth, in 

 which an apparently homogeneous crystal is not actually of the same handed- 

 ness, electrical sense, or orientation throughout. In the case of both elec- 

 trical and optical twinning (the only common twinning types in quartz), 

 the electric and optic axes in all parts remain parallel each to each. 



In a crystal which is only ELECTRICALLY TWINNED, the stone is 

 entirely of one handedness (either right or left), but one portion is of OP- 

 POSITE ELECTRICAL SENSE to another portion. This change in sense 

 of structure is NOT detectable by ordinary optical means. However, at 

 the surface it is detectable by, (1) the piezoelectric effect (determination of 

 electric charge on squeezing); (2) x-ray reflection intensities (using certain 

 sense determining planes, see Chapter III); and (3) most readily and ex- 

 tensively, by etch-pits and etch-pit figure techniques.^ Commonly elec- 

 trical twins are sufficiently large that they may be separated near a twinning 

 boundary and both parts used. 



In a crystal which is only OPTICALLY TWINNED, one portion of the 

 crystal is of OPPOSITE HANDEDNESS and electrical sense to another 

 portion. This change in handedness of structure is detectable by optical 

 means (i.e. by examining between crossed polarizing filters). Optical 

 twinning may also be detected, at the surface, by the etch technique in the 

 same manner as electrical twinning. L^sually a stone will be mainly of one 

 handedness with only small, thin, interlayered growths -of opposite handed- 

 ness, thus making it impossible to use both handed portions separately. 



Further discussion of both electrical and optical twinning will be found in 

 Chapter V, where means for simultaneously detecting both are described. 



4.5 Cracks 



Many quartz stones contain cracks which are not readily seen by a casual 

 surface examination. As mentioned above river quartz commonly exhibits 

 a network of shallow cracks extending inward from the surface, caused by 

 bumping. All types of stones commonly contain one or more cracks, es- 



' In finished plates, of course, the effects of twinning are also determined by measuring 

 the resulting piezoelectric and elastic constants of the plate (i.e., their effect on frequency 

 and activity). 



