There may be occasions, however, when the sample is limited to a very small, 

 pure single crystal, insufficient in amount to grind into powder. Under such 

 circumstances, a single crystal ranging from 0.5 mm to several hundredths of 

 a millimeter in cross section and from several millimeters to about 0.3 to 0.5 

 mm is mounted on the end of a small glass rod or wire, with one crystallographic 

 axis approximately parallel to the axis of the rod so that it can be mounted and 

 adjusted in the goniometer head of the single-crystal camera to turn about this 

 axis. Patterns are recorded successively with alternate rotation about the three 



Focusing 



N - circle 



\ 



X-ray 



target 



1 'Counter 



mechanism 



Sample 



/ : 



Figure 9-4. Schematic diagram showing principles of the counter-recorder type of apparatus. 



crystallographic axes according to procedures found in standard texts (Buerger, 

 1942). From these patterns unit-cell calculations are made. Under adverse 

 conditions it may be impossible to obtain patterns about all the crystallographic 

 axes, whereupon it may be necessary to calculate the dimensions of the entire 

 cell from a single rotation pattern by means of the reciprocal lattice. A dis- 

 cussion of this concept is beyond the scope of this chapter, but it may be found 

 in text books on X-ray-diffraction techniques (Clark, 1955; Davey, 1934; and 

 Bunn, 1946). 



Powders 



For powder patterns it is usually recommended that several milligrams of 

 representative material be crushed and ground in an agate (or mullite) mortar 

 until the entire specimen will pass a 200-mesh silk bolting cloth or screen. 

 The writer has observed that if the sample is turned or oscillated during ex- 

 posure to the X-rays it will be sufficient to grind the sample until highlights 



155 



