ELECTKON ^IICKOSCOPY 



crystals iiiul fra»;moiils easily studied at 

 lower nuignificatiou. Many of the particles 

 in mineral dusts, paints and pigments, fine- 

 grained rocks, ceramic bodies and other syn- 

 thetic aggregates fall in this group. Thus, 

 Figure 4 pictures quartz crystals that have 

 grown in pai-allc^l to near-parallel orientation 

 in a minute fissure in chert, a high-silica rock. 

 Although, for minerals of this group, the 

 moi-phological characteristics and behavior 

 of the finest particles can frequently be pre- 

 dicted from larger specimens of the same 

 material, the electron microscope still pro- 

 vides vital information as to size limitations 

 and distribution of the fine particles; as to 

 surface features, textural relationships, nu- 

 cleation and growth phenomena; and as to 

 chemical and physical stability under vari- 

 ous conditions. 



The second group, smaller in number, con- 

 sists of the "dwarfs" of the mineral kingdom 



which, because of restrictions imposed by 

 their crystal chemistry, rarely attain suffi- 

 cient size to be studied effectively with the 

 light microscope. It was at this group that 

 the electron microscope was first "aimed" 

 because it was here that the resolution of the 

 instrument was most needed to provide basic 

 information of the type long known for 

 coarse-grained minerals. The clay minerals 

 were among the first investigated (6, 7, 8, 9) 

 and have been more thoroughly subjected to 

 electron microscopic study than any other 

 mineral family (see, for example, 10, 11, 12, 

 13, 14). Not only do these minerals fall in 

 just the right size range but, because of the 

 large number of commercial applications of 

 clay and clay products, detailed high magni- 

 fication study has been industrially as well as 

 scientifically important. The morphological 

 complexity of the minerals is revealed by a 

 comparison of Figures 1 and 5, replicas of two 



Fig. 4. Quartz crystals in chert, Caddo Gap, Arkansas. Particles on most fracture surfaces in this 

 rock do not show the crystal forms exhibited here. X 12,100. 



190 



