MINERALS 



members of the kaolin group, halloysite and 

 kaolinite, respectively, which have the same 

 chemical composition but crystallize as tubes 

 or laths in the former case (15), and hexago- 

 nal plates in the latter. A similar situation 

 exists in the serpentine minerals where tubes 

 of chrysotile (Figure 2) contrast sharply with 

 laths and flakes of antigorite (16, 17, 18, 19). 

 Such striking morphological differences 

 within single mineral groups reflect differ- 

 ences in environment and growth conditions, 

 and obviously must be taken into account in 

 any application which depends on the physi- 

 cal attributes of the materials. 



Figure 3, referred to previously, provides 

 further evidence of the large morphological 

 variation among clay minerals. Of all the 

 clays, those of the so-called montmorillonite 

 group demonstrate the greatest morphologi- 

 cal variation with change of hydration state 

 (20, 21, 22). Precise measurement shows that 

 individual sheets approach unit cell thick- 



o 



ness of 10 to 30 A yet have a real extent of 

 the order of hundreds of sc^uare microns. 

 Such characteristics are basic to the applica- 

 bility of bentonite in oil-well drilling-mud and 

 as filler in many industrial materials, as well 

 as in many areas of colloid chemistr}^ agron- 

 omy and mineralogy where ion exchange and 

 surface behavior of such materials are of 

 interest. 



Synthetic minerals occur in both groups 

 discussed above but deserve particular men- 

 tion because of the important role played 

 by the electron microscope in their study. 

 In the field of geochemistry, particularly, 

 where much emphasis is placed on high tem- 

 perature, high pressure phase equilibrium 

 studies, conditions are frec^uently not con- 

 ducive to the formation of large crystals and 

 the products may be visible only at high 

 magnification. Such is the case for synthetic 

 chrysotile (Figure 6) produced from Mg-Si 

 gels under hydrothermal conditions at a tem- 

 perature of 450°C and a pressure of 40,000 

 psi (23). The tubular character of the fibers 

 is apparent, as is also the "cone-in-cone" 



Fig. 5. Kaolinite-coated paper. The character- 

 istic platy form and hexagonal outline and cleav- 

 age of the clay particles are apparent in this 

 platinum-carbon replica of a sheet of high-gloss 

 paper. X 15,000. 



growth feature only rarely observed in natu- 

 ral material. 



Minerals are now being synthesized under 

 a wide variety of conditions, many for use in 

 numerous industrial applications, and the 

 electron microscope has an important role in 

 the research involved with the development, 

 production and control of the product. Fig- 

 ure 7 shows an aggregate of crystals of a 

 synthetic zeolite, the mineral group so im- 

 portant in water softening. 



Whether the minerals be synthetic or nat- 



191 



