ELECTKO.N >llC;H()SCOPY 



Fig. 1. Halloysite clay, Wendover, Utah. 

 Platinum-carbon replica of a fracture surface 

 showing tubular and lath-shaped crystals. The 

 scale in all illustrations is one micron except where 

 otherwise indicated. (All micrographs not other- 

 wise expedited were taken by Joseph J. Comer, 

 electron microscopist. College of Mineral Indus- 

 tries, The Pennsylvania State University.) 

 X 84 ,300. 



ogy of fine particles that could be, or already 

 were, dispersed for direct observation. Thus, 

 as soon as the instrument became available 

 it began to fill the long standing needs of the 

 mineralogist concerned with the shape, size, 

 concentration, interaction, behavior, pro- 

 duction, separation, and use of particles 

 measured in microns. The effect in areas of 

 research such as silicosis and clay mineralogy 

 was immediate and dramatic; and although 

 new technicjues have greatly extended the 

 area of applicability of electron microscopy, 

 many present day problems can be soh'ed by 

 the simple technique of directly viewing par- 

 ticles that are dispersed upon a suitable sub- 

 strate. 



Two techniques provide additional infor- 



mation lo the scientist interested in the na- 

 ture of particles in dispersed systems : shadow 

 casting and freeze drying. The former soon 

 became standard procedure in electron mi- 

 croscope laboratories because it produced 

 additional contrast and provided a means of 

 measuring particle thickness. The freeze-dry 

 technicjue is invaluable when the morphology 

 and interaction of particles in suspension is 

 important. In Figure 3 the convolutions 

 of the Wyoming bentonite flakes suspended 

 in water were "captured" when the em- 

 bedding water droplet was suddenly frozen. 

 Had the particles been allowed to settle 

 and the water removed by evaporation 

 the resulting shape of the films would bear 

 no relationship to that which, to a large 

 extent, controls the behavior of clay-water 

 bodies. In this illustration note the effect of 



Fig. 2. Chrysotile asbestos, Transvaal. Ultra- 

 thin section showing circular cross sections of 

 tubular crystals. The scale is 0.1 micron. X94,000. 

 {Courtesy Robert V. Rice, Electron Microscope 

 Laboratory , Mellon Institute) 



188 



