MINERALS 



method of investigation (56, 57). The vastly 

 different results produced by etching syn- 

 thetic NaCl with ethyl alcohol as compared 

 to water are illustrated in Figure 12 (58). An 

 entirely different type of surface reaction is 

 shown in Figure 13 which pictures crystals of 

 mullite which have formed from a kaolinite 

 clay surface (see Figure 5) held at a tempera- 

 ture of 1130°C for 20 hours. The arrange- 

 ment of the mullite needles results from the 

 pseudo-hexagonal symmetry of the clay min- 

 eral. Similar application of electron micros- 

 copy to the study of minerals in various 

 ceramic materials has been extensive (see 59 

 for references). 



The type of result that may be obtained 

 from a study of fine-grained aggregates is 

 illustrated in Figures 1, 4 and 14. Whereas 

 the first tAvo show textural and crystallo- 

 graphic features of particles in monomineral- 



lic specimens, the pyrite in Figure 14 occurs 

 in a fine-grained shale containing many min- 

 eral varieties. The area pictured here is part 

 of a pyrite nodule made up of an assemblage 

 of pyritohedrons in parallel orientation. 

 Other examples of electron microscopy in 

 petrographic studies involve studies of such 

 subjects as slate (60), ore textures (61, 62), 

 meteorites (63), and glacial till (64) as well 

 as cherts and fine-grained limestones. 



Despite the extensive research done with 

 the electron microscope in the study of 

 minerals and rocks (for general articles see 

 65, 66, 67, 68, 69), the potential of the instru- 

 ment is far from realized. One reason has 

 been the tendency to regard the areas cov- 

 ered by electron and light microscopy as 

 separate rather than overlapping fields. With 

 the increased application of the replica tech- 

 nique supplemented by devices designed to 



Fig. 12a 



195 



