MINERALS 



shadow-casting in giving depth to the field 

 and reveahng the size, shape and texture of 

 the smaller particles that make up the 

 twisted flakes. 



The de^'elopment of replfca methods ex- 

 tended the applicability of the electron mi- 

 croscope to particles of any size and to the 

 study of surfaces, thus removing many of the 

 limitations of the instrument for mineralogi- 

 cal and petrographic work. As a result, elec- 

 tron microscopy began to play a more signifi- 

 cant role in studies of crystal growth and 

 solution, chemical reactivity and physical 

 wear, high temperature reactions, texture of 

 natural and artificial mineral aggregates, and 

 applications of minerals in many industrial 

 uses. Early replica work, particularly using 

 plastic films, was restricted to relatively 

 smooth, impermeable and non-porous sur- 

 faces. Later a very important forward step 

 was the successful apphcation of the plati- 

 num-carbon replica technique to clays (2, 3) 

 and other porous substances, thereby extend- 

 ing the method to all mineral materials, in- 

 cluding rocks and soils. Thus, Figure 1 is a 

 platinum-carbon replica of a fractured sur- 

 face of the halloysite clay. The technique is 

 especially suitable for minerals of this type 

 where sensitivity to hydration state may re- 

 sult in severe morphological changes if prep- 

 aration procedures become too involved and 

 time consuming. Thus, replicas of fractured 

 surfaces may indicate a very different parti- 

 cle size distribution than that revealed when 

 particles have been dispersed and allowed to 

 settle on a substrate. 



Thin sectioning for the electron micro- 

 scope is a technique that, until recently, has 

 been of limited use to mineralogists because 

 of the hardness and brittleness of the mate- 

 rials involved. However, improvements in 

 diamond knives have permitted an impor- 

 tant "breakthrough" in this area (4) and it 

 is to be expected that sectioning of minerals 

 and mineral aggregates will provide impor- 

 tant new data in the future. Figure 2 demon- 

 strates how effectively application of this 



h 



-I 



Fig. 3. Wyoming beutonite, Upton, Wyoming. 

 Freeze-dry preparation of montmorillonite clay 

 from aqueous suspension. X 20 ,200. 



technique to a bundle of chrysotile asbestos 

 fibers supports previous data as to the tubu- 

 lar morphology and the packing arrange- 

 ment of the individual crystals as seen in 

 cross section (5). 



Fine-grained Minerals 



Mineral particles that belong in the size 

 range appropriate to effective electron micro- 

 scope study of morphological features fall 

 into two categories. In one are those that 

 are simply small scale representatives of min- 

 erals which also occur in the form of coarser 



189 



