216 



Subsurface Geologic Methods 



On the other hand, the X-ray-diffraction method is sometimes con- 

 sidered rather limited as an analytic tool because the relative sensitivity 

 requires that an appreciable amount of a constituent (from one to thirty 

 per cent) ^^ must be present in a mixture before its presence can be 

 detected. However, the use of improved techniques will do much to correct 

 this situation. Some materials with patterns having reasonably low back- 

 ground intensities and fairly strong lines can readily be detected in con- 

 centrations as low as one-half to one percent, whereas other materials 

 with weaker patterns, such as the montmorillonite-type clays, can be 



Figure 93. Schematic diagram of conventional powder camera. 



detected when present in amounts ranging from five to six percent of the 

 sample. It has been reported that special treatment of this clay with 

 glycerol permits detection in amounts as low as one percent. ^^ Limitation 

 of the number of detectable constituents in mixtures due to crowding of 

 lines has also been considered a disadvantage by some workers using 

 small-diameter cameras with large pinhole systems. ^^ The use of larger 

 camera (10 to 20 cm.) diameters, smaller pinhole systems, and longer 



■"' Brosky, S., P. T. L. Netos, Fittsburgh Testing Laboratories, Pittsburgh, Pennsylvania. 

 '' Kelley, W. P., Cation Exchange in Soils: Am. Chem. Soc. Mon. 109, New York, Reinhold Publish- 

 ing Corporation, 1948. 



^- Broskey, S., op. cit. , 



