222 Subsurface Geologic Methods 



Before the method of mounting the powdered specimen is selected, 

 the optimum thickness of the sample to be used should be determined. 

 The proper thickness can be calculated if sufficient information is avail- 

 able concerning the specimen. Otherwise, the optimum thickness usually 

 can be estimated approximately by an experienced operator from the 

 amount of the undiffracted X-ray beam that penetrates trial specimens, 

 as determined with a fluorescent screen. This thickness can be calculated 

 from the equation :^^ 



2 

 t=— 



where jx is the linear absorption coefficient calculated from the mass 

 absorption coefficient according to the relationship: 





d being the density of the material, p the elemental fraction in the com- 

 pound and ^ the mass absorption coefficients of the elements for the 



wave length of the radiation used. The values for ^ can be found in 



table form in volume 2 of "International Tabellen zur Bestimming von 

 Kristallstrukturen," pages 577 and 578. 



For NaCl the optimum thickness for copper radiation is found to be 



'2;<7)= 



and 



dYp(~] =2.165 [.396X30.9+.604X103.4]=171 



t=^=m\l cm. 



This result indicates that the optimum sample thickness is usually con- 

 siderably less than that generally recommended for capillary mounting.^^ 



If too thick a sample is used, a distorted pattern is obtained. Thus, 

 it is obvious that sample thickness becomes important when deciding on 

 a suitable mounting technique. Another very important factor to be con- 

 sidered in connection with the mounting of the specimen is the amount 

 of material available. 



For materials of high atomic weight the optimum thickness may be 

 so small as to necessitate dilution of the crystalline material with amorph- 

 ous diluents such as flour, cornstarch, or gum tragacanth.^* ^^ In any 



^ Buerger, M. J., op. cit., p. 182. 



*' Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 

 Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 



" Davey, W. P., op. cit. 



* Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 

 Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 



