236 



Subsurface Geologic Methods 



will still be possible if suitable unit-cell data are available. In this case, 

 this would involve careful checking of the refractive indices, obtaining the 

 birefringence, and, if possible, such information as would enable one to 

 classify the constituent as isotropic, uniaxial, or biaxial. Now further 

 checking of the list of possible constituents obtained by the above pro- 

 cedure against the list for which powder data are available would readily 



Figure 99. Comparison of powder data of unknown with possible 

 unit-cell data. 



reduce the possibilities so that it would become feasible to apply the 

 reciprocal-lattice method shown in figure 99. 



All unidentified lines of the pattern of the unknown are converted 

 to lines with reciprocal radii by means of the above equation and then 

 drawn on transparent paper or plastic. The unit-cell dimensions of the 

 possible constituents are then converted to reciprocal dimensions, and these 



