the refractive indices, obtaining the birefringence, and, if possible, such informa- 

 tion 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 procedure against the list for which powder data are available will 

 readily reduce the possibilities so that it will become feasible to apply the 

 reciprocal-lattice method shown in Figure 9-11. 



All unidentified lines of the pattern of the unknown are converted to lines 

 having reciprocal radii by means of the above equation and the resulting circles 

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

 constituents are then converted to reciprocal dimensions, and these reciprocal 

 three-dimensional nets are drawn on separate sheets of paper. For Figure 9-11 

 the unit-cell dimensions are those of cordierite: namely a = 17.1 A, b = 9.78 A, 

 c = 9.33 A, in the orthorhombic crystal system. The experimental data are then 

 superimposed on these various possible nets and the experimental lines (rings) 

 checked for agreement with the net intersections. If a reasonable number of 

 lines show ^agreement, then any lines not identified by the net intersections direct- 

 ly as (hOO), (OkO), (001), (hkO), (hOl), and (Okl) are checked for (hkl) agree- 

 ment (dotted triangles in Figure 9-11 represent coincidence of (hkl) net inter- 

 sections with experimental data lines) . Coincidence of one or more net inter- 

 sections with every experimental powder-data line identifies the second constituent 

 in the mixture as cordierite. 



EMISSION X-RAY Within recent years emission X-ray spectro- 



SPECTROGRAPHIC graphic analysis has been developed to the 



ANALYSIS extent that now it is capable of analyzing for 



all elements with atomic number greater than 

 11. With suitable standards such analyses can be made in considerably less time 

 than is normally required for wet methods. The accuracy of these analyses 

 compares favorably with that of the wet methods and the range of detection for 

 most elements extends from .01 or .02 percent to 100 percent. Since even a 

 preliminary discussion of this method is beyond the scope of this chapter, the 

 reader is referred to the standard texts and periodicals that described the various 

 phases of the method (Klug, 1954; A.S.T.M., S.T.P. 157, 1953; Cullity, 1956; 

 The Norelco Reporter v. I, II, III, etc., 1953). 



APPLICATIONS The value of X-ray diffraction methods, par- 



ticularly as a research tool, need no longer be 

 questioned. A brief examination of the technical journals will quickly verify this 

 statement. Naturally, the greatest effectiveness is derived when X-ray data are 



172 



