Subsurface Laboratory Methods 237 



reciprocal three-dimensional nets are drawn on separate sheets of paper. 

 For figure 99 the unit-cell dimensions are those of cordierite: namely, a = 

 17.1 A., b = 9.78 A., c = 9.33 A., and 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 directly as (hOO), (kOO), 

 (100), (hkO), (hOl) and (Okl) are checked for (hkl) agreement (dotted 

 triangles in figure 99 represent coincidence of (hkl) net intersections 

 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. 



Special Problems of Identification 



The value of the X-ray-diffraction method, especially as a research 

 tool, cannot be questioned. Its greatest effectiveness is derived when the 

 X-ray-diffraction data are supplemented by physical and chemical deter- 

 minations made by other methods; but the method can be used inde- 

 pendently to great advantage in many problems. In some investigations 

 X-ray-diffraction analyses are more rapid and efficient than are alterna- 

 tive methods; in other investigations, X-ray-diffraction analysis only will 

 yield the necessary information. Jn plates 8 and 9 are examples of ma- 

 terials that are difficult to analyze or identify by methods other than X-ray- 

 diffraction analysis. 



Among the clays (pi. 8) some members of the montmorillonite 

 group ^^ ^^ show remarkable similarity, and at present information is 

 insufficient to permit positive differentiation of the several members of 

 the group on the basis of the X-ray-diffraction patterns alone. However, 

 if the clays are calcined at temperatures determined from experimental 

 studies or from thermal-dehydration ^° or differential-thermal ^^ ^^ analyses, 

 their identity can be established definitely from X-ray-diffraction studies. 

 Furthermore, when Dupont household Duco cement is used as a binder 

 for the powdered montmorillonite-type clay samples, the diameter and 

 sharpness of the innermost line in the pattern give some information con- 

 cerning the identity of the adsorbed cations. Preliminary observations in- 

 dicate that a broad diffuse line represents a mixture of cations, whereas a 

 sharp narrow line represents a relatively pure single cation. For the 

 potassium ion, the line position corresponds to approximately 11.9 A., for 

 the sodium ion approximately 12.9 A., and for the calcium ion approxi- 

 mately 15.5 A. Likewise, the general degree of expansion or contraction 



^3 Grim, R. E., Modem Concepts of Clay Minerals: Jour. Geology, vol. 50, no. 3, pp. 225 S., 1942i. 



■** Ross, C. S., and Hendricks, S. B., Minerals of the Montmorillonite Group: U. S. Geol. Survey 

 Prof. Paper 205-B, 1943 



^^ Nutting, P. G., Some Standard Thermal Dehydration Curves of Minerals: U. S. Dept. Interior Prof. 

 Paper, 197-E. 



^^ Grim, R. E., and Rowland, R. A., Differential Thermal Analysis of Clay Minerals and Other 

 Hydrous Minerals: Am. Mineralogist, voi. 27, pp. 746, 801 ff., 1942. 



^^ Grim, R. E., Differential Thermal Curves of Prepared Mixtures of Clay Minerals: Am. Mineralogist, 

 vol. 32, pp. 493 ff., 1947. 



