238 Subsurface Geologic Methods 



of the over-all pattern is an indication of the chemical nature of the 

 middle, gibbsite or brucite, sheet of the three-layer packet. For an element 

 with a small atomic radius (aluminum) an expanded pattern is obtained, 

 whereas for an element with a large radius (ferrous iron) a contracted 

 pattern results. In general, most clays can readily be recognized and 

 identified without preliminary treatment, as is shown by plate 8. The 

 X-ray-diffraction method is particularly valuable in the analysis of shales, 

 because they are frequently so fine-grained and so heterogeneous in com- 

 position as to preclude adequate microscopic analysis. The optimum par- 

 ticle size for X-ray-diffraction studies ranges from about 10"^ cm. to 10'^ 

 cm., which lies just beyond the limit of the microscope. Plate 9 illustrates 

 differences between various shales. 



However, the X-ray-diffraction method is not a panacea for all prob- 

 lems, and its utility is usually considerably enhanced if it is used in con- 

 nection with other methods, especially microscopic, spectrographic, and 

 chemical procedures. This is particularly true for investigation of certain 

 types of complex minerals or mixtures. There is no doubt that the method 

 becomes more effective and efficient as the mixture becomes simpler or 

 the unknown material purer; and, consequently, it is at times advisable 

 or even necessary to concentrate or purify the constituents for separate 

 study before a mixture can be satisfactorily analyzed. Purification and 

 concentration of ingredients are especially valuable in the investigation 

 of substances whose pattern is not sufficiently distinctive to permit use of 

 merely a few isolated lines. Optical data obtained from microscopic 

 measurements can often reduce time and labor by aiding in the selection 

 of standard patterns to be used in the comparison; of course, positive 

 identification may be accomplished on some materials by microscopy alone. 

 Much time can be saved in a laboratory by using the X-ray method only 

 if satisfactory answers cannot be obtained from microscopic studies. 



The fact that nature is not particular, as regards chemical composi- 

 tion, when forming crystals is being recognized by men of science.^^ ^* ^^ 

 Very important properties are associated with apparently insignificant 

 changes in chemical composition of minerals. Frequently, when once a 

 geometric space arrangement of a crystal has been started, nature will 

 continue with the building process using indiscriminately any atoms or 

 ions available that are reasonably similar in size as long as the over-all 

 structure is kept electrically neutral. Crystals that have extensive sub- 

 stitution have been referred to as "half-breed" and "stuffed" crystals,^® 

 depending on the mechanism by which the structure maintains neutrality. 

 As a result of such partial substitutions the refractive indices of some 



" Thompson, J. B., Jr., Role of Aluminum in Rock-Forming Silicates: Am. Mineralogist, vol. 33, 

 pp. 209 ff., 1948. 



"^ Buerger, M. J., Crystals Based on the Silica Structures: Am. Mineralogist, vol. 33, pp. 751 ff., 

 1948. 



^ Barshad, J., Vermiculite and Its Relations to Biotite as Revealed by Base Exchange Reaction, 

 X-ray Analysis, Differential Thermal Curves, and Water Content: Am. Mineralogist, vol 33, pp. 

 655 ff., 1948. 



'* Buerger, M. J., op. cit. 



