Subsurface Laboratory Methods 223 



case, however, these diluents should be avoided or kept to a minimum 

 since some of them (e.g., raw cornstarch) produce a crystalline pattern 

 of their own, or an amorphous pattern with very broad lines (halos) . 

 These superposed patterns of the diluents often cause a considerably 

 localized background fog, with consequent difl&culty in observing lines in 

 the regions of the amorphous bands. 



It is recommended that the ground and diluted samples be packed 

 into capillary tubes with an inside diameter of 0.4 to 0.6 mm. and made 

 of plastic materials (materials with amorphous patterns) or glass con- 

 taining elements of only low atomic weight. The plastic materials are 

 preferred to glass, as measurements on Pyrex tubes with wall thickness 

 just sufi&cient to permit careful handling show forty- to fifty-percent 

 absorption of the CuKa radiation. Longer wave lengths are absorbed to 

 an even greater extent. Glass appears to be suitable for MoKa radiation; 

 however, as will be shown later, Mo radiation is not desirable for use in 

 the identification of components of mixtures. 



Another mounting method recommended for long-wave-length studies 

 on materials of low atomic weight consists in mixing the powder of the 

 unknown with about ten percent (by volume) of gum of tragacanth or 

 collodion and extruding it as a rod approximately 0.5 mm. in diameter. 



An excellent method of mineral specimen preparation used by some 

 of the most prominent workers in the field, although it is usually not de- 

 scribed in standard texts nor recommended in the American Society for 

 Testing Materials procedures,^'' consists in mixing the powder of the 

 unknown with a minimum of Dupont household Duco cement (or other 

 plastic cements) and then rolling the plastic mass between two microscope 

 slides to form a thin rod of the desired thickness. Thickness can be care- 

 fully controlled by inserting the microscope slides in a jig which holds 

 them a fixed, predetermined distance apart. The cement acts as binder 

 and diluent, and if kept to a minimum generally will not affect the back- 

 ground of the diffraction pattern. The writer has found this method to be 

 particularly desirable for identifying montmorillonite-type clays, as the 

 Duco cement conditions the clay so that it needs not be specifically 

 treated "^ ^^ ^^ to be differentiated from other materials, such as muscovite 

 or illite. 



Platy or fibrous crystals may become oriented in the cement during 

 rolling of the rod. The lack of random orientation changes the circular 

 lines of the pattern to arcs, especially the lines formed at a small angle 

 to the beam. On film patterns orientation effects usually do not cause 

 any difiiculty where qualitative identification is the objective, so long as 



^' Tentative Recommended Practice jot Identification of Crystalline Materials by the Hanawalt X-ray 

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



" Jackson, M. L., and Hellman, N. N., X-ray Diffraction Procedure for Positive Differentiation of 

 Montmorillonite from Hydrous Mica: Soil Sci. Soc. Am. Proc, vol. 6, pp. 133 £f., 1941. 



' Hellman, N. N., Aldrich, D. G., and Jackson, M. L., Further Note on an X-ray Diffraction Pro- 

 cedure for the Positive Differentiation of Montmorillonite from Hydrous Mica: Soil Sci. Soc. Am Proc. 

 ''ol. 7, p. 194, 1942. 



Bradley, W. F., Diagnostic Criteria for Clay Minerals: Am. Mineralogist, vol. 30, pp. 704 £f., 1946. 



