Subsurface Laboratory Methods 215 



Scope 



A complete discussion of the various methods of recording diffrac- 

 tion patterns is obviously beyond the scope of this section, and the reader 

 is referred to the original papers and standard texts.^ 3 4 5 6 7 'p}^jg 

 discussion will be limited to the information required by geologists for 

 the identification of geologic materials. Such information includes the 

 advantages and disadvantages of the X-ray-diffraction method; the funda- 

 mentals of recording the data with apparatus employing photographic 

 film or Geiger-counter circuits; the selection and preparation of the mate- 

 rial to be investigated; the selection and processing of the films; the con- 

 version of the data into usable form; the interpretation of the data; and 

 when the method can be applied advantageously to geologic problems. 



The interpretation of the data requires (1) the conversion of the 

 lines in the X-ray powder pattern to their corresponding interplanar dis- 

 tances so that Hanawalt's ^ method employing the card file of X-ray- 

 diffraction data® can be used; or (2) an extended series of standard 

 patterns which are used for direct comparison if complementary data, such 

 as optical measurements, are available to limit the unknown to a work- 

 able number of possible materials; or finally (3) the application of the 

 reciprocal lattice to make use of unit-cell data, if powder-diffraction data 

 are lacking and the unit-cell data available. 



Advantages and Limitations 



The X-ray-diffraction method is advantageous for the analysis of un- 

 known materials, especially mineralogic, because it reveals the state of 

 chemical combination of the constituent elements. Furthermore, the meth- 

 od is nondestructive, and the sample can be used for further studies by 

 other methods. Moreover, satisfactory results can be obtained from very 

 limited amounts of material. Small clusters of powder approximately 

 0.1 to 0.2 mm. in diameter produce good patterns without necessitating 

 objectionably long exposures. Under extreme conditions, suitable patterns 

 have been obtained from samples that consisted of only a few micrograms 

 of material. Likewise, only limited accuracy in measurements is necessary 

 when making a qualitative analysis. Furthermore, a file of diffraction 

 patterns constitutes a permanent record, which can always be examined 

 and checked by anyone versed in the field. 



^ Hull., A, W., A New Method of X-ray Crystal Analysis: Phys. Rev., vol. 10, pp. 661 ff., 1917. 



'Hull, A. W., A New Method of Chemical Analysis: Am. Chem. Soc. Jour,, vol. 41, pp. 1168 £F., 

 1919. 



* Clark, G. L., Applied X-Rays, New York, McGraw-Hill Book Co., Ino., 1940. 



' Davey, W. P., Study of Crystal Structure and Its Applications, New York, McGraw-Hill Book Co., 

 Inc., 1934. 



'Barrett, C. S., Structure of Metals, New York, McGraw-Hill Book Co.., Inc., 1943. 



' Bunn, C. W., Chemical Crystallography (Interpretation of Data) , New York, Oxford Univ. Press, 

 1946. 



' Hanawalt, J. D., Rinne, H. W., and Frevel, L. K., Ind. and Eng. Chemistry, Anal. Ed., vol. 10, 

 pp 457 ff., 1938. 



' Card file index and first supplement compiled under the joint supervision of the American Society 

 for Testing Materials and the American Society for X-ray and Electron Diffraction. These are available 

 from the American Society for Testing Materials, 260 S. Broad Street, Philadelphia, Pennsylvania. 



