Subsurface Laboratory Methods 231 



strong lines of the patterns of minor constituents frequently cannot be 

 found in the pattern of the mixture. 



Identification of Minerals and Components of Mixtures 



Single Crystals 



If X-ray-diffraction data have been obtained from single-crystal rota- 

 tion patterns and unit-cell dimensions calculated, the identity of the com- 

 pound usually can be determined. However, the determination may not 

 be simple because as yet unit-cell data have not been compiled into tables 

 according to some regular order (decreasing or increasing) of the cell 

 dimensions along the three axes. However, if the unit-cell dimensions 

 of the unknown are found to correspond to those of a previously de- 

 scribed compound, the identification can be considered to be reliably 

 established. In fact, unit-cell data are about the most reliable type of 

 X-ray data available for identifying organic materials; and it is most 

 regrettable that no one has undertaken the task of compiling them into 

 some systematic form based upon dimensions. Recently R. W. G. Wyckoff ^^ 

 initiated a continuous loose-leaf system for compiling unit-cell and other 

 crystallographic data according to compound classification. This compila- 

 tion will aid in the identification of compounds, but the diflficulties of 

 accomplishing an identification from unit-cell dimensions alone will be 

 manifest. 



Powders and Fine-Grained Materials 



If the X-ray-diffraction data were obtained from powder patterns, 

 the process of qualitative and semiquantitative identification is consider- 

 ably simpler than if only single-crystal rotation patterns were available. 

 For identification of a specimen from a powder-diffraction pattern, the 

 radii, or diameters, of all lines in the pattern are measured, and the inter- 

 planar spacings calculated. The details of the procedure to be followed 

 depend on the nature of the unknown and on the amount of other data 

 available, such as optical and physical properties and chemical analyses. 



If the unknown represents a pure compound or a mixture composed 

 essentially of one constituent with only minor amounts of other ingredi- 

 ents and nothing is known concerning the identity of the compound or 

 the principal ingredient, the Hanawalt method of identification is used. 



The Hanawalt method, recommended by the American Society for 

 Testing Materials, is based upon a card-file index system catalogued 

 according to the three strongest lines in the pattern. After the pattern 

 of the unknown has been measured, converted into interplanar spacings, 

 the intensity of the lines estimated, and at least the three strongest lines 

 (more if the three strongest lines are not outstanding) identified, the group 

 of cards representing materials for which the strongest line corresponds 

 to the same interplanar spacing as does the strongest line in the pattern 



^^ Wyckofif, R. W. Gv, Crystal Structures, New York, Interscience Publishers, Inc., 1948. 



