234 Subsurface Geologic Methods 



tains the formulas (chemical and structural for organic compounds), 

 name, and complete pattern data. The cards are arranged into small 

 Hanawalt groups of convenient size for values of the strongest line, and 

 each group is arranged in numerical sequence according to the values of 

 the second strongest line. This difference between the old and the revised- 

 card indices will, of course, alter the above-described procedure somewhat 

 when the revised index is used. With the revised index, the search of the 

 diffraction-data file starts with two lines chosen from the unknown pattern 

 as the strongest and second strongest. If this choice does not locate a 

 corresponding X-ray pattern, it is necessary to reverse the order of the 

 lines and search again. 



It may even be necessary to try various other combinations of strong 

 lines in the pattern before the identification can be made. For those who 

 wish to continue the original method of searching the data file, the Society 

 offers additional sets of the revised cards at reduced prices. A numerical 

 index is also supplied with the revised sets of cards. This index has listings 

 arranged in Hanawalt groups with three variations for the three strongest 

 lines in each pattern: namely, first, second, third; second, first, third; and 

 third, first, second. 



When considerable investigation is being carried out in a limited 

 field, or if sufficient optical or other data are available so that the possible 

 compounds in unidentified specimens are relatively small, it frequently 

 is advantageous to build up a file of patterns of standard materials. These 

 patterns can then be used for identifying unknowns by direct comparison 

 with their patterns. Plate 7 illustrates this method. However, it is to be 

 strongly emphasized that extreme caution must be observed in selecting the 

 materials for these standard patterns. Errors in identification are found 

 frequently even for specimens obtained from established museum and 

 private mineral collections. 



Direct comparison of patterns, when used together with the Hanawalt 

 method described above, is the most satsifactory for identification of 

 materials, both in accuracy and time saved in the analysis. Occasionally, 

 the Hanawalt method fails for mixtures because several strong lines of 

 different ingredients fall in juxtaposition on the pattern and consequently 

 are considered as a single broad line in the interpretation of the data, 

 thus considerably displacing the position of the line in question. If the 

 probable constitution of the mixture can be surmised, direct comparison 

 with standard patterns will immediately disclose such situations, and errors 

 and time-consuming labor are avoided. 



Sometimes unit-cell data are available in the literature when powder 

 data are lacking. Unit-cell data for a known material can be used to 

 establish the identity of an unknown material from which a powder-dif- 

 fraction pattern has been obtained. This method is practicable only if 

 some clue suggests the identity of the unknown, and the number of known 

 materials to be compared with the unknown is small. The comparison of 



