232 Subsurface Geologic Methods 



is selected from the index. The subgroup for which the second-most- 

 intense line corresponds to the same interplanar spacing as does the 

 second-strongest line in the pattern of the unknown is then examined for 

 correspondence between the third line of the cards and the third-strongest 

 line in the pattern. Finally, the entire pattern of the unknown is checked 

 against the pattern selected from the card index. This procedure is illus- 

 trated in plate 6. However, because of differences between the techniques 

 used in obtaining the data for the card index and that used by the oper- 

 ator in obtaining the pattern of the unknown, or because of variations 

 found in the patterns of some types of materials (to be discussed later) , 

 the operator should regard correspondence within ±: 0.05 A. as a satis- 

 factory match for interplanar spacing in comparing his patterns with 

 those recorded in the index. This same possible variation should be 

 allowed in selecting the groups of cards for comparison. 



Should the foregoing procedure be unsuccessful or if the specimen 

 to be identified is known to be a mixture of several ingredients all in 

 only small or moderate concentration, a somewhat different method of 

 identification must be used. In mixtures, each of the three strongest lines 

 may belong to patterns of different constituents so that the above procedure 

 (outlined in plate 6) could not be used. For relatively simple mixtures, 

 the procedure above may work if more (ten or twelve) of the strongest 

 lines are used in searching the card index. In general, however, only the 

 strongest line of the pattern can be used as a guide for selecting the 

 group of cards for comparison. All of the lines on each card of the 

 selected group are compared with the pattern of the unknown; bearing 

 in mind, of course, that at least all the strongest lines must be found in 

 the pattern of the unknown, with proper relative intensity. Checking of 

 only a few lines on a card is usually sufficient to indicate whether or not 

 agreement exists. When a card identifies part of the pattern of the mix- 

 ture, the lines belonging to the pattern of the identified constituent are 

 marked (on the pattern or a corresponding tabulation of data). The 

 procedure is now repeated for the remainder of the pattern, again start- 

 ing with the strongest remaining line. In this way all the constituents 

 of the mixture can be identified, provided their patterns are catalogued 

 in the index, when fluorescent scattering is small (recognized by light back- 

 ground in the X-ray pattern). The relative amounts of the ingredients 

 present are deduced from the relative intensities of the lines in the pattern, 

 as compared to the intensities of the lines in the pattern of the pure con- 

 stituents, the exposure times, of course, being the same for all patterns. A 

 series of underexposed patterns (1, 2^, 5, etc. percent of the total exposure 

 time) of the pure constituent in question will be of considerable help in 

 estimating these intensities. 



If the absolute proportion of each compound in the mixture is to 

 be determined, a synthetic specimen must be prepared from the identified 

 pure materials in such proportions that the synthetic mixture yields a 



