than would be the equivalent complete circular line, lower percentages of platy 

 or fibrous minerals can be detected in mixtures than would otherwise be observed. 



Furthermore, use of the Duco cement permits preparation of a thinner 

 sample than would the recommended capillaries and consequently makes it 

 possible to obtain a pattern of narrow, sharp lines with maximum resolution. 

 With this type of sample mounting, several lines are frequently obtained at the 

 average position of a single broad line reported in the literature. 



Other methods, such as affixing the powder to strings, hair, wire, and glass 

 rods, have been suggested also (E43-42T, 1942). These mounts commonly 

 produce abnormal effects and do not appear desirable because of the difficulties 

 involved in obtaining representative samples and the large amount of foreign 

 material (the rod and binder) included in the sample. With glass rods, double 

 lines frequently are obtained in the pattern, a condition that is very undesirable, 

 especially in analysis of mixtures. 



If only a very limited amount of material is available, a small lump 0.1 

 to 0.2 mm in diameter can be mounted with mucilage or Duco cement on the 

 end of a very thin glass rod. For very fine-grained materials in which the 

 particles are randomly distributed, a powder diffraction pattern is obtained. 

 If the particles are not arranged randomly, the materials first should be crushed 

 with a microspatula and the powder worked into a tiny ball with a binder. Such 

 samples require no more than a few micrograms of material and produce 

 satisfactory patterns at approximately double the usual exposure time. 



Flat4ilm cameras (fig. 9-6) can readily be modified to study materials 

 in petrographic thin sections that are not identifiable by microscopic methods. 

 The area on which the pattern is obtained is approximately 0.005 inch in 

 diameter, or about twice the thickness of an average sheet of paper. For this 

 procedure the slide is warmed to soften the mounting medium, and the thin 

 section slid over so that the region to be studied projects over the edge of the 

 glass slide. The cover glass is retracted at the same time. The specimen is 

 mounted in the camera under the petrographic microscope to insure centering of 

 the selected area in the beam. The sample is rotated during the exposure to pro- 

 duce smooth, uniform lines in the pattern. After the pattern has been recorded, 

 the slide is again warmed, and the thin section returned to its original position 

 and covered with the original cover glass. 



At present, reliable methods for mounting powder samples for studies with 

 the Geiger-counter apparatus seem to be lacking. Carl (1947) has described 

 a method which he found to yield satsifactory quantitative accuracy. Whatever 

 method is selected, it should be remembered that different materials pack 

 differently into the holder, and the operator should first check his technique 

 on a series of synthetic samples of known composition before attempting to 

 use it quantitatively or on unknown specimens. 



159 



