224 SUBSUKFACE GEOLOGIC METHODS 



the film is wide enough to include all orientation arcs. On the other 

 hand, these effects frequently are advantageous in that they give some 

 idea concerning the orientation of the crystallographic planes producing 

 these arcs. Moreover, because the orientation arcs are darker than would 

 be tlie equivalent complete circular line, lower percentages of platy or 

 fibrous minerals can be detected in mixtures than would otherwise be 

 detected. 



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 with narrow, sharp lines with maxi- 

 mum 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.^*' 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 condi- 

 tion 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 mate- 

 rials in which the particles are randomly distributed, a powder diffraction 

 is obtained. If the particles are not arranged randomly, the materials 

 first should be crushed with a miscrospatula 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. 



Recently a camera has been developed in the Bureau of Reclamation 

 laboratories to study materials in petrographic thin sections that are not 

 identifiable by microscopic methods. The area on which the pattern 

 is obtained is approximately 0.008 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 

 produce 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 



^^ Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 

 Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 19-12. 



