ETCH TECHNIQUE 25 



Choice of one of the four above methods of examining etched surfaces for 

 twinning and orientation, depends upon many factors, as will be noted in the 

 following section. The pinhole method is used wherever possible because of 

 the simplicity of the optical system and the brilliance of the figures obtained. 



5.5 Etch-Figure Instruments 



Herein are described several instruments which have been designed for 

 shop use in determining orientation and twinning of etched quartz sections 

 and slabs. Their basic principles of operation are as described above. The 

 nomenclature of handedness, sense of axes, sense of cuts, natural faces, etc. 

 is according to Fig. 5.4, as explained at the end of Section 5.2. 



The etch-figures and reflection patterns obtained on these instruments 

 van*' with the preparation of the specimen (i.e. the type of grind and the type 

 of etch). A complete study of these factors would include a variation of the 

 grind from a ver\- coarse grind to polishing (and include saw-cut surface), 

 and a variation of the etching time from short to very long, and the strength 

 and kind of etching agent. Here chosen for illustration are the simplest 

 practical preparations, namely, the coarsest grind usable, and the shortest 

 etching time (in hydrofluoric acid). The etch-flgures are thus markedly 

 different than some which have appeared in the literature. Further, the 

 photographic reproduction of etch-figures on paper, is not exact due to the 

 limited contrast range of the paper. Thus in the accompanying illustrations 

 detail is lost in the brilliant portions of the etch-figures in order to show de- 

 tails in the weaker portions, and vice-versa. ^^ 



5.51 The Reflection Oriascope 



Fig. 5.11 shows diagrammatically a reflection "Oriascope", which may be 

 used on specimens with a single flat etched surface. By the reflection prin- 

 ciple of Section 5.41 figures are obtained on a viewing screen. Due to the 

 relatively weak figures obtained by reflection from weakly etched surfaces, 

 the viewing screen must be enclosed in a well blackened enclosure, and 

 viewed through an eye chute. The screen is ruled with appropriate lines, 

 relative to which the figure is aligned by turning the specimen on the table. 

 The table is mounted so that when the specimen is properly oriented, the 

 table may be slid to the right or left over a marking template, and marked 

 through the template with appropriate lines to indicate the desired axial 

 orientations of the specimen. 



When used with Z-cut sections it is necessary to have two marking 

 templates, one for each handedness of the quartz, since the three-fold figures 



"Apparent shifts in etch-figure orientation, with etching time for example, are not 

 to be considered as resulting from an orientation shift of the individual etch-pit-/ace/j, 

 but as a shift in the relative areas of differently oriented facets. See Figs. 5.12 and 5.17. 



