CHAPTER II 



Methods For Specifying Quartz Crystal Orientation and 

 Their Determination by Optical Means 



By W. L. BOND 



2.1 Quartz and its Axes • 



The chemist describes quartz as silicon dioxide, Si02, crystallized in 

 hard, brittle, glass-like, six sided prisms, often with pyramidal terminations; 

 melting point 1750° Centigrade, density 2.65, hardness on Moh's scale 7. It 

 transforms from alpha to beta quartz at 573°C under atmospheric pressure. 

 Under stress it transforms at lower temperatures. Alpha quartz is in- 

 soluble in ordinary acids but soluble in hydrofluoric acid; and in hot alkalis. 

 At first glance we might say that it had hexagonal symmetry but if we 

 etch two adjacent pyramid faces we find that the microscopic etch pits 

 are of different shape, hence the faces cannot be equivalent. It has three 

 axes of two-fold symmetry and one axis of three-fold symmetry. Let us 

 also remark that it does not have a center of symmetry or a six-fold axis. 

 Figure 2.1 shows us that the three two-fold axes are perpendicular to the 

 three-fold axis and are 120° apart. If they were not like this, they would 

 not be self-consistent. 



As we examine more and more quartz crystals we find that there is a 

 tendency for pyramid faces to be alternately large and small, the larger 

 faces being brighter than the smaller faces. Also the etch pits of alternate 

 faces are similar. (The etch pit study is a powerful tool in determining 

 crystal symmetry.) Further, two other "kinds" of faces are quite 

 commonly found. If we draw such a crystal as though equivalent faces 

 were of equal size we get such a picture as Fig. 2.2. It is an idealized figure 

 used to illustrate the symmetry of quartz. The prism faces are marked 

 w, the six faces marked r "constitute the primary rhombohedron" — the 

 ones we called the large bright pyramid faces. The crystallographer 

 thinks of these six faces as pieces of the faces of a rhombohedron. (A 

 crystallographer's rhombohedron is like a cube stood on one corner, then 

 the opposite corner pushed in a little towards the other, or pulled away 

 from it. He thinks of it always as standing on this corner, as Fig. 2.3.) 

 The z faces constitute a second rhombohedron — the secondary rhom- 

 bohedron or minor pyramid faces. The s and x faces illustrate a further 

 property of quartz. Figure 2.3 differs from its mirror image so that we 



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