Methods for Specifying Quartz Crystal Orientation and Their Determination by Optical Means

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 insoluble 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.