Topaz nearly normal to acute bisectrix.
Figure 1 shows a thin topaz crystal cut nearly normal to the acute bisectrix viewed with its optic plane oriented at 45°. One pole of optic axis is located on the left, just on the border of the figure, the other pole being slightly out of the field of view. The 2V angle of this topaz is estimated between 50° and 60°. For that microscope stage position, the isogyres are out of the field. The addition of a wave plate produces a decrease of the interference colors between the melatopes along the optic plane (figure 2 b). This observation is the characteristic of a positive optical sign. If this crystal is rotated by 90°, we observe an addition of the retardations between the optic axes and so, high interference colors (figure 1c). This is just the opposite to what was described for the muscovite mineral.
In figure 2, we have a thick topaz crystal, in fact the same as the one used to make the pictures in figure 1 just just before the abrasion step. The 0° figure (2a) has high interference colors except around the visible melatope on the left side. The dark isogyres indicate the position of the center of figure slightly displaced to the right due to the cutting angle. A slight rotation of the stage illustrates the movement of the isogyres (figure 2b). At 45 °, the isogyres leave the field so the faint high order isochromes appear ( figure 2c). The pattern of the isochromes is better visible in monochromatic light (628 nm) in figure 3a and b.
Topaz nearly parallel to optic plane.