Biotite is a mineral belonging to the mica family. It is normally biaxial but often found with a very low angle between the optic axis. The section below, nearly cut normal to the acute bisectrix has a so small 2V angle that it appears as a uniaxial mineral. As the color of biotite is so strong (it appears brown or black in thick section), it modifies the normal interference colors by adding a strong yellow hue to the figures. The rules applied to the uniaxial conoscopic figures can be used to determine the optical sign. The addition of a wave plate and a quarter wave plate in figures 3 and 4 gives the characteristic of a negative crystal optics.
Phlogopite which is also a member of the biotite group is shown below in figures 5 and 6. It is also pseudo uniaxial. Its deep green color also modified the appearance of the interference colors. As shown in figure 6, its optic sign is also negative.
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Figure 1. Biotite crystal nearly normal to acute bisectrix. |
Figure 2. Same crystal rotated counterclockwise by an angle of 90°. |
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Figure 3. Addition of wave plate to the crystal in position 2. |
Figure 4. Same with addition of a quarter wave plate. |
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Figure 5. Phlogopite crystal cute normal to acute bisectrix. |
Figure 6. Same Phlogopite crystal with addition of a wave plate. |
