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.


Figure 1. Biotite crystal nearly normal to acute bisectrix.

Figure 2. Same crystal rotated counterclockwise by an angle of 90.

Figure 3. Addition of wave plate to the crystal in position 2.

Figure 4. Same with addition of a quarter wave plate.

Figure 5. Phlogopite crystal cute normal to acute bisectrix.

Figure 6. Same Phlogopite crystal with addition of a wave plate.


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