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.