When a mineral
thin-section is rotated on a microscope
stage, some coloured minerals change colour between two 'extremes', each of which is seen twice during a full 360-degree rotation.
Such a mineral is said to be pleochroic, and ferromagnesian minerals such as biotite possess this property.
Pleochroism is explained by the unequal absorption of light by the mineral in different orientations. For example, in a longitudinal section of biotite, when plane polarized light enters the mineral, considerable absorption of light occours and the biotite appears dark brown. If the mineral section is then rotated 90 degrees, much less absorption of light occours and the colour seems to be pale yellow.
In this example, the colour change is due to the angle of cleavage in the mineral section to the vibrational direction of the light entering it. When the cleavage is parallel to the vibrational direction, the mineral appears dark since most light is absorbed. When rotated, the cleavage is at right angles to the vibrational direction and less light is absorbed, hence the lighter colour.