Under cross-polarized light, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye maple extinction, a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section.
When viewed under cross-polarized light, biotite displays extinction patterns that align with its cleavage planes, sometimes exhibiting a distinctive "bird's eye maple" pattern due to the deformation of the mineral's lamellae during the preparation of thin sections.