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.
The description explains how biotite appears under cross-polarized light, highlighting its characteristic extinction pattern and the "bird's eye maple" appearance that results from the deformation of its lamellae during thin section preparation.