Figure 50 (a) Biotite, showing its strong basal cleavage (larger crystal is 7.5 cm across). (b) Plane-polarised light image of biotite in granite. The biotite displays strong brown pleochroism. The cleavage traces are also obvious and run NW-SE along the length of the grain. Dark circles in the biotite surround small grains of the mineral zircon, which contains small amounts of radioactive uranium. The dark circles are called pleochroic haloes and are due to radiation damage in the mineral structure (field of view 5.5 mm across). (c) The same field of view as in (b) between crossed polars; although biotite has second-order interference colours, they are masked by its strong body colour.