In the biotite-phlogopite solid solution series, the term biotite refers to minerals with Mg/(Mg+Fe) < 0.67.  Biotite is a common pleochroic mineral in many igneous and metamorphic rocks, and views A and B, at right angles to one another, illustrate the light-brown to dark brown pleochroism of this sample.  Other examples may be pleochroic in shades of green and yellow.  Extinction is parallel to the prominent micaceous cleavage and pinacoid surfaces (note that the EW oriented grain in the upper left of C is extinct in D), and grains near extinction have a “crinkly” or “mottled” appearance, sometimes referred to as a “birds-eye maple” pattern.  Muscovite, talc and some chlorites also have this crinkly appearance.  The maximum interference colours of third order in D are typical, but this can be difficult to discern in dark brown grains.  The green mineral in A, B and C is chlorite, replacing some of the biotite.  Also visible, especially in A, are dark “pleochroic halos”.  Sample is from the Hepburn Lake Batholith, Wopmay Orogen, N.W.T. Canada. All views 2.2 mm across.  A, B, C ppl, D x-nicols.