Biotite schist;
Isle of Mull, Scotland
In this specimen of coarse-grained schist you can see much
more clearly the flakes of dark mica, lined up parallel to
each other, and separated by light-coloured material, which
is mostly millimetre-sized quartz crystals. In the photo
above you are looking sideways at the schistosity, so you
see the thin edges of the mica flakes.