Schisrosity develops only partly because the
different minerals separate into layers. The main
reason is that the continuous squeezing and shearing
of the rock during metamorphism allows crystals only
to grow in one plane, at right angles to the pressure.
Because the crystals that develop are tabular flakes such as mica, or needle-like crystals such
as amphiboles, this layering effect is even more marked. Whenever schist is split, it tends to
break along layers of mica within the rock, giving the slightly misleading impression that it
is entirely made of glistening mica. Quartz is also an important constituent, but the quartz
layers tend only to be seen clearly when the rock is
curacross the grain.