The extinction angle of a given grain is the angle between any specified crystallographic
direction and either of the two vibration directions. It is an important distinguishing character
for different minerals (clino-pyroxenes). As with interference colors, a mineral in different
orientations will show different kinds of extinction. It is important to record either the nature
of the extinction shown by most grains of a mineral (i.e., parallel or inclined) and if inclined
extinction is displayed to record the maximum extinction angle shown. The following
generalizations apply:
* Minerals belonging to the tetragonal, hexagonal, or orthorhombic crystal
systems will in general
show straight (parallel) extinction.
* Minerals belonging to the monoclinic system will often show inclined extinction
but may
sometimes show straight extinction.
* Minerals belonging to the triclinic system will in general show inclined
extinction.