Extinction (optical mineralogy)

Extinction is a term used in optical mineralogy and petrology, which describes when cross-polarized light dims, as viewed through a thin section of a mineral in a petrographic microscope.

Isotropic minerals, opaque (metallic) minerals, and amorphous materials (glass) do not allow light transmission under cross-polarized light (i.e. constant extinction).

Anisotropic minerals specifically will show one extinction for each 90 degrees of stage rotation.

To find this, simply line up the cleavage lines/long direction with one of the crosshairs in the microscope, and turn the mineral until the extinction occurs.

You can help Wikipedia by expanding it.This optics-related article is a stub.

A sand grain of volcanic glass under the petrographic microscope . Its amorphous nature makes it go extinct in cross-polarized light (bottom frame), and thus does not have an extinction angle. Scale box in millimeters.
Undulose extinction in quartz .