Extinction (optical mineralogy)
![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.](/Images/godic/202501/12/LvMS-Lvv0944.jpg")
![Undulose extinction in quartz.](/Images/godic/202501/12/Quartz-zhaseni0944.gif")
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, or amorphous materials (glass) show no light (i.e. constant extinction). Anisotropic minerals will show one extinction for each 90 degrees of stage rotation.