Distinguishing between the two mineral groups with the microscope can be accomplished quickly by crossing the polars, with the following being obvious:

  1. All isotropic minerals will appear dark, and stay dark on rotation of the stage.
  2. Anisotropic minerals will allow some light to pass, and thus will be generally light, unless in specific orientations.

Why are isotropic materials dark?

  1. Isotropic minerals do no affect the polarization direction of the light which has passed through lower polarizer;
  2. Light which passes through the mineral is absorbed by the upper polar.

Why do anisotropic minerals not appear dark and stay dark as the stage is rotated?

  1. Anisotropic minerals do affect the polarization of light passing through them, so some component of the light is able to pass through the upper polar.
  2. Anisotropic minerals will appear dark or extinct every 90° of rotation of the microscope stage.
  3. Any grains which are extinct will become light again, under crossed polars as the stage is rotated slightly.

To see the difference between Isotropic vs. Anisotriopic minerals viewed with the petrographic microscope look atthe following images:

  1. Image 1 - plane light view of a metamorphic rock containing three garnet grains, in a matrix of biotite, muscovite, quartz and a large stauroite grain at the top of the image.
  2. Image 2 - Crossed polar view of the same image. Note that the three garnet grains are 'extinct" or black, while the remainnder of the minerals allow some light to pass.