Radiation produced by decay of U and Th in allanite, zircon and some other accessory minerals, which damages their crystal structures, can also damage the structure of coexisting minerals. One common phenomenon that results from this is enhancement of the pleochroic colours of the host mineral.  This effect is shown in images A and B in a grain of biotite.  In A, in which the {001} cleavage is oriented north - south (perpendicular to the polarizer), the grain is pale brown.  Slightly darker brown circles are apparent around tiny high relief grains of zircon, some of which are so small that they only appear as black dots.  In B, the grain is rotated 90 degrees from A, and the {001} cleavage is parallel to the polarizer.  In this orientation, the entire grain is darker shade of brown than in A, and the dark circles around the tiny zircons (pleochroic halos) are much darker, and many more of them are obvious.  Images C and D illustrate the same effect in a small porphyroblast of chlorite.  A and B from a kyanite mica schist from Fernleigh, Ontario and C and D are from a garnet schist from the Odjick Formation, N.W.T.  Views are 2.2 mm across.  A and C ppl, B and D x-nicols.