Sometimes, radioactive minerals are included in other minerals. Radioactive decay may then cause burn marks, called pleochroic halos, or just halos. Two examples are shown below.

Biotite With a Halo Around a Zircon Inclusion

This sample is a biotite-cordierite-orthopyroxene gneiss from near Sioux Lookout, Ontario. The field of view, about 2 mm across, is dominated by olive-green-brown biotite.

The largest grain of biotite contains a pelochroic halo (burn mark) around an included zircon. Although hard to identify in this view, the clear to light green, high relief grains to the left and beneath the biotite are orthopyroxene. The large clear (PP) grain to the lower right is cordierite. which, in this view, looks like quartz.

The biotite grain in the lower left shows one good cleavage, but the large grain in the center does not because the view is more or less perpendicular to the cleavage. It does, however, show a partially developed "birds eye" pattern in XP light.

 Cordierite, Orthopyroxene and Biotite from near Kazabazua, Quebec

The photos above are of a spectacular rock from near Kazabazua, Quebec. It contains primarily cordierite and orthopyroxene with lesser amounts of bioite. The cordierite is twinned (and in XP looks a lot like plagioclase) and contains pleochoric halos around zircon inclusions. The halos appear as "burn" marks in PP light. The orthopyroxene is blocky and fractured, shows high relief (PP), and has first order interference colors (XP). It is pleochroic, with color ranging from light pink to light green. Biotite is also pleochroic and here exists as flakes in various shades of brown (PP) and has upper 2nd order interference colors (XP).