a colored zone, usually showing pleochroic discoloration, that occurs around small inclusions of radioactive minerals, such as zircon, pyrochlore, monazite, and thorite, in translucent, colorless, or light-colored grains of other minerals, for example, micas, amphiboles, fluorites, quartz, and cassiterite.
Pleochroic halos form as a result of a change in the color of the enclosing mineral caused by the radioactive emission, primarily of α and β-particles, of the inclusion. The change in color is linked to the change in the charge of the atom-chromophore in the crystal lattice (for example, Fe2+ into Fe3+) or to different types of defects in the crystals (in quartz, fluorite, and other minerals). The diameter of pleochroic halos is small and corresponds to the possible trajectory length of the a-particle (α few microns) or β-particle (up to 2–3 mm). Pleochroic halos are observed in thin splinters using a polarization microscope.