Pleochroic halos are dark rings of various radii seen in mica and other minerals.
There is general agreement that alpha particles emitted by radioactive isotopes create the halos. The radii of the rings are proportional to the alpha particle energy, and can thus identify the isotopes in the mineral. Some halos, however, are apparently formed by very short-lived polonium isotopes without any trace of parent uranium isotopes. York argues the case for selective local chemical concentration of polonium from fluids in the surrounding rocks. The captured polonium atoms decay almost immediately while the fluid containing the parent atoms passes on.