Radiohaloes, or pleochroic haloes, are concentric rings (actually spheres) of crystal damage centering on a mineral grain containing radiogenic isotopes such as Uranium. Alpha particles, slowing as their energy decreases, cause the damage to the crystal's lattice structure. The diameter of each ring is proportional to the energy of the particles that formed it.
Haloes are found in many minerals; biotites are most used, because they break into thin flakes which show the haloes (cross sections of damage spheres) easily.
2010 Arthur V. Chadwick, Ph.D.