Pleochroic halo


Pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks. The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure. The inclusions are typically zircon, apatite, or titanite which can accommodate uranium or thorium within their crystal structures. One explanation is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particles' energy.

Production

follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence.
IsotopeHalf-lifeEnergy in MeV
U-238 years4.196
U-234 years4.776
Th-23075,400 years4.6876
Ra-2261,599 years4.784
Rn-2223.823 days5.4897
Po-2183.04 minutes5.181
Po-214163.7 microseconds7.686
Po-210138.4 days5.304
Pb-206stable0

The final characteristics of a pleochroic halo depends upon the initial isotope, and the size of each ring of a halo is dependent upon the alpha decay energy. A pleochroic halo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a halo formed from polonium has only one, two, or three rings depending on which isotope the starting material is. In U-238 haloes, U-234, and Ra-226 rings coincide with the Th-230 to form one ring; Rn-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a petrographic microscope.