This article is about the geophysical phenomenon. For the creationist interpretation, see Robert V. Gentry. For the astronomical phenomenon, see radio halo.
Pleochroic halos around crystals of zircons in a sample of biotite
Apleochroic halo, or radiohalo, is a microscopic, spherical shell of discolouration (pleochroism) within minerals such as biotite that occurs in granite and other igneous rocks. The halo is a zone 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 uraniumorthorium within their crystal structures.[1] 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.[2]
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.[3] 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.[4]
^Pal, Dipak C. (2004). "Concentric rings of radioactive halo in chlorite, Turamdih uranium deposit, Singhbhum Shear Zone, Eastern India: a possible result of 238U chain decay". Current Science. 87 (5): 662–667.
Collins, L.G. (1997). "Polonium Halos and Myrmekite in Pegmatite and Granite". In Hunt, C. W.; Collins, L. G.; Skobelin, E. A. (eds.). Expanding Geospheres, Energy And Mass Transfers From Earth's Interior. Calgary: Polar Publishing Company. pp. 128–140.
Schnier, C (2002). "Indications for the existence of superheavy elements in radioactive halos". Journal of Radioanalytical and Nuclear Chemistry. 253 (2): 209–216. doi:10.1023/A:1019633305770. S2CID120109166.
