Radiohalos or 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 (Faure 1986). 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 particle's energy (Henderson & Bateson 1934). The phenomenon of radiohalos has been known to geologists since the early part of the 20th century, but wider interest has been prompted by the unsupported claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992).
Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Because of their continuous energy distribution and greater range, beta particles cannot form distinct rings.)
The final characteristics of the radiohalo depends upon the initial isotope, and the size of each ring of a radiohalo is dependent upon the alpha decay energy. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a radiohalo formed from polonium has only one, two, or three rings depending on which isotope is the starting material (Weber 2010). 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 light microscope (Pal 2004).
Robert V. Gentry studied halos which appeared to have arisen from Po-218 rather than U-238 and concluded that solid rock must have been created with these polonium inclusions, which decayed with a half-life of 3 minutes. They could not have been formed from molten rock which took many millennia to cool (the standard theory) because polonium decays in a few minutes. This is taken by creationists as evidence that the Earth was formed instantaneously (Gentry 1992).
Critics of Gentry, including Thomas A. Baillieul (Baillieul 2005) and John Brawley (Brawley 1992), have pointed out that Po-218 is a decay product of radon, which as a gas can be given off by a grain of uranium in one part of the rock and migrate to another part of the rock to form a uraniumless halo. Apparently a large number of radon atoms are caught or absorbed at a particular point. This phenomenon has not yet been observed experimentally, but is supported by the fact that Gentry's "polonium halos" are found along microscopic cracks in rocks that also contain uranium halos (Wakefield 1988).
Gentry's work has been continued and expanded by the creationist Radioisotopes and the Age of the Earth (R.A.T.E.) project that was operating between 1997 and 2005 (Wieland 2003). However, most scientists such as Collins (1997), Wakefield (1988) have offered rebuttals of the radiohalo evidence for a young Earth.
Gentry's hypothesis has not appeared in any peer reviewed journals, and the scientific consensus is that Gentry's work is not supported by facts. In addition, Gentry's hypothesis has not proven to provide any predictive power.
Baillieul, T.A. (2005), "Polonium Haloes" Refuted: A Review of "Radioactive Halos in a Radio-Chronological and Cosmological Perspective" by Robert V. Gentry, TalkOrigins Archive (published 2001–2005)
Brawley, J. (1992), Evolution's Tiny Violences: The Po-Halo Mystery, TalkOrigins Archive
Collins, L.G. (1997), "Polonium Halos and Myrmekite in Pegmatite and Granite", in Hunt, C. W., Collins, L. G., and Skobelin, E. A., Expanding Geospheres, Energy And Mass Transfers From Earth’s Interior, Calgary: Polar Publishing Company, pp. 128–140 CS1 maint: Uses editors parameter (link).
Durrani, S.A.; Fremlin, J.H.; Durrani, S. A. (1979), "Polonium Haloes in Mica", Nature (published October 1979), 278 (5702): 333–335, Bibcode:1979Natur.278..333H, doi:10.1038/278333a0 .
Ellenberger, C.L., with reply by Gentry, R.V. 1984. "Polonium Halos Redux," Physics Today. December 1984. pp. 91–92
Ellenberger, C.L. 1986. "Absolute Dating," unanswered surrebuttal to Gentry, Physics Today. March 1986. pp. 152, 156
Faure, Gunter (1986), Principles of Isotope Geology, Wiley, pp. 354–355 .
Gentry, R.V. (1970), "Giant Radioactive Halos: Indicators of Unknown Alpha-Radioactivity?" (PDF), Science (published August 1970), 169 (3946): 670–673, Bibcode:1970Sci...169..670G, doi:10.1126/science.169.3946.670, PMID 17791843 .
Gentry, R.V. (1975), "Spectacle Haloes", Nature (published October 1975), 258 (5532): 269–270, Bibcode:1975Natur.258..269G, doi:10.1038/258269c0 .
Gentry, R.V. (1973), "Radioactive Halos", Annual Review of Nuclear Science (published October 1973), 23 (1): 347–362, Bibcode:1973ARNPS..23..347G, doi:10.1146/annurev.ns.23.120173.002023 .
Gentry, R.V. (1974), "Radiohalos in a Radiochronological and Cosmological Perspective" (PDF), Science (published October 1974), 184 (4132): 62–66, Bibcode:1974Sci...184...62G, doi:10.1126/science.184.4132.62, PMID 17734632 .
Gentry, R.V. (1992), Creation's Tiny Mystery, Earth Science Associates (published 2004) .
Henderson, G.H.; Bateson, S. (1934), "A Quantitative Study of Pleochroic Haloes, I", Proceedings of the Royal Society of London, Series A, Containing Papers of a Mathematical and Physical Character, 145 (855): 563–581, Bibcode:1934RSPSA.145..563H, doi:10.1098/rspa.1934.0120, JSTOR 2935523 .
Henderson, G. H., "A quantitative study of pleochroic halos: V, The genesis of halos", Proc. Roy. Soc. A, 173:250–264, 1939.
Henderson, G. H., and F. W. Sparks, "A quantitative study of pleochroic halos, IV, New types of halos", Proc. Roy. Soc. A, 173:238–249, 1939.
Lide, David R. (Ed.) (2001), CRC Handbook of Chemistry and Physics, 82nd Ed., London: CRC Press, ISBN 0-8493-0482-2
Moazed, Cyrus; Richard M. Spector; Richard F. Ward, 1973, Polonium Radiohalos: An Alternate Interpretation, Science, Vol. 180, pp. 1272–1274.
Odom, L.A., and Rink, W.J., 1989, "Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle", Science, v. 246, pp. 107–109.
Osmon, P., 1986, "Gentry’s pleochroic halos: Creation/Evolution," Newsletter, Feser, Karl D., Editor, v. 6, no. 1, Concord College, Athens, West Virginia
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" (PDF), Current Science, 87 (published 10 September 2004), n5: 662–667 .
Schadewald, R., 1987. "Gentry’s tiny mystery, Creation/Evolution" Newsletter, Fezer, Karl D, Editor, v. 4, no. 2 & 3. Concord College. Athens. West Virginia, p 20.
Schnier, C (2002), "Indications for the existence of superheavy elements in radioactive halos", Journal of Radioanalytical and Nuclear Chemistry (published August 2002), 253 (2): 209–216, doi:10.1023/A:1019633305770 .
Wakefield, J. R. (1988), "The geology of 'Gentry's Tiny Mystery'", Journal of Geological Education, 36: 161–175 .
Wakefield, J. R., 1987–88, "Gentry’s Tiny Mystery - unsupported by geology," Creation/Evolution, v. 22, p. 13–33.
Weber, B. (2010). "Halos und weitere radioaktive Erscheinungen im Wölsendorfer Fluorit (in German)". Der Aufschluss. 61: 107–118.
York, D., 1979, Pleochroic Halos and Geochronology, EOS, v. 60, no. 33, pp. 617–618, Aug. 14, 1979 (publication of the American Geophysical Union).
