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). The most widely accepted 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, as John Joly (1917), but wider interest was prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992). These claims are rejected by the scientific community as an example of creationist pseudoscience (Wakefield 1988).
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.)
|Isotope||Half-life||Energy in MeV|
|U-238||×109 years 4.47||4.196|
|U-234||×105 years 2.455||4.776|
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; Ra-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a lighted microscope (Pal 2004), but Ra-222 and Po-210 rings can be distinguished by other means (Gentry 1974).
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 has not been proved 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 Radioactivity and the Age of the Earth (R.A.T.E.) project that was operating between 1997 and 2005 (Wieland 2003). However, Collins (1997), Wakefield (1988) and others have repeatedly offered rebuttals of the radiohalo evidence for a young Earth in peer-reviewed publications.
- 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.
- Durrani, S.A.; Fremlin, J.H.; Durrani, S. A. (1979), "Polonium Haloes in Mica", Nature (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 (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 (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 (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 (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 (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 (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).
Favoring a young earth interpretation
- Polonium radiohalos: still "a very tiny mystery", from the Institute for Creation Research
- Fingerprints of Creation
- Radiohalos and Diamonds: Are Diamonds Really for Ever?
Favoring an old earth interpretation
- Geology of Gentry's "Tiny Mystery", J. Richard Wakefield, Journal of Geological Education, May 1988.
- Examining radiohalides, R. H. Brown, H. G. Coffin, L. J. Gibson, A. A. Roth, and C. L. Webster, Origins 15:32-38 (1988).
- Origin of Polonium Halos, Lorence G Collins, Barbara J Collins, Reports of the National Center for Science Education Vol. 30 Issue 5:11-16 (2010).