Pleochroic halo: Difference between revisions
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The final characteristics of the radiohalos depend upon the initial isotope. The U-234 and Ra-226 rings coincide, with the Th-230 ring merely thickening it,{{harv|Pal|2004}} so it is hard to tell which one of those isotopes started the halo, but it is easy to tell a polonium halo from a uranium halo. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable, while a radiohalo formed from polonium can have only one, two, or three rings depending on which isotope is the starting material.{{Fact|date=May 2008}} However, a halo formed from Ra-222 is not distinguishable from one formed from Po-210.{{harv|Wakefield|1988}}{{harv|Pal|2004}} |
The final characteristics of the radiohalos depend upon the initial isotope. The U-234 and Ra-226 rings coincide, with the Th-230 ring merely thickening it,{{harv|Pal|2004}} so it is hard to tell which one of those isotopes started the halo, but it is easy to tell a polonium halo from a uranium halo. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable, while a radiohalo formed from polonium can have only one, two, or three rings depending on which isotope is the starting material.{{Fact|date=May 2008}} However, a halo formed from Ra-222 is not distinguishable from one formed from Po-210.{{harv|Wakefield|1988}}{{harv|Pal|2004}} |
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[[Robert V. Gentry]] has found considerable evidence that giant radiohalos found in Madagascar mica result from the decay chain of unidentified primordial [[Superheavy element]]s in the earth's crust {{harv|Gentry|1970}}. Gentry's work was continued by Christian Schnier {{harv|Schnier|2002}} who postulated the presence of two long-lived superheavy elements, one with an [[atomic mass]] of between 280 and 300, and another with an atomic mass greater than 300. |
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==Controversy== |
==Controversy== |
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{{seealso|Creation geophysics}} |
{{seealso|Creation geophysics}} |
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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.{{Fact|date=May 2008}} 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 [[creationist]]s as evidence that the Earth was formed instantaneously {{harv|Gentry|1992}}. |
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Critics of Gentry, including Thomas A. Baillieul {{harv|Baillieul|2005}} and John Brawley {{harv|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 adsorbed 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 {{harv|Wakefield|1988}}. |
Critics of Gentry, including Thomas A. Baillieul {{harv|Baillieul|2005}} and John Brawley {{harv|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 adsorbed 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 {{harv|Wakefield|1988}}. |
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*{{citation| last=Faure|first=Gunter |title=Principles of Isotope Geology|year=1986|publisher=Wiley |pages= 354–355 }}. |
*{{citation| last=Faure|first=Gunter |title=Principles of Isotope Geology|year=1986|publisher=Wiley |pages= 354–355 }}. |
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*{{citation | last=Gentry|first=R.V. | title=Giant Radioactive Halos: Indicators of Unknown Alpha-Radioactivity? | journal=Science |publication-date=August 1970 |year=1970| | volume= 169 | pages=670-673 | url=http://www.halos.com/reports/science-1970-giant-halos.pdf}}. |
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*{{citation | last=Gentry|first=R.V. | title=Spectacle Haloes | journal=Nature | publication-date=October 1975| year=1975 | volume=258 | pages=269–270}}. |
*{{citation | last=Gentry|first=R.V. | title=Spectacle Haloes | journal=Nature | publication-date=October 1975| year=1975 | volume=258 | pages=269–270}}. |
Revision as of 22:02, 30 January 2009
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 sphene 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, 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). The claims are contested by the mainstream scientific community as an example of creationist pseudoscience (Wakefield 1988).
Production
Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (According to the standard theory, beta particles do not discolour the rock, although Baillieul (2005) suggests the need to reexamine the possible role of beta emission.)
Isotope | Half-life | Energy in MeV |
---|---|---|
U238 | 4.47×109 years | 4.196 |
U234 | 2.455×105 years | 4.776 |
Th230 | 75400 years | 4.6876 |
Ra226 | 1599 years | 4.784 |
Rn222 | 3.823 days | 5.4897 |
Po218 | 3.04 minutes | 5.181 |
Po214 | 163.7 microseconds | 7.686 |
Po210 | 138.4 days | 5.304 |
Pb206 | stable | 0 |
The final characteristics of the radiohalos depend upon the initial isotope. The U-234 and Ra-226 rings coincide, with the Th-230 ring merely thickening it,(Pal 2004) so it is hard to tell which one of those isotopes started the halo, but it is easy to tell a polonium halo from a uranium halo. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable, while a radiohalo formed from polonium can have only one, two, or three rings depending on which isotope is the starting material.[citation needed] However, a halo formed from Ra-222 is not distinguishable from one formed from Po-210.(Wakefield 1988)(Pal 2004)
Robert V. Gentry has found considerable evidence that giant radiohalos found in Madagascar mica result from the decay chain of unidentified primordial Superheavy elements in the earth's crust (Gentry 1970). Gentry's work was continued by Christian Schnier (Schnier 2002) who postulated the presence of two long-lived superheavy elements, one with an atomic mass of between 280 and 300, and another with an atomic mass greater than 300.
Controversy
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.[citation needed] 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 adsorbed 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 creationist 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 and soundly rebutted the radiohalo evidence for a young Earth in peer-reviewed publications.
Citations
- 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)
{{citation}}
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- 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", Expanding Geospheres, Energy And Mass Transfers From Earth’s Interior, Calgary: Polar Publishing Company, pp. 128–140
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- Durrani, S.A.; Fremlin, J.H. (October 1979), "Polonium Haloes in Mica", Nature, 278: 333–335, 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, 169 (published August 1970): 670–673
{{citation}}
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(help).
- Gentry, R.V. (1975), "Spectacle Haloes", Nature, 258 (published October 1975): 269–270.
- Gentry, R.V. (October 1973), "Radioactive Halos", Annual Review of Nuclear Science, 23: 347–362, doi:10.1146/annurev.ns.23.120173.002023.
- Gentry, R.V. (October 1974), "Radiohalos in a Radiochronological and Cosmological Perspective", Science, 184: 62–66, doi:10.1126/science.184.4132.62.
- 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, doi:10.1098/rspa.1934.0120
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and|issue=
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- 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.
- 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 n5 (published 10 September 2004): 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 (August 2002), "Indications for the existence of superheavy elements in radioactive halos", Journal of Radioanalytical and Nuclear Chemistry, 253: 209–216, doi:10.1023/A:1019633305770
{{citation}}
: CS1 maint: extra punctuation (link).
- 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.
- Wieland, C. (21 August 2003), RATE group reveals exciting breakthroughs!, Answers in Genesis
- Moazed, Cyrus; Richard M. Spector; Richard F. Ward, 1973, Polonium Radiohalos: An Alternate Interpretation, Science, Vol. 180, pp. 1272-1274.
- York, D., 1979, Pleochroic Halos and Geochronology, EOS, v. 60, no. 33, pp. 617-618, Aug. 14, 1979 (publication of the American Geophysical Union).
External links
Favoring a young earth interpretation
- Polonium radiohalos: still "a very tiny mystery", from the Institute for Creation Research
- Fingerprints of Creation
Disputing a young 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(1):32-38 (1988).
- Talkorigins FAQ on the criticisms of Gentry's interpretation
- Answers In Creation expose showing that radiohalos do not support a young earth