Radiohalo
Encyclopedia
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 structure
s. The most widely accepted explanation is that the discolouration is caused by alpha particle
s emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy . 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 . These claims are rejected by the scientific community as an example of creationist
pseudoscience
.
-238 follows a sequence of decay
through thorium
, radium
, radon
, polonium
, and lead
. These are the alpha-emitting isotope
s in the sequence. (Because of their continuous energy distribution and greater range, beta particle
s 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. 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 , but Ra-222 and Po-210 rings can be distinguished by other means .
Giant radiohaloes caused some excitement when Robert V. Gentry
proposed that they resulted from the decay chain of unidentified primordial Superheavy elements .
Critics of Gentry, including Thomas A. Baillieul and John Brawley , 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 .
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 . However, , and others have repeatedly offered rebuttals of the radiohalo evidence for a young Earth in peer-reviewed publications.
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
that occur in granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
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
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, or titanite
Titanite
Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...
which can accommodate uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
or thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
within their crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s. The most widely accepted explanation is that the discolouration is caused by alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy . 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
Robert V. Gentry
Robert V. Gentry is a nuclear physicist and young earth creationist, known for his claims that radiohalos provide evidence for a young age of the Earth. He is a member of the Seventh-day Adventist Church.-Career:...
that radiohalos in biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
are evidence for a young earth . These claims are rejected by the scientific community as an example of creationist
Creationism
Creationism is the religious beliefthat humanity, life, the Earth, and the universe are the creation of a supernatural being, most often referring to the Abrahamic god. As science developed from the 18th century onwards, various views developed which aimed to reconcile science with the Genesis...
pseudoscience
Pseudoscience
Pseudoscience is a claim, belief, or practice which is presented as scientific, but which does not adhere to a valid scientific method, lacks supporting evidence or plausibility, cannot be reliably tested, or otherwise lacks scientific status...
.
Production
UraniumUranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
-238 follows a sequence of decay
Decay chain
In nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations...
through thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
, radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
, radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
, polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
, and lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. These are the alpha-emitting isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s in the sequence. (Because of their continuous energy distribution and greater range, beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
s cannot form distinct rings.)
| Isotope | Half-life | Energy in MeV |
|---|---|---|
| U-238 | years | 4.196 |
| U-234 | years | 4.776 |
| Th-230 | 75400 years | 4.6876 |
| Ra-226 | 1599 years | 4.784 |
| Rn-222 | 3.823 days | 5.4897 |
| Po-218 | 3.04 minutes | 5.181 |
| Po-214 | 163.7 microseconds | 7.686 |
| Po-210 | 138.4 days | 5.304 |
| Pb-206 | stable | 0 |
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. 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 , but Ra-222 and Po-210 rings can be distinguished by other means .
Giant radiohaloes caused some excitement when Robert V. Gentry
Robert V. Gentry
Robert V. Gentry is a nuclear physicist and young earth creationist, known for his claims that radiohalos provide evidence for a young age of the Earth. He is a member of the Seventh-day Adventist Church.-Career:...
proposed that they resulted from the decay chain of unidentified primordial Superheavy elements .
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. 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 .Critics of Gentry, including Thomas A. Baillieul and John Brawley , have pointed out that Po-218 is a decay product of radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
, 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 .
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 . However, , and others have repeatedly offered rebuttals of the radiohalo evidence for a young Earth in peer-reviewed publications.
Citations
..- Ellenberger, C.L.C. Leroy EllenbergerCharles Leroy Ellenberger is perhaps best known as a one-time advocate, but now a prolific critic of controversial writer Immanuel Velikovsky and his works on catastrophism. He first read Worlds in Collision in August 1969 after discovering it while browsing in the B. Dalton's Bookstore in...
, with reply by Gentry, R.V. 1984. "Polonium Halos Redux," Physics Today. December 1984. pp. 91–92 - Ellenberger, C.L.C. Leroy EllenbergerCharles Leroy Ellenberger is perhaps best known as a one-time advocate, but now a prolific critic of controversial writer Immanuel Velikovsky and his works on catastrophism. He first read Worlds in Collision in August 1969 after discovering it while browsing in the B. Dalton's Bookstore in...
1986. "Absolute Dating," unanswered surrebuttal to Gentry, Physics Today. March 1986. pp. 152, 156....... - 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.
- 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.
- 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...
- Wakefield, J. R., 1987–88, "Gentry’s Tiny Mystery - unsupported by geology," Creation/Evolution, v. 22, p. 13–33.
- 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).
Favoring a young earth interpretation
- Polonium radiohalos: still "a very tiny mystery", from the Institute for Creation ResearchInstitute for Creation ResearchThe Institute for Creation Research is a Christian institution in Dallas, Texas that specializes in education, research, and media promotion of Creation Science and Biblical creationism. The ICR adopts the Bible as an inerrant and literal documentary of scientific and historical fact as well as...
- 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 15:32-38 (1988).
- Polonium Halo FAQs, TalkOrigins ArchiveTalkOrigins ArchiveThe TalkOrigins Archive is a website that presents mainstream science perspectives on the antievolution claims of young-earth, old-earth, and "intelligent design" creationists...
- Thousands ... Not Billions book review, Answers in Creation