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20.2 Radiohaloes

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

A thin film of polonium on a stainless-steel d...
A thin film of polonium on a stainless-steel disk, the form in which it is sold as an alpha-particle source for scientific use. (Photo credit: Wikipedia)

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.

In the 1970s, young Earth creationist Robert V. Gentryproposed that radiohaloes in certain granites (such as biotite) represented evidence for the Earth being created instantaneously rather than gradually. This idea has been criticized by physicists and geologists on many grounds including that the rocks Gentry studied were not primordial and that the radionuclides in question need not have been in the rocks initially.

Thomas A. Baillieul, a geologist and retired senior environmental scientist with the United States Department of Energy, noted that Gentry was a physicist with no background in geology and given the absence of this background, Gentry had misrepresented the geological context from which the specimens were collected. He noted that Gentry relied on old research, long before radioisotopes were thoroughly understood; that his assumption that a Polonium isotope caused the rings was speculative; and that Gentry falsely argued that the half-life of radioactive elements varies with time.

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