Radiohalo

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, 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).