Pleochroic halo

A pleochroic halo, or radiohalo, is a microscopic, spherical shell of discolouration (pleochroism) within minerals such as biotite that occurs in granite and other igneous rocks. The halo is a zone 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.^[1] One explanation is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particles' energy.^[2]

^ Faure, Gunter (1986). Principles of Isotope Geology. Wiley. pp. 354–355.
^ Henderson, G.H.; Bateson, S. (1934). "A Quantitative Study of Pleochroic Haloes, I". Proceedings of the Royal Society of London A. 145 (855): 563–581. Bibcode:1934RSPSA.145..563H. doi:10.1098/rspa.1934.0120. JSTOR 2935523.

[1] Faure, Gunter (1986). Principles of Isotope Geology. Wiley. pp. 354–355.

[2] Henderson, G.H.; Bateson, S. (1934). "A Quantitative Study of Pleochroic Haloes, I". Proceedings of the Royal Society of London A. 145 (855): 563–581. Bibcode:1934RSPSA.145..563H. doi:10.1098/rspa.1934.0120. JSTOR 2935523.

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