Definition of pleochroic halo
Any of the small, concentrically colored aureoles in minerals--e.g., micas, chlorites, fluorite, and cordierite--centered by minute inclusions of minerals containing radioactive elements, such as zircon, uraninite and monazite. This discoloration results from crystal structural radiation damage from alpha decay. The term is not entirely appropriate as true pleochroism is not really involved. Radiohalo and Radiation-induced colour halos are better terms. They are usually microscopic (<100 microns).
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.
Odom, A. L.; Rink, W. J. (1989). "Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle". Science. 246 (4926): 107–109. doi:10.1126/science.246.4926.107. https://science.sciencemag.org/content/246/4926/107