Definition of pleochroic halo

i. A minute zone of color or darkening surrounding and produced by a radioactive mineral crystal or inclusion.



Ref: AGI

ii. Any of the concentrically colored aureoles in minerals--e.g., micas, fluorite, and cordierite--centered by minute grains of minerals containing radioactive elements, such as zircon and monazite. This discoloration results from crystal structural radiation damage from alpha decay.