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pleochroic halo The dark, strongly pleochroic (see PLEOCHROISM) zone often seen around certain radioactive inclusions (e.g. zircon, apatite, and sphene) in some minerals (e.g. biotite, tourmaline, amphibole, chlorite, muscovite, cordierite, and fluorite). The halo is created by the interaction of alpha particles with the atoms of the crystal lattice. Some haloes contain concentric rings with radii of 10–15 μm, the radii indicating the various kinetic energies of the different sets of alpha particles.