Pleochroic Halo

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.

Because most of the energy of an alpha particle is absorbed at the end of its path length in a mineral, these colour centres are produced most intensely around the inclusion. Pleochroic halos are commonly found in the minerals biotite, fluorite, and amphiboles; the most common inclusions are the minerals zircon, xenotime, apatite, and monazite.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London


Photo
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Pleochroic halo around zircon enclosed in Biotite. XPL image. 20x (Field of view = 1mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 20x (Field of view = 1mm)
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Pleochroic halo around zircon enclosed in Biotite. XPL image. 20x (Field of view = 1mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 20x (Field of view = 1mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic halo around zircon enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic halo around zircon enclosed in Biotite. XPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic halo around zircon enclosed in Biotite. PPL image. 10x (Field of view = 2mm)
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Pleochroic haloes around zircons enclosed in Biotite. PPL image. 10x (Field of view = 2mm)