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

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

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