Pleochroic halo

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Pleochroic halos around crystals of zircons in a sample of biotite

Pleochroic halos (also referred to as radiohalos) are microscopic, spherical shells of discolouration (pleochroism) 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.[1] One explanation is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy.[2]

Production[edit]

Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Because of their continuous energy distribution and greater range, beta particles cannot form distinct rings.)

Isotope Half-life Energy in MeV
U-238 4.47×109 years 4.196
U-234 2.455×105 years 4.776
Th-230 75,400 years 4.6876
Ra-226 1,599 years 4.784
Rn-222 3.823 days 5.4897
Po-218 3.04 minutes 5.181
Po-214 163.7 microseconds 7.686
Po-210 138.4 days 5.304
Pb-206 stable 0

The final characteristics of a pleochroic halo depends upon the initial isotope, and the size of each ring of a halo is dependent upon the alpha decay energy. A pleochroic halo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a halo formed from polonium has only one, two, or three rings depending on which isotope the starting material is.[3] In U-238 haloes, U-234, and Ra-226 rings coincide with the Th-230 to form one ring; Rn-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a petrographic microscope.[4]

References[edit]

  1. ^ Faure, Gunter (1986). Principles of Isotope Geology. Wiley. pp. 354–355.
  2. ^ 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.
  3. ^ Weber, B. (2010). "Halos und weitere radioaktive Erscheinungen im Wölsendorfer Fluorit (in German)". Der Aufschluss. 61: 107–118.[permanent dead link]
  4. ^ Pal, Dipak C. (2004). "Concentric rings of radioactive halo in chlorite, Turamdih uranium deposit, Singhbhum Shear Zone, Eastern India: a possible result of 238U chain decay". Current Science. 87 (5): 662–667.

Further reading[edit]

  1. Collins, L.G. (1997). "Polonium Halos and Myrmekite in Pegmatite and Granite". In Hunt, C. W.; Collins, L. G.; Skobelin, E. A. (eds.). Expanding Geospheres, Energy And Mass Transfers From Earth's Interior. Calgary: Polar Publishing Company. pp. 128–140.
  2. Durrani, S.A.; Fremlin, J.H.; Durrani, S. A. (1979). "Polonium Haloes in Mica". Nature (published October 1979). 278 (5702): 333–335. Bibcode:1979Natur.278..333H. doi:10.1038/278333a0.
  3. 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.
  4. "A quantitative study of pleochroic haloes. V. The genesis of haloes". Proceedings of the Royal Society of London A. 173 (953): 250–264. 1939. doi:10.1098/rspa.1939.0143.
  5. Lide, David R., ed. (2001). CRC Handbook of Chemistry and Physics (82nd ed.). London: CRC Press. ISBN 0-8493-0482-2.
  6. Moazed, C.; Spector, R. M.; Ward, R. F. (1973). "Polonium Radiohalos: An Alternate Interpretation". Science. 180 (4092): 1272–1274. doi:10.1126/science.180.4092.1272.
  7. 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.
  8. Schnier, C (2002). "Indications for the existence of superheavy elements in radioactive halos". Journal of Radioanalytical and Nuclear Chemistry. 253 (2): 209–216. doi:10.1023/A:1019633305770.
  9. York, Derek (1979). "Polonium halos and geochronology". Eos, Transactions American Geophysical Union. 60 (33): 617. doi:10.1029/EO060i033p00617.

External links[edit]