A pleochroic halo, or radiohalo, is a microscopic, spherical shell of discolouration (pleochroism) within minerals such as biotite that occurs in granite and other igneous rocks. The halo is a zone 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 particles' energy.[2]
Production
editUranium-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- ^ Faure, Gunter (1986). Principles of Isotope Geology. Wiley. pp. 354–355.
- ^ 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.
- ^ Weber, B. (2010). "Halos und weitere radioaktive Erscheinungen im Wölsendorfer Fluorit (in German)". Der Aufschluss. 61: 107–118.[permanent dead link]
- ^ 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- 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.
- Durrani, S.A.; Fremlin, J.H.; Durrani, S. A. (1979). "Polonium Haloes in Mica". Nature. 278 (5702) (published October 1979): 333–335. Bibcode:1979Natur.278..333H. doi:10.1038/278333a0. S2CID 4260888.
- 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.
- Henderson, G. H. (1939). "A quantitative study of pleochroic haloes. V. The genesis of haloes". Proceedings of the Royal Society of London A. 173 (953): 250–264. Bibcode:1939RSPSA.173..250H. doi:10.1098/rspa.1939.0143.
- Lide, D. R., ed. (2001). CRC Handbook of Chemistry and Physics (82nd ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0482-2.
- Moazed, C.; Spector, R. M.; Ward, R. F. (1973). "Polonium Radiohalos: An Alternate Interpretation". Science. 180 (4092): 1272–1274. Bibcode:1973Sci...180.1272M. doi:10.1126/science.180.4092.1272. PMID 17759119. S2CID 32535868.
- Odom, A. L.; Rink, W. J. (1989). "Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle". Science. 246 (4926): 107–109. Bibcode:1989Sci...246..107L. doi:10.1126/science.246.4926.107. PMID 17837769. S2CID 1639793.
- 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. S2CID 120109166.
- York, Derek (1979). "Polonium halos and geochronology". Eos, Transactions American Geophysical Union. 60 (33): 617–618. Bibcode:1979EOSTr..60..617Y. doi:10.1029/EO060i033p00617.
External links
edit- Geology of Gentry's "Tiny Mystery", J. Richard Wakefield, Journal of Geological Education, May 1988.
- Polonium Halo FAQs, TalkOrigins Archive