{{Short description|Geological phenomenon}} {{about|the geophysical phenomenon|the creationist interpretation|Robert V. Gentry|the astronomical phenomenon|radio halo}} [[File:Dark crown.JPG|thumb|Pleochroic halos around crystals of zircons in a sample of biotite]] 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.<ref>{{cite book | last=Faure |first=Gunter |title=Principles of Isotope Geology |year=1986 |publisher=Wiley |pages=354–355 }}</ref> 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.<ref>{{cite journal |first1=G.H. |last1=Henderson |first2=S. |last2=Bateson |title=A Quantitative Study of Pleochroic Haloes, I |journal=Proceedings of the Royal Society of London A |year=1934 |volume=145 |pages=563–581 |issue=855 |doi=10.1098/rspa.1934.0120 |jstor=2935523 |bibcode = 1934RSPSA.145..563H |doi-access=free }}</ref>
==Production== 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.)
{|class="wikitable" style="text-align:center" |- ! Isotope !! Half-life !! Energy in MeV |- |U-238||{{val|4.47|e=9}} years||4.196 |- |U-234||{{val|2.455|e=5}} 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.<ref>{{cite journal |title=Halos und weitere radioaktive Erscheinungen im Wölsendorfer Fluorit (in German) |journal=Der Aufschluss |volume=61 |pages=107–118 |year=2010 |last=Weber |first=B. |url=http://www.vfmg.de/aufschluss_2-2010_Weber.htm }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> 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.<ref>{{cite journal |last=Pal |first=Dipak C. | title=Concentric rings of radioactive halo in chlorite, Turamdih uranium deposit, Singhbhum Shear Zone, Eastern India: a possible result of 238U chain decay |journal=Current Science |year=2004 |volume=87 |issue=5 |pages=662–667}}</ref>
==References== {{Reflist}}
===Further reading=== #{{cite book |first=L.G. |last=Collins |chapter-url=http://www.csun.edu/~vcgeo005/revised8.htm |chapter=Polonium Halos and Myrmekite in Pegmatite and Granite |title=Expanding Geospheres, Energy And Mass Transfers From Earth's Interior |pages=128–140 |editor=Hunt, C. W. |editor2=Collins, L. G. |editor3=Skobelin, E. A. |publisher=Polar Publishing Company |location=Calgary |year=1997}} #{{cite journal |first1=S.A. |last1=Durrani |author-link2=John H. Fremlin |first2=J.H. |last2=Fremlin |title=Polonium Haloes in Mica |journal=Nature |publication-date=October 1979 |volume=278 |issue=5702 |pages=333–335 |doi=10.1038/278333a0 |year=1979 |last3=Durrani |first3=S. A. |bibcode = 1979Natur.278..333H |s2cid=4260888 }} #{{cite journal |first1=G.H. |last1=Henderson |first2=S. |last2=Bateson |title=A Quantitative Study of Pleochroic Haloes, I |journal=Proceedings of the Royal Society of London A |year=1934 |volume=145 |pages=563–581 |issue=855 |doi=10.1098/rspa.1934.0120 |jstor=2935523 |bibcode = 1934RSPSA.145..563H |doi-access=free }} #{{cite journal |doi=10.1098/rspa.1939.0143 |title=A quantitative study of pleochroic haloes. V. The genesis of haloes |journal=Proceedings of the Royal Society of London A |volume=173 |issue=953 |pages=250–264 |year=1939|bibcode=1939RSPSA.173..250H |doi-access=free |last1=Henderson |first1=G. H. }} #{{RubberBible82nd}} #{{cite journal |doi=10.1126/science.180.4092.1272 |title=Polonium Radiohalos: An Alternate Interpretation |journal=Science |volume=180 |issue=4092 |pages=1272–1274 |year=1973 |last1=Moazed |first1=C. |last2=Spector |first2=R. M. |last3=Ward |first3=R. F.|pmid=17759119 |bibcode=1973Sci...180.1272M |s2cid=32535868 }} #{{cite journal |doi=10.1126/science.246.4926.107 |title=Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle |journal=Science |volume=246 |issue=4926 |pages=107–109 |year=1989 |last1=Odom |first1=A. L. |last2=Rink |first2=W. J.|pmid=17837769 |bibcode=1989Sci...246..107L |s2cid=1639793 }} #{{cite journal |first=C |last=Schnier |title= Indications for the existence of superheavy elements in radioactive halos |journal=Journal of Radioanalytical and Nuclear Chemistry |volume=253 |issue=2 |pages=209–216 |doi=10.1023/A:1019633305770 |year=2002|bibcode=2002JRNC..253..209S |s2cid=120109166 }} #{{cite journal |doi=10.1029/EO060i033p00617 |title=Polonium halos and geochronology |journal=Eos, Transactions American Geophysical Union |volume=60 |issue=33 |pages=617–618 |year=1979 |last1=York |first1=Derek|bibcode=1979EOSTr..60..617Y }}
==External links== * [http://www.csun.edu/~vcgeo005/gentry/tiny.htm ''Geology of Gentry's "Tiny Mystery"''], J. Richard Wakefield, ''Journal of Geological Education'', May 1988. * [http://www.talkorigins.org/faqs/po-halos/default.htm Polonium Halo FAQs], TalkOrigins Archive
Category:Radioactive minerals Category:Radiometric dating Category:Lead Category:Polonium Category:Radon Category:Radium Category:Thorium Category:Uranium