how can we explain that a cristal of zircon observed in a granite thin section (like in the following image) seems not affected while the surrounding biotite has formed a desintegration aureole? seems not affected, though the zircon contains the radioactive elements and would have been crossed (and modified) by radiations ?
img from https://planet-terre.ens-lyon.fr/ressource/gneiss-icartien-Port-Beni.xml
Pleochroic halos in biotite are very common. It does not necessarily have to be only around zircon but also around inclusions of other minerals with elevated Th or U content (monazite, xenotime, apatite, etc.) and the surrounding mineral does not have to be only biotite, but sometimes, for example, amphibole or fluorite.The point is that different minerals have different sensitivity to radiation. The crystal structure of zircon is very compact and durable. Conversely, the biotite structure contains interlayer spaces as well as divalent iron in the octahedral layer, which is prone to oxidation. Some radiation damage can also occur with zircon, but it is not so noticeable (hydrozircon?).
Historically addressed, U haloes were first discovered by ROSENBUSCH in 1877 in andalusite, cordierite and mica and the concentrically shaped dark cocards have been named by him „pleochroitic haloes“.
Mügge and JOLY found out in 1907 and 1909 that the radii of the same size resultant from Alpha emission
1913 Kasimir FAJANS and Frederick SODDY related it to the
Uranium-radium decay chain with alpha and beta radiating elements
238 U(a) > 234 Th (b) > 234 Pa (b) > 234 U(a) > 230 Th (a) > 226 Ra (a) > 222 Rn (a) > 218Po (a) > 214 Pb (b) > 214 Bi (b) > 214 Po (a) > 210 Pb (b) > 210 Bi (b) > 210 Po (a) > 206 Pb (stabile)
The stepwise approach to the topic has been given in the sequence of photographs attached below for clarity of my text.
HGD
Thank you Dr Matysek and thank you Dr Dill. Very interesting answers, a new and fascinating subject for me. I will transmit with pleasure to my student who has motivated this post.
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