Imagine a very long pipe, kinked and twisted, bent here and there, with a few large bulges somewhere along its length.
You are to throw "superballs" (those little rubbery balls that bounce like crazy) into the end of the pipe.
As you do, the balls will bounce down the length of the pipe, behaving as one might expect upon meeting these kinks and bends and bulges.
Radon atoms are inert. Radon is a "noble" gas; it's outermost electron "shell" is filled; it has no appreciable "electronegativity" nor "electropositivity."
It will not stick to any of the atoms along the length of the "pipe" (a crack, or crystal plane separation, or other channel in the biotite mica).
The source of these radon atoms is a radioactive particle, usually of uranium, releasing these atoms as they appear as decay products of the atom one step above them in the U238 decay chain.
(The source is "you", throwing "superballs" (radon222 atoms) into the "pipe" (the crack or channel in the mica).
Now, since radon222 has roughly four days (on average; its 'half-life') to live before it decays into the next step of the decay chain, whereupon it will suddenly become electronegative (it will lose its "noble" status).
During this time, it can bounce around, using the host mica's thermal energy to do so (not being at absolute zero, the mica is vibrating).
At the same time, it can be pushed along the crack or channel by new atoms of radon222 being manufactured back "behind" it in the source radioactive particle. It is behaving like a gas atom (which, of course, it is).
Now imagine that the first of these decays about halfway down the crack, and "sticks" there. (It is no longer inert.)
What effect might this have on the crack, and thus on the next radon atom to bounce down there? Well, it should narrow the crack a little where it decayed.
Now, bearing in mind our original pipe, with a few bends here and there, imagine what happens to the balls that you throw down the pipe:
When a ball meets a bend or a narrow spot, it has less chance of making it further down the pipe than it would if the obstruction were not present.
Thus, although it might well make it past this point, to bounce further on down the pipe, it will be delayed here at this bend. Likewise, any sort of obstructions, including wide places larger in diameter than the rest of the "pipe," will over time collect decayed radon atoms, narrowing the pipe OR closing down wide spots. (It's as possible to build up a spot that makes it easier to get past the spot, rather than harder.)
Now, suppose our superballs are radon atoms, and suppose they do behave as above described. What should we see in the mica, and do we see it?
1) We should see cracks with haloes strung out along them, said haloes showing a range of different densities, according to how long radon atoms were delayed, to decay while 'trying' to get past the obstructions, in the spots where we now should see them.
2) We might see these differentially darkened haloes seemingly unreasonably laid out, as well.
Do we see these things?
Well, I can't speak for you (hence no "we"), but I certainly do: See the "More" haloes link on the previous page.