The suggested decay chain for Polonium is as follows. (I have begun the chain at Radon-222, fifth in the series of alpha particle- emitting daughters of Uranium-238, both for brevity and for another reason which will shortly become clear.)

86 Rn 222 (86 protons, Radon, mass 222) decays in about four days to Polonium-218 with the emission of an alpha particle of 5,486,000 electron volts (5.486 MeV). (Please note this energy value.)

84 Po 218 (84 protons, Polonium, mass 218) decays, through two additional non-alpha-emitting (beta decay) steps involving Lead and Bismuth, over a total of about 45 minutes, to Polonium-214 with the emission of a 6.111 MeV alpha particle.

84 Po 214 (84 protons, Polonium, mass 214, arriving via Lead-214 (27 minutes) and Bismuth 214 (20 minutes)) decays through two additional (and time-consuming) steps taking about 21 years, to Polonium 210, with the emission of a 7.687 MeV alpha particle. This involves the immediate alpha decay (.000164 seconds) of the Po-214 nucleus to Lead 210, which has a half-life of 21 years, then via beta decay to Bismuth 210 with a 5 day half-life, and another beta decay to Po 210. (There is a small chance of the Bismuth 210 appearing in its isomeric form, which has a half-life of three million years.) This Polonium-214 alpha energy is the highest in the Uranium 238 decay chain, and consequently creates the largest, outermost, halo.

84 Po 210 (84 protons, Polonium, mass 210) decays with a half-life of 138 days directly to Lead 206 (stable), emitting an alpha particle of 5,305,000 electron-volts' energy. (Note the close similarity between this energy and that of the decay from Radon 222 to Polonium 218.)

82 Pb 206 (82 protons, Lead, mass 206). End of the line. Stable Lead. No further decay is possible.

At this point, one of those rare and welcome "Aha!" experiences occurred as I realized that Radon is:
* A gas,
* An inert gas,
* Produced a few atoms at a time in the U-238 decay chain, continuously and steadily over geologic time.

There was therefore no reason to think that Radon manufactured in any nearby Uranium mineral particle (uraninite, betafite, uranophane, etc.) would stay attached to the disintegrating particle; an atom with a filled outer shell would not 'attach' to the biotite crystal's atoms, nor would it be likely to remain attached to the disintegrating Uranium mineral inclusion. Moreover, with about four days to move around as single atoms subject to thermodynamic gas laws, it could wander literally anywhere in the mica permitted by the slightest crack, cavity, lattice discontinuity, or separation between crystal planes, "pushed" along by new Radon atoms forming back 'behind' it in the inclusion.