His book states, for example, that the halos are found "in clear
areas" of the mica "far from" any other source of radioactivity.
This is false.  He states that the halos are found "in granites."
This is totally misleading, being equivalent to saying that "rubber
tires are found in parking lots" without mentioning that there have
to be _cars_ in the parking lots; halos do not occur in "granites,"
they occur in _pegmatites_ (pockets of radically different
geochemistry from the rest of the granite massif.  He states that
no other explanation exists for the Po-218 halos, when his own
photomicrographs, and my own eyes, see clear evidence for their
production by migrating Radon-222 atoms from nearby radioactive
particles (the particles he says aren't nearby, but which are).

(By the way, "[my] piece of biotite" translates to several tens
of _pounds_ of biotite samples, which if carefully and thinly
split as is necessary if one is to detect all the halo rings,
would cover a space some thirty feet on a side.  (That's an educated
guess.  I've examined square _yards_ of biotite with 100x and
higher objectives, with fifty times as much yet to examine.)

I have seen and photographed _excellent_ examples of what ought
only to be migrated-radon "Polonium" halos, some with complete
three-level Po-218-type halos around the entire length of the crack.

What Gentry says I _should_ see, I do not see; what he says I
should _not_ see, I do see.  Significant also is the presence
in my samples of biotite from certain mines in North Carolina
which do not contain any detectable inclusions of radioactive
"nearby" particles.  These samples also contain _no_halos_
at all.  (No radioactives equals no halos, in other words.)

It goes on and on...