Pleochroic halos are microscopic, ring-like discolorations observed when
thin sections of certain minerals are examined by polarized light. They
were first discovered some years before the turn of the present century and
were named for their "pleochroism," a term introduced more than a century
ago to denote "the property of exhibiting different colors in different
directions by transmitted polarized light (Dana)."(1) But they remained an
unexplained curiosity of nature for many years.
It was not until 1907 that the mystery of their origin was cleared up. In that
year, John Joly, an Irish physicist and geophysicist, and O. Mόgge, a German
scientist, almost simultaneously came up with the explanation: The
discolorations are the result of radioactivity in tiny inclusions
(impurities) at the halo centers; prolonged alpha particle emission by atoms
undergoing radioactive decay produces a halo-like zone of damage surrounding
each inclusion, disrupting the surrounding crystal structure and giving rise
to the pleochroism.
More recently, in recognition of the actual cause of the halos, the term "radiohalo"
has come into use in place of "pleochroic halo.(2)
Historically, radiohalos helped to put the science of geochronology on a
quantitative footing. More recently, they have thrown the entire
discipline into disarray.
The idea of dating rocks radiometrically was first advanced in 1905 by Ernest
Rutherford.(3) He reasoned that the helium found in radioactive minerals
must come from the decay of radium and other radioactive elements. He
wrote: "If the rate of production of helium from known weights of the
different radioelements were experimentally known, it should thus be
possible to determine the interval required for the production of the amount
of helium observed in radioactive minerals, or, in other words, to
determine the age of the mineral."
Considering the possibility that helium, a gas, might tend to
escape from a source mineral over long periods of time, Rutherford also
suggested that lead, should it indeed prove to be an end-product of radioactive decay, as suggested by some then-recent work reported
by B. B. Boltwood,(4) might better serve as an indicator of age for
radioactive minerals. The role of lead as a final descendant in several
chains of radioactive decay was later firmly established.
So it was only a matter of a year or so between Rutherford's first proposal for
radiometric dating and the recognition of radiohalo origins by Joly and
Mόgge. And as early as 1909 Joly further suggested that the progressive
nature of halo discoloration might also provide a way to measure the ages of
certain minerals. Half a century was to pass before techniques sensitive
enough to test this idea were perfected, and then the results were
disappointing: Different crystals, even from the same rock, showed different
sensitivities to alpha-particle darkening; and it was found that
discoloration proceeds only up to a saturation point, beyond which the
process tends to reverse itself.(5)
But during this period when radiohalos were still untested as time-keepers, they
came into their own in a much more fundamental way.
In some of his earlier papers on pleochroic halos, Joly argued that halos of
varying sizes indicated that decay rates for uranium had not been
constant throughout geological time. But later he reversed himself and
insisted that pleochroic halos actually proved what everyone else had always
assumed - that radioactive decay rates have always been constant.(6)
As presented some years later by astronomer Otto Struve, the
argument goes like this: ". . . a uranium atom of atomic number 92
and atomic mass 238 (92U238) successively
releases eight alpha particles (helium nuclei) to become lead of
atomic number 82 and atomic weight 206 (82Pb206
) [See Figure 2]. Similarly, another isotope of uranium, U235,
loses seven alpha particles and decays to
Pb207. The disintegration of thorium (90Th232)
follows a like pattern, forming Pb208. In all three
cases, the end products are lead and the gas helium.
"There is excellent evidence that the rates of radioactive processes
measured in the laboratory at the present time are valid also for
the remote past. If a radioactive element and its decay products
are imbedded in a crystal, each alpha particle emitted during
disintegration travels a certain distance that depends only on the
rate of that particular decay step. The more rapid this rate, the
greater the energy of the alpha particles, and the farther they go
before being stopped and producing a color change in the crystal.
Emission of an alpha particle decreases the electric charge on the
nucleus (subscript) by two and the mass number (superscript) by
four. Emission of a beta particle (negative electron) increases the
charge by one and leaves the mass number unchanged. This list omits
several branch reactions. After Struve, Reference 7.
"Thus, it can be concluded that the rates of disintegration of
uranium and thorium are constant ... (7)
The first premise of this argument may seem strange when first
encountered. Is it true that the energy of an emitted alpha
particle depends on the rate of decay? Apparently so. This
relationship was discovered experimentally in the early years of
radioactivity studies, and for a long time it constituted something
of a paradox not only for the regularity of the increase in
alpha-particle energy with increase in decay rate (or decrease in
half-life; see Figure 1), but especially for the fact that none of
the observed energies was sufficient to explain how the alpha
particles escaped from radioactive nuclei in the first place. It
would not be until the late 1920's that quantum theorists would
claim to have dispelled this mystery in terms of probabilities.(8)
But does an observed relationship between alpha-particle-emission
energies and rates of nuclear decay prove anything about decay rates
in the distant past? Not directly, of course. Thus it was that
Joly suggested examining ancient minerals, or minerals of presumed
antiquity, to see whether or not their radiohalos exhibited
dimensions indicative of emission energies predictable on the
basis of modern decay rates.
Early investigators reported that some of the radiohalos indeed
showed rings in proper sequences and of the expected sizes.(9) And
when a theoretical explanation for the relationship between emission
energies and decay rates emerged, it was generally assumed that the
case for decay rates never wavering down through geological time was
at last ironclad.
But recent work on radiohalos has again cast this premise into
doubt. The man responsible for most of this work, Robert V. Gentry,
is understandably cautious in discussing its broadest implications,
but on one point he stands firm: Radiohalos do not support the idea
of constant decay rates.
If this is so, the findings of geochronology, to the extent they are
based on radiometric dating, are clearly questionable.
In the early 1960's, Gentry, now of the Chemistry Division of the
Oak Ridge National Laboratory, found himself wondering: "Can the
earth's age be measured by radioactive dating of its rocks, as most
geologists and geochemists believe? For some time I had been
intrigued with the thought that certain scientists were using the
terms 'age of the universe' and 'age of the earth' quite freely in
conjunction with each other. How did they know, I thought. Was the
earth actually as old as the universe? Were the earth's elements in
fact synthesized in some gigantic primeval nuclear event?"(10)
Noting that radiohalos "provide the only means for studying the
radioactive transformation of elements in the earth," Gentry set out
to review previous work on the subject, then began his own
painstaking study of thousands of halos in rocks from around the
world. Almost immediately he found that all was not in order in
this long neglected field.
Gentry discovered that, although uranium halos, for example, are
readily identifiable by the number and relative rough diameters of
their rings, their actual dimensions often vary substantially, even
within a single crystal.(11) While explanations might be contrived
to account for such variability and thus seemingly preserve the idea
of invariant decay rates, the evidence, on its face, denied the idea
of a decay "constant" valid for all time.
As Gentry explains in a more recent paper, "experimental
uncertainties in measuring U halo radii preclude establishing the
constancy of [the decay constant] to within 35 percent, and under
certain assumptions U halos provide no information at an in this
respect . . ." In a footnote, the author elaborates by pointing out
that if alpha-decay energies do not vary for given decay rates,
while radiohalo radii do vary (which they do), then there must also
be some variation in the decay constant. "In such a case," writes
Gentry, "halos furnish no proof that [the decay constant] is
Gentry's demonstration that Joly's conclusion about pleochroic halos
proving the constancy of radioactive decay rates was overdrawn was
only the beginning. Soon to be announced were findings even more
In 1966 Gentry reported "Abnormally Long Alpha-Particle Tracks in
Biotite (Mica).(13) He called attention to earlier investigations
disclosing the existence of halo rings much larger than could be
attributed to known alpha-emissions in the uranium and thorium decay
chains. The largest ring to be expected in these series would have
a radius of about 42 microns, due to 8.78-million-electron-volt
alpha particles from polonium-212. Yet radii as large as 75 microns
had been noted by others, (14) and now Gentry had similar findings
After examining and ruling out suggestions that the "giant halos"
might be due to protons from atoms of low atomic number, or to
extraordinarily large central inclusions, he pointed out that
certain known alpha-emitters - actinium-213, radium-216, radium-217,
and francium-215 might have given rise to the oversized halos. But
these radionuclides are unrelated to the uranium and thorium decay
chains. Gentry added: "This suggests the possibility of other decay
series which may have existed in times past ... [Furthermore,] any
attempt to trace the progenitors of At2l3, Ra2l6,
Ra217, and Fr215 through the transuranium
elements reveals a very interesting point: all possible precursors
of these nuclides have fairly short half-lives. Since these halos
would begin to form upon solidification of the host mineral, it is
conceivable that further study of the giant halo parent nuclides
(and their precursors) would establish a maximum time period
between nucleosynthesis and crustal formation."
This cautiously worded statement barely disguises the bombshell it
contains. Nearly every modern cosmological theory assumes that all
the heavy, radioactive elements were created "in the beginning" or
at least prior to the formation of the Earth and were assembled
as terrestrial elements as the planet formed. Gentry's studies
seemed to indicate that radionuclide-creation in some instances
occurred mere minutes before those nuclides became trapped in rocks
solidifying to form parts of the Earth's crust. If so, they must
been created on Earth long after the planet itself was formed.
In the years to follow, Gentry was to turn up much more radiohalo
evidence pointing to this same conclusion. And most of this
additional evidence is much more straightforward, not depending on
speculative identifications of halo-inclusion nuclides belonging to
unknown decay series.
Early in 1967 Gentry reported his observations of certain
intermediate-sized "Y halos" found in biotite of supposedly
Pre-Cambrian age from Canada.(15 ) The most likely parent nuclide
for these radiohalos, he concluded, was bismuth-211, an isotope with
a half-life of only 2.15 minutes.
In another paper that same year, Gentry described halos due to
polonium nuclides of atomic weights 210, 212, 214, and 218, all of
short half-lives.(16) Although these nuclides are ordinarily
members of familiar uranium and thorium decay series, none of the
halos reported gave any evidence that the usual polonium precursor
elements had ever been present in their central inclusions, or "radiocenters,"
as Gentry was later to call them.
In a less-technical article published in Medical Opinion and
Review,(17) Gentry discussed at some length the problem posed by
the "parentless-polonium" halos. Earlier workers, in attributing
the halos to polonium and noting the puzzling absence of uranium and
thorium from the radiocenters, had speculated that polonium had
precipitated from uranium-bearing solutions flowing in microscopic
channels through the mineral grains; this neatly sidestepped the
unthinkable idea of parentless polonium. But Gentry's examination
of many of these halos had turned up no sign whatever, either of
cracks or conduits in which solutions might once have flowed, or of
the staining characteristic of such flow. More importantly, "the
absence of uranium fission tracks [which are always present around
uranium radiocenters] after the mineral was etched with
hydrofluoric acid demonstrated that there was no uranium."
The studies again had disclosed "overwhelming evidence for the
presence of several different types of short-half-life radioactivity
in crustal rocks around the world . . . Only elements with extremely
long half-lives could have survived the time span from
nucleosynthesis to a period several billion years later when the
crustal rocks of the earth finally solidified. Yet, the polonium
and other variant halos of very short half-life existed side by side
with the uranium and thorium halos in the same piece of mica. . ."
By 1970 Gentry could report finding additional halos that seemed to
require explanation in terms of "unknown alpha radioactivity which
may no longer be present in the earth's crust"(18) or perhaps even
New evidence that many polonium halos were "parentless" was reported
in 1971.(20) This was the discovery that an overabundance of
radiogenic lead Pb-206, which would be predictable for parentless
polonium halos, was actually present in polonium radiocenters. When
a decay series starts with U-238, a parallel decay series stemming
from U-235 also gets under way: the two uranium isotopes are always
found together in nature. The U-238 ultimately decays to Pb-206,
while the U-235 decays to Pb-207. Thus, inclusions initially
containing uranium should later contain the two leads, mixed in
proportions that will depend upon how long uranium-decay has been in
progress, as well as on the initial ratio of U-238 to U-235 in the
inclusion. (Since this initial ratio for all occurrences of the two
uraniums is assumed to be a known, fixed ratio,(21) geochronologists
have come to put great faith in uranium-lead dating techniques.)
Assuming that none of the daughter products of either decay chain is
lost from the mineral under study, the ratio of Pb-206 to Pb-207
will vary from about 20 for a very young mineral to about 4 for a
mineral more than three billion years old. Gentry points out that
"the theoretical maximum possible radiogenic 206 Pb/207 Pb ratio, on
the basis of an instantaneous production of Pb from U decay, would
be 21.8.(22) Yet in polonium radiocenters Gentry found that the
ratio ranges from about 20 to as high as 60.
Such high values are explainable only if parentless polonium decayed
to produce most of the Pb-206.
Thus, in the course of research spanning little more than half a
dozen years, Gentry had cast doubt on the theoretical foundations of
radiometric dating by demonstrating that the constancy of decay
rates cannot be proved by radiohalos, and he had turned up strong
evidence that seemed to imply an inconceivable process the
crystallization of terrestrial rocks within minutes of the creation
of radioelements trapped within them.
Such disturbing results could hardly be ignored or left
unchallenged. In 1973, three Wayne State University (Michigan)
professors sought to deflate Gentry's findings. (23 ) These
investigators two physicists and a geologist - frankly stated
what they saw as the need for their challenge: The existence of
polonium halos, unsupported by uranium-decay, would cause
"apparently insuperable geological problems . . ." But their effort
was not up to the task, and Gentry turned their attack aside with
The problem remains. And again it seems, to judge from the silence
that has settled over the subject, that another in a long list of
geological "anomalies" is in danger of being swept under the rug.
Can Velikovsky's work throw light on this problem? I believe so.
Furthermore, it seems entirely reasonable to suspect that Gentry's
studies of radiohalos have unearthed strong, new support for the
thesis of Worlds in Collision.
First, consider Velikovsky's conclusion that interplanetary electric
discharges in historical times produced a certain amount of element
building (nuclear fusion) on Earth. (The possibility that similar
discharges to the Moon may be held responsible for concentrations of
radioactivity on that body has been discussed elsewhere.(25)) Might
we not imagine that new polonium (and uranium, too, for that matter)
was created on Earth, and in place, by powerful thunderbolts?
It seems unnecessary to speculate that all parentless polonium must
date from recent (historical) electrical events. If fusion was
achieved catastrophically in historical times, it undoubtedly was
achieved in earlier times as well, perhaps under comparable
circumstances or perhaps on vastly larger scales. The point to be
considered is that electric discharges of cosmic proportions should
be capable of creating new elements; even atmospheric lightning is
credited with producing radionuclides,(26) and all artificial
element-creation starting with the first fusion reaction ever
achieved in the laboratory producing technetium from molybdenum, in
1937 has involved harnessing the forces of the electric
Of course, creating parentless polonium (or any other heavy element)
is one thing; getting it into crustal rocks before it decays seems
quite another. But here, too, we may be able to draw clues from
Velikovsky. The same electrical disturbances that led to
interplanetary arcing and sparking may have temporarily prolonged
the half-lives of heavy elements that now decay in mere moments.
All matter, as far as we know, is electrically constituted. Atomic
nuclei may be said to be "constituted" in spite of electrical forces
tending to disrupt them; but electrical forces are there to be
reckoned with. If all this is true, what role might environmental
electrification play in setting the rules for nuclear stability,
radioactive-decay rates, and energies of particle-emissions in decay
"If, according to our present interpretation, the elements are made
up of electropositive and electronegative particles, the
combinations which these particles will form will depend upon the
relative numbers of the two kinds. We suppose that every possible
stable grouping of these ultimate (for our purpose) particles will
exist. Some will be very stable and some will be upon the verge of
instability. The most stable groups will be the most numerous. A
relatively small change in the total number of either electrons or
protons will cause some of the elements to become unstable . . ."
These words were written almost 50 years ago by Fernando Sanford,
then professor of physics, emeritus, at Stanford University a
highly respected physicist who happened to stray from doctrinaire
paths thanks to a penchant for letting evidence take precedence over
preconception. The quoted passage is part of an inquiry as to "what
phenomena should be observed on an electrified planet which would
not appear on an unelectrified one"; in this passage he is
describing conditions that actually prevail on Earth today and help
to demonstrate its intense electrification.(28)
We may interpret Sanford's reference to the relative numbers of the
two kinds of charged particles as a reference to the state of
electrification the ambient electrical stress of the Earth.
Now what if the Earth's state of electrification were altered, even
if only temporarily? It would seem to follow that decay rates for
radionuclides might well differ radically from today's norms.
Polonium isotopes now exhibiting very little stability might then
acquire briefly, but long enough half-lives in keeping with
the evidence of the Earth's crustal rocks.
This is speculation, certainly, but speculation that might be
checked experimentally by actually subjecting radionuclides to
unaccustomed electrical stress and seeing whether or not they behave
It may also be highly significant that radiohalos are found only in
plutonic rocks granites, pegmatites, and the like and in
metamorphosed rocks in close association with plutonic rocks.
These rocks are themselves the subject of a centuries-old debate as
to their origin.(29) At issue is what used to be called "the
granite problem," an intriguing mystery much too complex to discuss
here, but whose ultimate solution may also require consideration of
physical forces well-known today but seldom, if ever, mentioned in
the entire course of training of a professional geologist.(30)
In their continuing searches for "the oldest" materials to be found
in the crust of the Earth, the finding of which brings publicity and
prestige, geochronologists ardently seek samples of rock of plutonic
origin from every continent. Yet so long as the origins of such
rocks evade understanding, so long as doubt remains as to the
reality of decay "constants," so long as the evidence for
element-creation on Earth is ignored, and so long as geophysicists
fail to examine the full spectrum of physics, all announcements of
success" in this endeavor must remain questionable. And in the
meantime, the explanation of discrepant radiohalos seems to require
consideration of Earth history in terms of cosmic quietude
interrupted ftom time to time by episodes of Velikovskian violence.
NOTES AND REFERENCES
1. Glossary of Geology and Related Sciences (Washington:
American Geological Institute, 1960), p. 225.
2. In the late 1960's R. V. Gentry referred to pleochroic halos as
radioactive halos, but by 1971 (Science 173, 727) he had
simplified the term to radiohalos.
3. E. Rutherford, Radioactive Transformations (New York:
Charles Scribner's Sons, 1906); quoted here from excerpts
republished in Source Book in Geology 1900-1950, K. F.
Mather, ed., (Cambridge: Harvard University Press, 1967), 115-119.
4. B. Boltwood (as cited by Rutherford), Philosophical
Magazine, April 1905; American Journal of Science,
5. Cf., H. Faul, Ages of Rocks, Planets, and Stars (New
York: McGraw-Hill, 1966), pp. 21-22.
6. J. Joly, Nature 109 (1922), 480.
7. O. Struve, "Finding the Age of the Earth," Sky &
Telescope 18 (June 1959), 433-435.
8. G. Gamow wrote in his book, A Star Called the Sun
(New York: Bantam Books, 1965, p. 111): "Using wave mechanics, the
author was able to explain the 'paradox' of alpha decay of
radioactive nuclei, and to give a perfect [sic] theoretical
interpretation of the dependence between the energies of the
emitted alpha particles and the half-lives of . . . elements . .
9. CL, 0. Struve, op. cit.
10. R. Gentry, "Cosmology and Earth's Invisible Reahn,"
Medical Opinion & Review (October 1967), 65-79.
11. R. Gentry, American Journal of Physics 33 (1965),
12. R. Gentry, Science 184 (5 April 1974), 62-66.
13. R. Gentry, Applied Physics Letters 8 (1 February
14. (As cited by Gentry): E. Wiman, Bulletin, Geological
Institute, University of Uppsala 23 (1930), 1; also H. Hirschi,
15. R. Gentry, Nature (February 4, 1967), 487-489.
16. R. Gentry, abstract in American Physical Society Bulletin
12 (1967), 32.
17. R. Gentry, Medical Opinion & Review (October 1967),
18. R. Gentry, abstract in EOS, Transactions, American
Geophysical Union 51 (April 1970), 338.
19. R. Gentry, Science 169 (14 August 1970), 670-673.
Quite recently Gentry's name has been prominent in on again-off again
reports of finding evidence for extinct superheavy elements; see, e. g.,
reports by A. L. Robinson, Science 193 (16 July 1976), 219-220
and Science 195 (4 February 1977), 473-474.
20. R. Gentry, Science 173 (20 August 1971), 727-731.
21. The abundances of U-235 and U-238 are such that about 7 of
every 1000 uranium atoms found on Earth are U-235; the remaining 993 are
U-238. TMs, within very narrow limits, is an observational fact.
22. R. Gentry, op. cit.
23. C. Moazed, R. M. Spector, and R. F. Ward, Science 180
(22 June 1973), 1272-1274.
24. R. Gentry, Science 184 (5 April 1974), 62-66.
25. R. Juergens, Pensee, Vol. 4, No. 3, Summer 1974,4546.
26. It has been suggested that lightning may be held accountable
for the creation of carbon14 and introducing an unknown into
27. AR of man's devices for accelerating ions to fusion
energies are basically modeled after the capabilities of natural
28. F. Sanford, Terrestrial Electricity (Palo Alto:
Stanford University Press, and London: Oxford University Press, 1931;
reproduced in 1967 by microfilm-xerography by University Microfilms,
Ann Arbor), Chapter III: "Electrical Charges of the Earth and the Sun."
29. Cf., M. Walton, Science 131 (4 March 1960), 635-645;
see also references cited therein. In the near future I plan to prepare
a paper suggesting an answer to W. R Bucher's question ("The Crust of
the Earth," Scientific American, May 1950): "Where did the
granite come from, and where did the sediments [that formerly occupied
regions now filled with granite] go?" The question is one that has never
yet been answered satisfactorily, and I believe that the explanation for
this puzzling situation, as well as for the fact that the Earth's
radioactive elements are concentrated in granitic rocks, is to be found
in granite origins as a result of electrical breakdown of pre-existing
sediments during Earth catastrophes of the kind described by Velikovsky.
30. I refer, of course, to the forces pertaining to electrical