In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.
Sometimes the minerals segregate into distinct bands.
Metamorphic rocks represent alterations of precursor sedimentary, igneous, or other metamorphic rocks.
Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.
Radiation damage haloes around mineral inclusions are well known from the geological literature.
The crystals develop an interlocking texture with some of the trace minerals becoming completely surrounded by later forming crystals.
Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye.
These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles.
In numerous papers published in scientific journals in the 1970s and 1980s, Gentry built the case that the different alpha decay energies of various naturally occurring radioactive isotopes resulted in distinctly different halo diameters.
The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity.
Certain minerals, such as zircon and monazite, which form as common trace constituents in igneous rocks, have crystal structures which can accommodate varying amounts of the naturally occurring radioactive elements, uranium and thorium.
One such individual is Robert Gentry, who holds a Master's degree in Physics (and an honorary doctorate from the fundamentalist Columbia Union College).