When these minerals occur as inclusions in certain other minerals, most notably the mica family, they are often seen to develop discoloration, or "pleochroic" haloes.
The haloes are caused by radiation damage to the host mineral's crystalline structure.
Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time.
From these observations early attempts were made to use the dimensions of haloes as an age dating technique.
Granites, for example, have more than 10% quartz and abundant potassium feldspar.
Other plutonic rocks have less quartz and potassium, and different ratios of calcium and sodium feldspar minerals.
Granite is a well-known type of plutonic igneous rock, but there are many others as well.
Geologists distinguish these types of rock based on their chemical and mineralogical composition.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.When burial pressures and temperatures get too great, the rocks melt completely, becoming new igneous rocks.To fully understand Gentry's hypothesis a basic background in geology, mineralogy, and radiation physics is helpful.Sedimentary rocks are secondary in formation, being the product of precursor rocks (of any type).