ABSOLUTK DAllNG 



27 



po lo 



Fig. 4. Schematic drawing of the pleochroitic rings around an inclusion 

 of uranium. In normal rock-forming minerals the radius of the outer 

 sphere of RaC^ is about 30^. 



concentric rings (Fig. 4). In polarised light these rings show different 

 colour schemes from the host mineral, especially when rotated under 

 the polarising microscope; hence the name pleochroitic rings. These 

 rings are caused by irregularities in the crystal lattice of the host 

 mineral. And these, in turn, are due to various emissions from small 

 inclusions of decaying radioactive elements. Now in a given decay 

 process, the emanation accompanying the decay of a certain radio- 

 active element always has the same energy, because this is a nuclear 

 property innate to the type of atom constituting that element. A 

 certain emanation will always penetrate the same distance in the 

 crystal lattice of the host mineral. It will form a spherical zone of 

 disruption around the radioactive inclusion in the host mineral. In a 

 thin section of the rock, under the microscope, this sphere will, of 

 course, appear as a ring or a perfect circle. Now if ever the energy 

 of the emission of an inclusion of radioactive matter should have 

 changed throughout the life history of a given host mineral, it would 

 not have produced sharply outlined spheres of disturbance within 

 that host mineral. A slow change of energy of the radioactive decay 

 over the millions of years would have resulted in a blurred spot, not 

 in discrete rings. Therefore, because we find so many pleochroitic 



