The striking effect of *-radiation on five yellow and brown fluorites is
readily explained by the writer's theory. Atomic calcium, which causes
"F centers" in yellow-brown fluorite, is believed to coagulate into colloidal
aggregates which produce the blue color of irradiated fluorite.
Presumably fluorine must migrate in the crystal to permit this coagulation.
Mollwo's experiment of electron introduction into fluorite has
proved that fluorine migration is a feasible process.
However, not all brown and yellow fluorites can be considered to
contain "F centers." Specimens from Clay Center and Cave-in-Rock do
not show striking color change when irradiated. Their yellowish white
ultraviolet fluorescence suggests that color may be the result of enclosed
928 ROBERT D. ALLEN
hydrocarbons. Brown Weardale fluorite also fails to exhibit the irradiation
efiect. Perhaps as suggested by Pizibram (1938) the color is due to
bivalent europium which is known to be present.
Blue material from Livingston Co., Kentucky, is unusual in that it has
an unusually high refractive index without the presence of an impurity to
explain the index. Associated with the blue fluorite is yellow fluorite
whose index of refraction is identical within the limits of experimental
error. It is suggested that colloidal calcium in the blue specimen resulted
from coagulation of "F centers" formerly present.
The fact that artificial irradiation does not change the index of refraction
is regarded as support for the present hypothesis. If r-radiation
merely causes a coarsening; of pigment particle size it would not be
expected to efiect a change in index. No change in density would be experienced
by the irradiated substance.