Micro-crystallinity in radioactive minerals

Most natural minerals contain small quantities of uranium, thorium and their decay products. We have measured the emanation of two of these decay products, 222Rn (t1/2 = 3.8 days) and 220Rn (t1/2 = 55.6 s), from single large crystals of apatite, monazite, zircon and uraninite. The amounts released range from <0.01 to as high as 25%, which is to be expected only if the minerals are composed of micron size crystals, with gaps in between. Since the porosity of these nearly perfect crystals is small, these observations imply that the internal gaps/openings are very narrow (a few nm wide) but are connected efficiently to the surface. (author)