I have been searching certain fluorescent minerals, such as Andersonite, and all websites are saying how the mineral will glow if exposed to Ultraviolet light. Does enough ultraviolet light come through the atmosphere (from the sun), to make any natural minerals actually glow, and to keep glowing for a time if the light was removed? If there is not enough UV light, or if no such mineral exists, would it be possible for such a mineral to exist?
The lifetime of a fluorescence excited state is on the nanosecond to microsecond time scale. So once the excitation source light is removed, the emission of light will stop within microseconds.
Spectroscopic Characterization of the Uranium Carbonate Andersonite provides quantitative information on fluorescence lifetimes for Andersonite, and lifetimes are in the microsecond range.
Regarding phosphorescence, see Phosphorescence; or, The emission of light by minerals, plants, and animals (1862):
SEVERAL substances manifest the strange property of emitting light when they are placed in darkness, after having been exposed for some time to the direct rays of the sun. In some cases a very short exposure to sunlight is sufficient to excite the manifestation of this remarkable property, and in others the direct rays of the sun are not necessary : it suffices that the substance experimented upon be exposed to the dull light of a cloudy day. To this phenomenon the denomination of Phosphores- cence after insolation has been given.
' The substances which possess this property in the highest degree are the Bologna stone, or solar phosphorus, certain varieties of fluor-spar and carbonate of lime, some fossils, calcined shells or pearls, phosphate of lime, arseniate of lime, etc. Many diamonds shine with brilliancy in the dark if they have previously undergone an exposure of some seconds' duration only to solar light. But no substance surpasses in this respect sulphuret of barium.
It is now a long time since the cobbler of Bologna,, in Italy, astonished and amused his friends with a peculiar substance since known as Bologna phosphorus, Bologna stone, or Solar phosphorus, which shines brightly in the dark after having been placed in the sunlight for some time. This substance is sulphuret of barium. The cobbler prepared it by heating red-hot with charcoal a piece of sulphate of baryta, or Barytine, (Fig. I,) a stone which he...
I am not a minerals expert, and can't claim expertise on these particular materials. However, from a general physics / materials point of view I'm pretty sure the answer will be that,
- There is more than enough UV in sunlight to make the minerals fluoresce, but...
- The amount of visible light from the fluorescence is low enough to be undetectable by eye on top of the incoming sunlight.
It would be easy to test this by putting a black enclosure with a UV-admitting filter over the rocks and an observer's head... :-)
(if anybody with actual specific knowledge would like to confirm or deny this, I'd be very grateful!)