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Fluorescent Minerals

by Bill Fowler

On a table in a dark room, lay out a large collection of crystalline minerals of different types from different locations. Then shine a short-wave ultraviolet (UV) lamp on each of the specimens. Although UV radiation is not visible to the human eye, you'll nonetheless find that about 10% of the mineral specimens in a typical collection will emit a clearly visible glow in response to the UV lamp. A few of them may even be quite spectacular in this regard. This phenomenon is called fluorescence. Of those that do fluoresce, about 10% will continue to glow for a few seconds after the UV lamp is turned off. This property is usually termed phosphorescence or afterglow.

Fluorescence and phosphorescence in minerals can be of any color. This color is usually not related to the type of mineral but is instead dependent on the nature of the fluorescence activator that is present in the mineral. Thus, the fluorescent color can be identical for two or more different mineral types that happen to contain the same activator. Or the color can be different for two or more different specimens of the same mineral type but from different geographical locations.

Common fluorescence activators in minerals include point defects in the crystal structures, as well as atoms, ions (i.e., charged particles), and molecules that are present in minerals as impurities, e.g., hydrocarbons, manganese, titanium, europium, lead, uranium, and sulfur. The causes of mineral fluorescence are complex, and a complete understanding of them requires an indepth knowledge of atomic and molecular processes. Nevertheless, a simplified explanation can be given as follows. When a fluorescence activator is struck by photons of UV light, the activator absorbs the UV energy. This extra energy promotes the activator from its ground energy state (or ground energy level) to an excited energy state or level. This is an unstable condition for the activator, and thus it tries to find a way to throw off the excess energy and thereby return to the ground state For most atoms, ions, and molecules, there are a number of ways to accomplish this goal that do not involve fluorescence. But for fluorescence activators, the only way to get rid of the excess energy is to emit photons of visible light. It is these emitted photons of visible light that our eyes detect as fluorescence 0r phosphorescence.

Although the explanation of fluorescence is complex there is nothing at all complex about the observation 0f fluorescence ... it is purely and simply beautiful! The collector of fluorescent minerals loves to display his or her specimens in display boxes equipped with UV lamps. But the greatest thrill for the collector lies in discovering attractive new specimens in the field as they respond to a hand-held UV lamp for the first time. This is best done at night, but it's also possible to do it during the day if a portable dark-box or dark shroud is carried along.

There are now at least three types of UV lamps on the market, differing mainly in the wavelength of UV radiation emitted: long-wave, mid-wave, and short-wave It is generally true that the shorter the wavelength, the more expensive the lamp, and the more mineral specimens there are that can be excited to fluorescence by that wavelength.

Many minerals will fluoresce only under one of the three wavelengths, yet there are a few minerals that will fluoresce either the same color or a different color under each of the three wavelengths. So where does one find fluorescent minerals? It turns out that the potential for finding fluorescent minerals is significant at any location where one can find crystalline rocks and minerals. For example, I have found them all across Alabama in mines and quarries, on rock dams, in road cuts, in stream gravel, along railroad tracks, on prehistoric Indian sites in timber clearcuts, on the banks of major reservoirs, and in landscaped areas where ornamental rock was used But one of the best places to find them is in the mineral collection of a rockhound who has never shined a UV lamp on his or her specimens. If this includes you, then you need to borrow a lamp from someone and check it out. Discover a few fluorescent minerals in your collection, and it could shed a whole new light on your favorite hobby.

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