UV Light and Fluorescent Minerals

A Willemite specimen under UV light

Ultraviolet (or UV) is light with wavelengths shorter than that of visible light, measuring from about 25nm (nanometers) up to 400nm. Although we can't see ultraviolet light, when visible light is dim or absent we can see ultraviolet's brilliantly colorful effect on fluorescent minerals.

The ultraviolet range of light is divided into four categories: far ultraviolet (25nm to 250nm); short-wave (250nm to 300nm); mid-wave (300nm to 350nm); and long-wave (350nm to 400nm). Of these, short-wave and long-wave UV are the ones of most interest to collectors of fluorescent minerals. Ninety percent of these minerals respond most strongly to short-wave radiation, though a respectable number respond to long-wave (also known as black light). Some minerals respond to both, and the different colors brought out by the two wavelengths can help identify a specimen.

Ultraviolet light has many uses other than causing colorful displays in fluorescent minerals. Short-wave, mid-wave, and long-wave UV are vital in numerous laboratory applications, such as chromatography and the analysis of biological samples. Short-wave and mid-wave UV kill many bacteria, and so are used in sterilization, as well as for a variety of medical treatments. Carpet cleaning companies use long-wave UV to locate pet urine stains (you can do this too, using a bottomless box or black drape to provide the necessary darkness). Security, forensic, and industrial quality control applications for ultraviolet light are common.

Fluorescent Minerals

Under the rays of a UV lamp many ordinary items fluoresce: teeth, white shirts, many inks and plastics, crankcase oil, some woods -- even scorpions, a fact which has caused problems for more than one prospector who was out hunting the desert at night! But it is the response of fluorescent minerals that excites the interest of collectors. Calcite, which is often a dull white in ordinary daylight, may fluoresce red or orange red, pink, or green. Fluorite is pretty on its own, but really comes alive under UV: bright pale yellow, deep green, blue-green, blue, violet-blue, red-violet, orange, cream, and bluish white.

The fluorescent color variations displayed by a mineral are generally due to impurities known as activators. The activator (or activators, in some cases) absorbs ultraviolet radiation, and in reaction emits a combination of visible light (color) and minute amounts of heat. The activator manganese makes calcite fluoresce red or orange-red; fluorite, a deep green; and fluorapatite, yellow. The uranyl ion makes most minerals in which it is found fluoresce some shade of green.

Sometimes the activator electrons get stuck in their high-energy state, and the mineral will continue to glow after the UV light is switched off. This is called phosphorescence. The variety of Willemite found in Franklin, NJ is an excellent example of a mineral that is both fluorescent and phosphorescent, but there are many others. Calcite is often phosphorescent. Applying heat to some UV-charged minerals (by holding them briefly under a hot water tap, for example) will make the phosphorescence even more dramatic.

How UV Lamps Work

The lamps used for this hobby (or passion!) are similar in appearance and function to ordinary fluorescent lights -- glass tubes filled with argon gas and a little mercury. Electrical current causes the mercury to emit ultraviolet light. In the ordinary fluorescent lamp, a coating on the inside of the tube turns this into visible light. Short-wave tubes are uncoated, and made of a special quartz or high-silica glass, as most kinds of glass block short-wave transmission. Long-wave tubes are regular glass coated with a long-wave-emitting phosphor instead of a visible light-emitting one.

Both short-wave and long-wave tubes require filters to block the small amount of visible light the tubes would otherwise produce. Short-wave tubes use a separate filter, which must be made of a specialized kind of glass (one of the factors that makes short-wave lamps more expensive than long-wave ones). Some long-wave tubes incorporate the filter as part of the glass, while others use a separate, inexpensive filter.

Collecting Fluorescent Minerals

Often the most satisfying way of acquiring specimens is to find them yourself. One good way to start is to join a local rock club. If none is available, or you prefer to go it alone, a good book on fluorescent minerals, combined with a rockhounding guide to your area can get you started. Always be sure that you know the status of the area where you plan on collecting, and get permission when it is needed! A good resource for both the beginning and the experienced prospector is the Fluorescent Mineral Society.

Rechargeable 6 watt UV lampSmall handheld 4 watt UV lamps are convenient and easy to carry. They are a good place to start when you are new to the hobby. The larger 6 watt handhelds will be more satisfying to the committed prospector, as they will trigger fluorescence in specimens from several feet away, a great boon when you are out hunting at night. Multiband or combo lamps allow the user to switch between short-wave and long-wave light. This can provide a significant savings in the cost of your equipment, though some decrease in ultraviolet output can be expected.

Most of the other tools useful to the UV mineral prospector are standard rockhounding tools: hammers, chisels, pry bar, field bag, safety equipment, and the like. Safety glasses or goggles are particularly useful, as they will protect your eyes from short-wave radiation as well as flying rock chips. A good flashlight should go into your field bag or pocket along with your UV lamp(s) for night trips, of course.

Some way of creating a dark space for testing possible fluorescent specimens is necessary when prospecting during daylight hours. Commercial viewing bags are available, though a black plastic sheet does a pretty decent job and is both lightweight and inexpensive.

When wrapping your prizes (which you should always do before you put them in your field bag, to protect them from getting banged up), either wrap them first in plastic wrap, or be sure that you've tested the newspaper you plan to use with your short-wave and long-wave lamps, for fluorescent dyes.

Remember when you're collecting at night to be careful to not pick up any prettily glowing scorpions!

Hints for Using UV Lamps

Handheld UV lamps run on 12 volt DC, or direct current. In practical terms this means one of the filaments in the tube will wear out before the other. If you reverse the tube when the output begins to dim, you can extend its life.

Another thing to keep in mind -- a safety suggestion this time -- is that short-wave UV is the wavelength that causes sunburn. While it is very hard to get enough exposure from a hobbyist's lamp to burn skin, looking directly into a short-wave light for an extended period of time can certainly cause temporary damage to your eyes. Avoid this! Displaying short-wave specimens is safe as long as you have them behind glass (but not in a fish tank! Fish have no eyelids, and thus no protection at all from being burned by the short-wave radiation).

Much of the information in this article comes from Ultraviolet Light and Fluorescent Minerals, by Thomas Warren, et al.

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