by Bruce H. Fine
We all experience fluorescence more than we realize. Ever notice how *bright* white clothing appears in sunlight? The reason is that many laundry detergents and bleaches contain trace amounts of fluorescent dyes in them. Why? Since there is a component of UV light present in bright sunlight, clothes washed in these detergents appear, due to the fluorescent affect, to be brighter than clothes not washed with fluorescent soaps.
Light is a form of energy. To create light, another form of energy must be supplied. There are two common ways for this to occur, incandescence and luminescence.
Incandescence is light from heat energy. If you heat something to a high enough temperature, it will begin to glow. When an electric stove's heater or metal in a flame begin to glow "red hot", that is incandescence. When the tungsten filament of an ordinary incandescent light bulb is heated still hotter, it glows brightly "white hot" by the same means. The sun and stars glow by incandescence.
Luminescence is "cold light", light from other sources of energy, which can take place at normal and lower temperatures. In luminescence, some energy source kicks an electron of an atom out of its "ground" (lowest-energy) state into an "excited" (higher-energy) state; then the electron gives back the energy in the form of light so it can fall back to its "ground" state.
Phosphorescence is delayed luminescence or "afterglow". When an electron is kicked into a high-energy state, it may get trapped there for some time. In some cases, the electrons escape the trap in time; in other cases they remain trapped until some trigger gets them unstuck. Many glow-in-the-dark products, especially toys for children, involve substances that receive energy from light, and emit the energy again as light later.
Triboluminescence is phosphorescence that is triggered by mechanical action or electroluminescence excited by electricity generated by mechanical action. Some minerals glow when hit or scratched, as you can see by banging two quartz pebbles together in the dark.
Thermoluminescence is phosphorescence triggered by temperatures above a certain point. This should not be confused with incandescence, which occurs at higher temperatures; in thermoluminescence, heat is not the primary source of the energy, only the trigger for the release of energy that originally came from another source. It may be that all phosphorescences have a minimum temperature; but many have a minimum triggering temperature below normal temperatures and are not normally thought of as thermoluminescences.
Fluorescence and, a related phenomena called phosphorescence, are properties of materials that emit visible light when exposed to UltraViolet (UV) light and/or continue to emit such light after exposure to UV light.
Ultraviolet is closest to and just shorter than visible light in wavelength. Ultraviolet can be subdivided according to wavelength. From lowest to highest: longwave ultraviolet (UVA or near ultraviolet), middle-wave ultraviolet (UVB), short-wave ultraviolet (UVC), and extreme ultraviolet.
Longwave ultraviolet is part of sunlight. It is the lowest-frequency ultraviolet, and thus the nearest to visible light. Longwave ultraviolet passes easily through most transparent types of glass and plastic. Longwave ultraviolet lights are available, and they are the cheapest and longest-lasting ultraviolet lights. They cause some fluorescent minerals (perhaps 15%) to exhibit fluorescence.
Midwave ultraviolet is also part of sunlight. Longer wavelengths of midwave ultraviolet cause suntans, while shorter wavelengths of midwave cause sunburn. Midwave, especially shorter wavelengths, are partially stopped by clear glass. Midwave ultraviolet light is passed thru short-wave ultraviolet filters. Midwave tubes have recently become widely available, some collectors are starting to use midwave to study mineral fluorescence.
Short-wave ultraviolet is emitted by the sun, but it is stopped in the upper atmosphere of the earth by the ozone layer. Short-wave ultraviolet can also cause burns resembling sunburns (they are often called sunburns, even though the sun did not cause them). Short-wave ultraviolet is almost completely stopped by most forms of glass or plastic. Quartz or special glasses must be used in short-wave tubes to let the short-wave UV escape the tube. Short-wave ultraviolet over time cause failure in the short-wave filter used in short-wave ultraviolet lights; this process is called solarization. Short-wave ultraviolet is the most popular for seeing mineral fluorescence, causing fluorescence in perhaps 90% of fluorescent minerals.
Extreme ultraviolet is also emitted by the sun, but is stopped in the upper atmosphere, and in so doing forms ozone from the atmosphere's oxygen. It is this high ozone layer that stops part of the sun's middle-wave ultraviolet rays and all of its short-wave ultraviolet rays, and which may be in danger from some commercial chemicals. Extreme ultraviolet is closest to X-rays in frequency, and as with X-rays there is no practical equipment for its use. Few substances are transparent to extreme ultraviolet, and even air stops it within a fairly short distance.
Fluorescent minerals respond best to either short-wave UV light, which has a wavelength of 254 nanometers (nm), or longwave UV, at 366nm. Some minerals may fluoresce under both wavelengths with the same or a similar color, while some may show different colors under each. Most respond best to only one of the two. Well over 3600 mineral species have been identified at this time. Something over 500 of them are known to fluoresce visibly in some specimens. Arizona is a excellent location for fluorescent minerals hunting. There are over 140 mineral found in Arizona that have been know to Fluoresce at other locations around the world. Below are some of those minerals.
ADAMITE, ALLOPHANE, ANALIME, ANHYDRITE, ARAGONITE, AUSTINITE, AUSTINITE, BARITE, BASALUMINATE, BUSSAMITE, BAYLEYITE, BECQUERELITE, BERYL, BLODITE, BOLTWOODITE, BRUCITE CALCITE, CALOMEL, CASSITERITE, CELESTINE, CERUSSITE, CHABAZITE, CHLORAPAITITE CHRYSOBERYL, CLINOHEDRITE, COLEMANITE COOKEITE CORUMDUM COTUNNITE COWIESITE CRISTOBALITE DOLOMIT,E DICKITE, DIOPSIDE, DUMONTITE, DUMORTRERITE, EDENITE, ELBAITE, EPSOMITE, ETTRINGITE, EUCRYOTITE, FERRIERIT,E FLUORAPATITE, FLUORITE, GEARKSUTIT,E GLAUBERITE, GMELINITE, GONNARDITE, GREENOCKITE, GROSSULAR, GYPSUM, GYROLITE, HALITE, HARMOTOME, HAWLEYITE, HECTORITE, HELVITE, HEMIMORPHITE, HEVLANDITE, HUNITE, HYDROCERUSSITE, HYDROXYHERDRITEHYDROMAGNESITE HYDROZINCITE JUNITOITE, KUTNAHORITE, LAMONTITE, LEPIDOLITE, LEUCITE, LEVYNE, LIEBIGITE, MAGNESITE, MAMMOTHITE, MANGANOAXINITE, MARGARITE, MARIALITE, MATLOCKITE, MESOLITE, META-AUTUNTIC, METATORBERNITE, META-ZEUNERITE MINUM , MORSSANITE, MONTMORILLONITE, MORDENITE, NATROALUNITE, NEPHELINE, OPAL, ORTHOCLASE, PECTOLITE, PHLOGOPITE, PHOSGENITE, PLAGIOCLASE, POWELLITE,PREHNITE, PYROMORPHITE, PYROPHYLLITE, QUARTZ, REALGEAR, RHODOCHROSITE, SABUGALITE, SANIDINE, SCHEELITE, SCHOEPITE, SCHROCKINGERIT, SEPIOLITE, SODIUM-ZIPPEITE, SPHALERITE, SPINEL, STEVENSITE, STILBITE, STOLZITE ,STRONTIANITE, SULFUR, SWARTZITE, TALMESSITE, THAUMASITE, THENARDITE, THOMSONITE, THORITE, TILASITE, TITANITE, TOBERMORITE, TOPAZ, TORBERNITE, TERMOLITE, TRIDYMITE, URANOCIRICITE, URANOPHAN,E URANOSPINATE, UVORVITE, VANDANITE, WICKENBURGITE, WILLEMITE, WITHERITE, WOLLASTONITE, WULFENITE, WURTZITE, XONOTIME, XONOLITE ZUENERITE, ZINCITE, ZIRCON, ZUNYITE.
The phenomenon known as fluorescence occurs at the subatomic level by a process called electron excitation. Electrons are subatomic particles that orbit the nucleus of an atom at specific distances known as electron shells. These shells are arranged in layers around the nucleus, the exact number of electrons and their shells depending on the type of atom (element). The electrons contained in the shells nearest the nucleus carry less energy than the electrons in the outer shells.
When certain atoms are exposed to ultraviolet (UV) light, a photon (particle of light energy) of UV will cause an electron residing in a lower-energy inner electron shell to be temporarily boosted to a higher-energy outer shell. In this condition, the electron is said to be excited. It will then drop back to its original inner electron shell, releasing its extra energy in the form of a photon of visible light. This visible light is the fluorescent color that our eyes perceive. The exact color depends on the wavelength of the visible light emitted, with the wavelength itself being dependent on the type of atom undergoing the electron excitation. The specific atoms which undergo the fluorescence are known as activators. They are usually present as impurities in the normal molecular structure of the mineral, but sometimes are an intrinsic part of the mineral's composition. In fluorescent minerals, very often the activators are cations, which are atoms or molecules which carry a net positive charge (due to the loss of one or more electrons, each of which display a negative charge). Because the activators are usually impurities, the same mineral species may fluoresce in some locations and not others, depending on whether the activator was present when the mineral was formed. It also may contain different activators depending on location, and therefore fluoresce in various colors. The intensity of the fluorescence depends on the concentration of the activator in the mineral, but too much activator may actually block fluorescence.
There are a few minerals that will fluoresce when pure. These are called "self-activated" minerals, and include scheelite, powellite, and several uranium minerals. Others suspected of being self-activated include benitoite, cerussite, anglesite and perhaps many other lead minerals.
The best time to hunt for fluorescence is at night. Your eyes become adapted to the dark and you can pick up a weak fluorescence at greater distances. Rock hunting at night has a excitement all it own. Walking carefully to place you foot securely on a rocky ledge or backing in to a cactus. By night you discover the real meaning of "invisible" fluorescent minerals. In day light a ordinary rock specimen show several types of minerals of little to no interest , but at night under a UV light certain unnoticed specks or transparent crystals become very dominant. Turn your flash light on it and there are gone, their color blends in so closely with the adjoining rocks they are lost to sight.
As a field collector of fluorescence minerals you have 2 special needs the first is a portable source of ultraviolet light, and the second a means of creating enough darkness to view the minerals around you. The first is easy there are many different kinds of UV light out there. The Second is a little harder a black piece of plastic can be used or a blanket, viewing boxes are not that hard to make but both add extra weight to your pack. Believe me the rocks weigh enough by them selfs. I find the best way is just to hunt at night. Fluorescence mineral hunting is much easier for the most part than regular rock hunting. This depends on the quality of the black light and the amount of darkness. Another reason that they are easier to find is that portable UV light are a fairly new thing and were not around when many of the mines were active. Many of the mine dumps are just covered with color.
This article is part of a presentation given by Bruce H. Fine at the February 14, 2003 meeting of the Calgary Rock and Lapidary Club.