Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It also occurs when molecules are excited to higher electronic states by energetic electron bombardment. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed “resonance fluorescence”. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region.
Gemstones, minerals, may have a distinctive fluorescence or may fluoresce differently under short-wave ultraviolet, long-wave ultraviolet, or X-rays.
Many types of calcite and amber will fluoresce under shortwave UV. Rubies, emeralds, and the Hope Diamond exhibit red fluorescence under short-wave UV light; diamonds also emit light under X-ray radiation.
Fluorescence in minerals is caused by a wide range of activators. In some cases, the concentration of the activator must be restricted to below a certain level, to prevent quenching of the fluorescent emission. Furthermore, the mineral must be free of impurities such as iron or copper, to prevent quenching of possible fluorescence. Divalent manganese, in concentrations of up to several percent, is responsible for the red or orange fluorescence of calcite, the green fluorescence of willemite, the yellow fluorescence of esperite, and the orange fluorescence of wollastonite and clinohedrite. Hexavalent uranium, in the form of the uranyl cation, fluoresces at all concentrations in a yellow green, and is the cause of fluorescence of minerals such as autunite or andersonite, and, at low concentration, is the cause of the fluorescence of such materials as some samples of hyalite opal. Trivalent chromium at low concentration is the source of the red fluorescence of ruby. Divalent europium is the source of the blue fluorescence, when seen in the mineral fluorite. Trivalent lanthanides such as terbium and dysprosium are the principal activators of the creamy yellow fluorescence exhibited by the yttrofluorite variety of the mineral fluorite, and contribute to the orange fluorescence of zircon. Powellite (calcium molybdate) and scheelite (calcium tungstate) fluoresce intrinsically in yellow and blue, respectively. When present together in solid solution, energy is transferred from the higher-energy tungsten to the lower-energy molybdenum, such that fairly low levels of molybdenum are sufficient to cause a yellow emission for scheelite, instead of blue. Low-iron sphalerite (zinc sulfide), fluoresces and phosphoresces in a range of colors, influenced by the presence of various trace impurities.
For our simplification, fluorescent minerals are broken down into two different groups, long wave and short wave. Mineral collectors use two types of UV light to test for fluorescence. The common “black light” is long-wave UV, with wavelengths around 365 nanometers. (Visible light ranges from violet at about 400 nm to red at about 700 nm.) Long-wave UV is the default among collectors: the equipment is inexpensive and there’s no particular hazard in using it (although you should keep it from your eyes like any bright light). You can get a pocket flashlight using ultraviolet LEDs for a few dollars. It leaks a lot of violet light as well, but it’s a good screening tool if you’re walking through a cave or are out at night exploring a tailings pile. More serious black lights filter out the visible part and
can produce spectacular light shows when your eyes are dark-adapted.
Short-wave UV, with wavelengths around 250 nm, excites fluorescence in a whole different set of minerals. The lamps are much more expensive, though. Moreover, the light itself is hazardous.
Short-wave UV causes skin burns and eye damage. Although all light is technically radiation, this stuff is really radiation. Short-wave UV is what’s used in sterilizing lamps and UV water purifiers. (The sun’s
short-wave UV radiation is filtered out by the ozone layer, high in the stratosphere.) These factors make short-wave UV fluorescence a minority hobby.
Common minerals that fluoresce are calcite, halite, sodalite, wulfenite, esperite, opal, willemite, selenite, benitoite, fluorite, and hardystonite.
Under short wave fluorescence, the following colors are observed;
- Halite – dark red
- Sodalite – light orange
- Wulfenite – brown
- Esperite – bright yellow
- Opal – light green
- Willemite – green
- Selenite – pale blue
- Benitoiote – light blue
- Fluorite – dark blue
- Hardystonite – bluish violet
- Calcite – light pink
Long wave UV light shows the following;
- Apatite – blue
- Barite – orange
- Cerussite – white
- Scapolite – orange and blue
- Scheelite – orange
- Sphalerite – red, yellow, and blue
Many minerals are fluorescent under both long wave and short wave;
- Apatite – orange, yellow
- Aragonite – yellow(sw), green, white
- Barite – red, orange(lw), yellow, violet, white
- Calcite – red, orange, yellow, green, blue, violet, and white
- Celestite – yellow, blue, white
- Cerussite – orange, yellow
- Fluorite – yellow, blue, white
- Gypsum – yellow, blue, white
- Scapolite – red(sw), orange(sw), yellow, blue(lw)
- Scheelite – orange(lw), yellow, blue(sw), white(sw)
- Sphalerite – red(lw), orange, yellow(lw), blue(lw)
- Witherite – yellow, blue, white
- Wollastonite – orange, yellow, blue, white
- Zircon – orange, yellow
**This information is compiled from several sources so they may or may not be as accurate as I would like. **
Mineral of the Month: June 2014
Photographic Images © 2003 James O. Hamblen