Fluorescence in Minerals
White light mineral displays are always well lit with bright lights, offering a beautiful range of colors that makes them attractive to collectors. But some minerals have a unique property - a hidden rainbow of color that is only revealed using special ultraviolet lights. Though UV light is invisible to the human eye, these special "fluorescent minerals" react to the UV light by releasing visible light and glow in every color of the rainbow. This property is known as fluorescence.
What is Fluorescence?
It takes some nifty science to make fluorescence happen. All light is created by some type of energy. Two familiar types of light are incandescence and luminescence. Incandescence is light from heat energy, such as the sun, a light bulb, stovetops, fire, etc. Luminescence is when light is generated without heat, typically by an action or reaction: a Wint-O-Green life saver sparks when it breaks apart, a lighting bug glows through a chemical reaction in its abdomen, static electricity creates flashes when there’s an imbalance of electrical charges.
We're interested in the type of luminescence generated by the absorption of light - specifically ultraviolet light. This is called photoluminescence and is divided into two categories, fluorescence and phosphorescence.
Typical fluorescent minerals include: aragonite, calcite, fluorite, powellite, scheelite, sodalite, willemite, and zircon. But almost any mineral can "glow" under UV light with the right conditions. Most minerals do not fluoresce when pure (minerals like scheelite are an exception). Impurities, usually called "activators", cause a mineral to fluoresce. Different activators, in varying quantities, along with other impurities (quenchers, such as iron) can make the same mineral fluoresce in different colors, or not fluoresce at all. The amount/type of activators and quenchers in a mineral determine the fluorescent color and brightness.
From the Oxford Dictionaries:
"Fluorescence is the visible or invisible radiation produced from certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light."
“Phosphorescence is the emission of radiation in a similar manner to fluorescence but on a longer timescale, so that emission continues after excitation ceases”
Time to get excited!
The atoms in some rocks, when exposed to ultraviolet light, are temporarily "excited" by the shorter wavelength (ultraviolet) light. As a result, an electron of each atom is kicked out of its low energy "ground" state into a higher energy "excited" state. But they are fickle little electrons; they don't remain excited for long. Once they get bored, the electrons fall back to their low energy state, giving off the excess energy as visible, longer wavelength light. This process is repeated over and over until the UV light is turned off. With phosphorescent minerals,the electrons are real slow to fall back into their low energy state, and continue to give off visible light over some period of time after the UV light is turned off. With few exceptions, the energy of the emitted light is less energetic (a longer, visible wavelength) than the invisible UV excitation light. For more in-depth research, read up on "Stokes Shift" - https://en.wikipedia.org/wiki/Stokes_shift. If you are really interested in Sir George Stokes, and his paper published in 1852, the full pdf of his research is posted here - On the Change of Refrangibility of Light
What is ultraviolet (UV)?
UV lights are the mainstay of the Fluorescent Mineral hobby. These lights are used in the field to collect these beautiful minerals and are an essential tool. UV lights are not only used by hobbyists to find these treasures but have been used by prospectors in the past to find minerals such as the fluorescent minerals uranium and scheelite - primary ores used for many purposes (see this blog post for more on radioactive minerals). In the early days of mining at Franklin NJ UV lights were an essential tool in locating the ore veins.
The form of electromagnetic radiation that is most widely used to observe fluorescence is ultraviolet radiation, as generated by a "black light" or ultraviolet lights. Ultraviolet light is that portion of the electromagnetic spectrum that lies beyond the purple edge of the visible spectrum and has wavelengths between 100 and 400 nm.
Note: 400nm is where UV ends and visible light starts, but recent discussions have revealed that most people can see down to 380nm and the UV bandpass filters used in our lights pass significant amounts of this band from 380nm to 400nm. From the Icnirp: "The wavelength range where optical radiation is visible does not have sharp borders. Here, the wavelength band of 380 nm to 780 nm is used. There is an overlap with the UV wavelength range that extends to 400 nm and in the upper range with Infrared." This is probably where a lot of the "blue bleed" in photos comes from, in spite of a proper UV lamp and filter being used.
Fluorescent minerals also can change their natural color after exposure to UV light. Those capable of this reversible color change by exposure to UV (or other energy sources), without any change in their essential composition, are said to be tenebrescent (from Latin – tenebrae, meaning shadows or darkness). Another term sometimes applied is photochromic – a material that undergoes a color change in the presence of photonic energy (such as glasses containing silver salts which automatically darken in sunlight). Other examples of tenebrescence in everyday life include light filters, coatings for windows/blinds, and even jewelry (see "What is Tenebrescence" in the blog).
The UV spectrum is further divided into ranges:
- UVA: 315-400nm
- UVB: 280-315nm
- UVC: 200-280nm
- Vacuum UV (VUV): 100-200nm
For the fluorescent mineral collector there are three useful wavelengths of ultraviolet light; Longwave, Midwave, and Shortwave. A few minerals fluoresce the same color in all wavelengths, others fluoresce in only one wavelength (usually SW), and yet others fluoresce different colors in different wavelengths.
(More on UV Mineral Lights can be found here.)
Longwave (LW) - (aka - blacklight)
Compared to Shortwave, only a relatively few minerals fluoresce in longwave (perhaps only 15% of the total). Sometimes a significant difference in fluorescing color can be observed between the two. Longwave UV is closest to visible light and is the type of UV light that most people are familiar with (everyone has seen "blacklight" bulbs used in discos, to light up posters, etc). The light easily passed through most types of glass and plastic. Thus Longwave lamps are fairly inexpensive compared to shortwave - and the filters are significantly cheaper. Examples can be viewed by accessing the database using the "Longwave Fluorescent Minerals Category" on Nature's Rainbows.
Recently advances in LED technology have resulted in a wide selection of longwave UV LEDs. Many are rather weak in power, and some are very expensive, but as in all technology these products will continue to improve and come down in price. Using some of the higher power LW LEDs available today amazing results are seen; a few folks have built LW flashlights which allow exploration in the daytime, and cause fluorescence not usually observed under fluorescent LW. A new flashlight, complete with copy-cats, has hit the market bringing powerful and affordable LED performance to the masses. Most recently (2022) new shortwave and midwave flashlights have appeared on the market.
Filter - Although ultraviolet light is "invisible", the bulbs (all - LW, MW, and SW) used to generate UV light emit a significant amount of bluish/white visible light. This visible light must be blocked (filtered, typically by a piece of "black glass") so that the light does not overwhelm the fluorescence. A longwave filter is relatively inexpensive - in fact, some fluorescent lamps have a filter built-in (such as the common "blacklight fluorescent bulb" used to illuminate posters and "glow" products. Even LW LEDs produce too much visible light.
One common Longwave UV source to use for mineral fluorescence is a special white phosphor coated BLB bulb, combined with an external visible light blocking filter (but recently LW LEDs have advanced to a point where they significantly "outshine" any equivalent fluorescent bulb). While this approach is considerably more expensive than using ordinary blacklight bulbs, it provides much purer UV light, allowing less visible light to mask the fluorescence. It should be noted that LW bulbs (and LEDs) are available in different wavelengths, generating 350nm, 365nm, etc. 365nm seems to be the "standard" LW wavelength for minerals, but it may be interesting to observe what a slight shift in wavelength would do.
The Midwave ultraviolet spectrum lies in between Longwave and Shortwave. Midwave UV is partially stopped by clear glass just as Shortwave UV. Midwave UV is passed by existing Shortwave ultraviolet filters, and, since midwave lights (tubes) have become more readily available, the properties of Minerals under midwave UV are starting to be noticed.
Certain Minerals that do not exhibit a strong Fluorescence under either LW or SW may FL strongly under MW. Occasionally a color change may occur, or fluorescence may be seen where none was observed under the other two lamps. Examples can be viewed in the Nature's Rainbows database using the "Midwave Fluorescent Minerals Category".
Shortwave UV is the most popular light source for displaying fluorescent minerals. The number of minerals that fluoresce under SW far exceeds those that fluoresce under LW or MW. But the lights (bulbs and filters) required for SW illumination are considerably more costly. Examples can be viewed in the Nature's Rainbows database using the "Shortwave Fluorescent Minerals Category".
Shortwave ultraviolet is almost completely stopped by most forms of glass or plastic. Quartz or silica glass must be used in shortwave tubes to let the shortwave UV escape the tube. SW ultraviolet can, over time, cloud the shortwave filter used in the lamp assemblies. This is called solarization. Additionally, the ultraviolet light source has a tendency to lose output power after several hundred hours of use; heating and repeated on/off cycling can further degrade some of the lights.
In addition to the SW bulb, a visible light blocking filter must be used to only allow the SW UV to pass. The best filters are made by Hoya Optics, a Japanese company. This filter glass is quite expensive, and is the heart of a good UV lamp.
Some shortwave bulbs produce significant amounts of ozone (used for water sterilization mostly). Ozone has that mid-summer "thunderstorm" smell and can be irritating (or even harmful in quantity). Worse, as it builds up in a lamp housing it can block UV. There are two types of UVC bulbs - low ozone (some will say "no ozone" but in actuality they still produce a little ozone) and ozone generating. The ozone smell is noticeable from either bulb, but the generating bulb will be very strong and really not useful in our hobby. Do not use bulbs meant to produce ozone.
The graph above shows the transmission performance of the most commonly used bandpass filter - Hoya U325c (Ultraviolet transmitting, visible absorbing): (click for larger image).