There are around 4,000 different types of minerals - approximately 15% of them are known to fluoresce. Impurities in the mineral (usually) cause this fluorescense - very few "pure" minerals are known to fluoresce. These impurities - "activators" - are the reason for different colors. But the presence of an activator does not mean the mineral will fluoresce - different minerals with the same activator may even fluoresce different colors. Some activators require another "coactivator" to cause fluorescence, and some impurities will quench (prevent) fluorescence. The bottom line - a piece of fluorite from one location may fluoresce brightly while one from another location may not fluoresce at all.
Known activators include: Manganese, Chromium, Iron, Titanium, Copper, Lead, Europium, Cerium, Uranyl, Tungstate, Molybdate, Sulfur, Nitrogen, and various Organic activators.
The few minerals that fluoresce when pure are called 'self-activated' minerals. These include Scheelite, Powellite, and many Uranium minerals. Two of these minerals are perhaps the primary reason the Fluorescent Mineral hobby exists today. Both Scheelite and Uranium were important minerals during WWII. Prospectors used ultraviolet lights to hunt out deposits of both minerals.
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 uranium and scheelite - primary ores used for many purposes. In the early days of mining at Franklin NJ UV lights were an essential tool in locating the ore veins.
Fluorescence is a "cold light" (luminescence) caused by electromagnetic radiation. There are many forms of electromagnetic radiation: visible light, radio, infrared, ultraviolet, X-rays, and gamma rays are all forms of electromagnetic radiation. All are like light in many ways; but most occur at wavelengths that the human eye cannot see.
Visible light can cause luminescence; shine a pure violet light at a ruby, and it glows red. It absorbs the violet light energy and uses it to create red light. But this light-induced luminescence is uncommon and hard to see. Radio and infrared supply too little energy to cause visible luminescence. X-rays can cause dramatic luminescence, but X-ray sources are expensive and dangerous - not practical for ordinary use.
The form of electromagnetic radiation that is most widely used to observe fluorescence is ultraviolet radiation, as generated by a "black light" or Ultra Violet 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.
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 "All about Tenebrescence").
The UV spectrum is further divided into ranges as follows:
Vacuum UV (VUV): 100-200nm
For the fluorescent mineral collector there are three useful wavelengths of ultraviolet light; Long wave, Mid wave, and Short wave. 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.
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 the "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.
Filter - Although ultraviolet light is "invisible", the lamps 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.
The best (most efficient) Longwave UV source to use for mineral fluorescence is a special white phosphor coated blacklight bulb, combined with an external visible light blocking filter. 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 are available in different phosphors generating 350nm, 365nm, etc. 350nm is the "standard" LW wavelength but it may be interesting to observe what a slight shift in wavelength would do.
Recently advances in LED technology have resulted in a wide selection of longwave UV LEDs. Most 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. Perhaps soon, a solid state shortwave UV light will be a practical alternative to fluorescent bulbs
Midwave (MW) - 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.
Shortwave (SW) - 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 lamps (bulbs and filters) required for SW illumination are considerably more costly.
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 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. Do not use bulbs which produce ozone.
CAUTION! Ultraviolet light is dangerous to your eyes and skin.
Even a short exposure can burn the eyes, causing severe irritation at the least and
possibly loss of sight. Never look into a UV light source with unprotected eyes.
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