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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). For example, the activator which causes the bright red fluorescence of calcite from Franklin and Sterling Hill, New Jersey, is the manganese cation, Mn+2. The "Mn" is the chemical symbol for the element manganese, and the "+2" indicates a manganese atom which has lost two electrons and therefore has a net positive charge. A cation which has lost two electrons is also referred to as divalent; three electrons, trivalent; four, quadrivalent, etc. Activators can also sometimes be anions (containing a net 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. This is demonstrated in the photograph at left, which shows the mineral calcite in different color phases from different locales. The intensity of the fluorescence depends on the concentration of the activator in the mineral, but too much activator may actually block fluorescence. In those mineral species where the activator is an integral part of the composition, such as scheelite and many uranium-containing minerals, a specimen will always fluoresce regardless of location.
In some cases a mineral specimen will continue to emit visible light for a period of less than a second to several minutes or more after the UV light source is taken away, with the luminosity gradually fading away. This is known as phosphorescence, and occurs because the excited electrons are slow in returning to their original electron shells.
Fluorescent minerals respond best to either shortwave 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.
