The mineral fluorite (\(\text{CaF}_{2}\)) is famous for its ability to glow when exposed to ultraviolet (UV) light, a phenomenon named “fluorescence” after the mineral itself. This property is a form of luminescence, where light energy is absorbed and then re-emitted as visible light. The glow is not an inherent trait of the pure calcium fluoride structure but depends on specific impurities locked within its crystalline lattice. Observing this effect requires a UV light source and a dark environment, as the emitted light is often subtle compared to daylight.
Understanding the Mechanism of Fluorescence
Fluorescence is a type of photoluminescence, a process that begins when a material absorbs photons, or packets of light energy. This absorbed energy transfers to the electrons within the atoms of the material, temporarily boosting them to a higher energy level, which is known as the excited state. Electrons naturally prefer their lower, or ground, energy state, so they immediately begin to drop back down to their original level.
As the electrons return to the ground state, they release the excess energy as a new photon of light, which is perceived as the glow. Since some energy is lost while the electron is in the excited state, the re-emitted light always has less energy and a longer wavelength than the light that was absorbed. High-energy, invisible ultraviolet light is thus converted into lower-energy, visible light, such as blue or green. This emission process is nearly instantaneous, occurring within nanoseconds of the initial absorption.
Why Fluorite Glows: Role of Trace Elements and UV Wavelength
The fluorescence in fluorite is an outcome of trace elements acting as “activators” within the crystal structure. These activators are typically rare earth elements, such as Europium (Eu) and Yttrium (Y), that substitute for calcium atoms in the lattice. Divalent Europium (\(\text{Eu}^{2+}\)) is the most common activator, responsible for the characteristic rich, blue or violet-blue fluorescence seen in many samples.
The color of the emitted glow depends entirely on the specific trace element present and its concentration. For example, while \(\text{Eu}^{2+}\) generally produces a blue glow, other elements like Dysprosium (\(\text{Dy}^{3+}\)) can lead to a yellow fluorescence, while Samarium (\(\text{Sm}^{2+}\)) can cause a red glow. This variability explains why fluorite specimens from different geographic locations can fluoresce in a wide spectrum of colors, including white, yellow, green, and red.
The specific wavelength of the UV light used also dictates the resulting glow. Longwave UV (LWUV), often referred to as a standard “black light” (around 365 nanometers), causes most fluorite specimens to glow a vibrant blue. Shortwave UV (SWUV), which is higher in energy (around 254 nanometers), causes fewer specimens to react. However, those that do may fluoresce a different color or with greater intensity compared to their LWUV reaction, which helps identify the specific activators present.
Fluorescence Versus Phosphorescence
Fluorescence is often confused with phosphorescence, but the distinction lies in the duration of the light emission after the excitation source is removed. In fluorescence, the light emission stops almost immediately, within a fraction of a second, the moment the UV lamp is turned off. The electron returns to its ground state instantly, and the glow vanishes.
Phosphorescence, however, involves a slight delay in the energy release, causing an “afterglow” that can last from seconds to hours. In phosphorescent materials, the excited electrons become temporarily trapped in an intermediate energy state before they can return to the ground state. This slow, delayed return is what creates the sustained glow-in-the-dark effect, which is commonly seen in items like plastic toys or certain mineral specimens.
Common Fluorescent Minerals Beyond Fluorite
While fluorite gave its name to the phenomenon, hundreds of other minerals also exhibit fluorescence under ultraviolet light. Calcite, a common carbonate mineral, is perhaps the most frequent fluorescent mineral found, often glowing in shades of pink, red, or orange, particularly under shortwave UV light.
Willemite, a zinc silicate mineral, exhibits a bright, vivid green fluorescence. Scheelite, a calcium tungstate mineral, typically emits a bright blue-white glow when exposed to shortwave UV. Other examples include some varieties of opal and aragonite, which can fluoresce green or yellow depending on the impurities they contain.