Fluorescence is a fascinating phenomenon in which certain minerals emit light when irradiated with ultraviolet (UV) light. This property results from the ability of atoms or ions within the mineral structure to absorb light energy and re-emit it in the form of visible light. Fluorescent minerals not only provide spectacular visual effects, but also valuable information about the chemical composition and geological conditions under which they were formed.

The fluorescence of minerals can appear in a variety of colors, from deep red and blue to vivid yellow and green. These colors are often the result of trace elements or defects in the mineral's crystal lattice that act as activators of fluorescence. For example, the presence of manganese, uranium, lead or other metal ions in low concentrations can promote fluorescence in specific colors.

Fluorescent minerals are not only of aesthetic interest; they also have practical applications in geology, mineralogy and other sciences. By studying fluorescent properties, researchers can gain valuable information about the formation and composition of rocks and deposits. These analyses also help to distinguish between different minerals and determine their origin.

In the following, we present a series of spectra of various minerals that fluoresce under UV light. The spectra were recorded with our DIY Czerny-Turner spectrometer CTS-150-C.

The fluorescence was excited using UV LEDs with a wavelength of 275 nm, as these are now inexpensive and readily available. UV LEDs also have a longer service life and are more energy-efficient than other UV light sources. In addition, they generate less heat than mercury vapor lamps, for example, which results in a more stable operating environment and reduces the risk of overheating. LEDs are small and lightweight, making them easy to integrate into compact and portable fluorescence devices. This is particularly useful for field studies or mobile use.

In mineralogy, fluorescence excitation is traditionally realized by using mercury vapor lamps with a wavelength of 254 nm, which must be taken into account when comparing the spectra. The excitation efficiency of many minerals at 275 nm may be lower than at 254 nm. Since many classical studies and references in mineralogy are based on 254 nm, results obtained with 275 nm LEDs can only be compared to a limited extent.

• Zircon
• Natrolite
• Calcite

Zircon from Vishnevogorsk / RUS

Zircon is a widespread mineral that occurs in many different geological environments. It is a zirconium silicate mineral with the chemical formula ZrSiO₄ and is characterized by its high hardness and chemical resistance. Zircon can occur in a variety of colors, including colorless, brown, green, red and yellow. When irradiated with UV light, zircon often shows a yellow fluorescence, which is usually caused by the presence of trace elements such as uranium, thorium or rare earths (e. g. europium) in the zircon's crystal lattice. These elements partially replace the zirconium in the crystal structure and lead to electron transitions that emit light in the visible range. The intensity and color nuance of the fluorescence can vary greatly and depends on the concentration and type of trace elements contained.

Vishnevogorsk, located in the southern Ural Mountains in Russia, is a well-known site for various minerals, including zircon. The yellow fluorescence of zircon from Vishnevogorsk is particularly interesting, as it is more intense and consistent compared to zircon from other regions.

Natrolite from Soutěsky, Děčín / CZ

Natrolite is a common mineral from the zeolite group, known for its needle-like crystals and striking fluorescence under UV light. Chemically, natrolite is a sodium aluminum silicate with the formula Na₂Al₂Si₃O₁₀-2H₂O. This mineral is prized for its interesting crystal structure and ability to fluoresce in different colors under UV light, with the lemon yellow fluorescence being particularly striking. The lemon-yellow fluorescence observed in natrolite from Soutěsky is particularly striking and is often caused by the presence of small amounts of activator ions such as iron or other metal ions. These ions can trigger electron transitions in the crystal lattice, leading to the emission of light in the yellow spectral range.

Soutěsky, a small town near Děčín in the northern Czech Republic, is known for its significant deposits of natrolite. The geological conditions in this region have led to the formation of natrolite crystals, which stand out due to their clear transparency and impressive fluorescence under UV light. This region is particularly famous for natrolite samples that show a strong lemon-yellow fluorescence when exposed to UV light, a characteristic that distinguishes them from natrolite from other locations.

Calcite from Vrančice / CZ

Calcite is one of the most common and diverse minerals on earth and belongs to the group of carbonates with the chemical formula CaCO₃. It occurs in a variety of colors and shapes and is known for its ability to fluoresce in different colors under UV light, depending on the trace elements and impurities it contains.

Vrančice, a region in the Czech Republic, is known for its significant calcite deposits. The calcite crystals mined there are often clear or white, but can also occur in various shades of color. This region is particularly notable for the calcites, which show a strong orange fluorescence when exposed to UV light. This is often caused by the presence of manganese (Mn²⁺) as an activator. Manganese ions in the crystal lattice partially replace calcium and lead to electron transitions that emit light in the orange spectral range.

The fluorescence of the calcite crystal present was quite weak. The measurement was therefore carried out using a spectrometer with a focal length of 100 mm. Although this leads to a higher sensitivity, it also results in a lower spectral resolution.

The reddish fluorescence of the calcite is clearly measurable between 550 nm and 710 nm. The weak additional signal below 550 nm and above 720 nm does not originate from the calcite, but from a slight fluorescence of the LED housing itself, which was not yet corrected during the measurement.