Introduction
Many minerals fluoresce in their natural state. The Fluorescent Mineral Society in the USA lists around 200 specimens with some fine images of minerals fluorescing under different wavelengths of UV light. And the Fluomin database lists well over 1100 types. Both databases have links to the RRUFF project which provides open access to information on minerals such as their infrared and Raman spectra and even X-ray diffraction data.
My own mineral collection is far more modest, limited to only thirty or so different specimens, collected over many years. Several of these fluoresce under UV light and with a small spectrometer, a UV lamp and an optical fibre probe, you can obtain and record their fluorescence spectra.
Experimental Technique
Fluorescence spectra of each mineral were obtained using this simple setup with a UV LED light source, an optical fibre reflection probe and a small spectrometer.
Fluorite
Fluorite, which is also known as fluorspar, is perhaps the best known mineral that fluoresces under the right conditions. Chemically, fluorite is calcium fluoride, CaF2. The scientific term “fluorescence” is actually derived from the word fluorite the mineral and not the other way round. This sample comes from Weardale, County Durham, England and exhibits extensive twinning which is common with the halite minerals such as fluorite. Twinning occurs during crystal growth when two separately developing crystals share common lattice points. So in this specimen we can see cubic crystals developing out of the face of another cubic crystal.
Calcite
Chemically, calcite is one of the main polymorphs of calcium carbonate, CaCO3, and is extremely common. This is a good example of of the rhombohedral form of the mineral from Morocco.
Cerussite
Cerussite is lead carbonate, PbCO3 and is an important source of metallic lead by its extraction from the ore. This small sample is just over 1 cm in length but already weighs around 10 g, far greater than an equivalently sized sample of calcite or similar mineral with a lower density. Under UV light, the mineral fluoresces in the yellow.
Mangano Calcite
Mangano calcite is a variety of calcite that is rich in manganese ions. So the chemical formula can be expressed as (Ca,Mn)CO3. This sample originates from Peru, but the mineral is widely distributed.
Under room light, my specimen is tinged slightly pink, although this is not evident from the photo. But under UV light the pink coloration is pronounced as the manganese ions in the mineral strongly fluoresce.
Travertine
Travertine is also a calcium carbonate based mineral, common around the world, that often forms as deposits around hot limestone springs. This sample comes from Yorkshire (UK) and the mineral exhibits broad band fluorecence across the spectrum, perceived by the eye as blue-white in colour.
Chalcedony
Chalcedony is a form of silicon dioxide (silica), SiO2. This nodule comes from Morocco. It fluoresces with a pale orange light when exposed to UV, as shown below.
Lapis Lazuli
Lapis Lazuli consists of a mixture of different minerals, so it is best described as a metamorphic rock. The main minerals in Lapis Lazuli are lazurite, calcite, diopside and pyrite. Because of its lustrous blue colour, which comes from lazurite, it is considered a semi-precious stone.
I was slightly surprised to see the above fluorescence spectrum with its multiple band structure. This is significantly different from the spectra of the other minerals recorded above. However, lapis lazuli does consist of a mixture of minerals, each with metallic impurities, which may account for this complex fluorescence band structure. And similar spectra in fluorescence have been observed before, as shown in this example from the fluomin database.
These are just a few examples from my own humble mineral collection. The interested reader can peruse the databases mentioned earlier for many more examples.
