Fluorescence measurements on cerussite

Cerussite (chemical formula: PbCO₃) is a mineral that belongs to the carbonate group. It is formed by the weathering of lead ores, especially galena (PbS), and occurs in many forms, including crystals, massive aggregates or acicular structures. Cerussite is known for its high density, distinct luster and sometimes fluorescence under UV light. It was first described in the 19th century in Tyrol, Austria, and has been a coveted collector's item for mineralogists and geologists ever since.

Some cerussite samples show intense fluorescence caused by trace elements such as manganese or rare earth elements and crystal defects. These elements act as activators that absorb the energy of UV light and then emit it again in the form of visible light, usually yellowish or greenish. Cerussite fluoresces more strongly in long-wave UV light (e. g. 365 nm) than in short-wave UV radiation.

Cerussite also shows phosphorescence, a form of luminescence in which the mineral continues to glow for a short time after the UV light is switched off. This is due to the delayed release of energy from the activators.

Recording a spectrum

Using our DIY spectrometers, we recorded the fluorescence of a cerussite sample from Stribro/Czech Republic, an interesting site known for its rich mining history. Here, cerussite is often found in association with other secondary lead minerals such as anglesite or pyromorphite.

For excitation, we used a 375 nm LED to illuminate the sample at an angle of 45 degrees. The emitted light was recorded with a light guide at an angle of 45 degrees and fed to the spectrometer for analysis.

The spectrometer used was our DIY spectrometer in Czerny-Turner design with a focal length of 150 mm, a slit size of 10 µm, a grating with 300 lp/mm and a CCD detector with 3648 pixels (e9u-LSMD-TCD1304-STD line scan camera).

As the fluorescence was only weakly pronounced in the existing sample, a relatively long integration time of 15 s was selected. The recorded spectrum shows a clear emission in the range of 425 – 700 nm, which corresponds to the typical yellowish glow that cerussite typically shows under UV light.

In the range between 400 nm und 425 nm, remnants of the light from the exciting UV LED can still be seen in the fluorescence spectrum. At a central wavelength of 375 nm, this has a half-width of 15 nm (see image). However, if the integration time of the spectrometer is sufficiently high, offshoots up to 420 nm can still be detected.

However, the sensitivity of the CCD sensor used only starts at approx. 400 nm at short wavelengths (partly due to the sensor cover glass), so that only a small and asymmetrical part of the LED spectrum is detected by the spectrometer (see spectrum below).

Would you also like to carry out this experiment in your practical course or class? Then please contact us by e-mail. We will be happy to help you with any questions or with obtaining the necessary components.