Fluorescence Rocks! Exploring Glowing Sodalite

Co-Authored by Dean J. Campbell* and Thomas Kahila*

*Bradley University, Peoria, Illinois

In 2017, rocks that exhibited a yellowish fluorescence when irradiated with UV light were found by Erik Rintamaki on the Upper Michigan beaches of Lake Superior. He named these rocks Yooperlites (derived from the “U.P.” of Michigan’s Upper Peninsula) and they became a bit of a sensation for collectors.^1,2 In fact, one of us recently found one of these Yooperlites on a trip to Upper Michigan. These rocks are comprised of fluorescent sodalite minerals in syenite, an igneous rock. The syenite is believed to have originated from 1.1 billion year old intrusive igneous rocks in Canada that were later pushed down to the beaches by much more recent continental glaciation.³ Although Central Illinois has predominantly sedimentary rock, we suspected there might still be a chance of finding fluorescent sodalite among igneous rocks that have been transported to this area. For example, continental glaciation has reached down to Central Illinois from the north. We found sodalite by searching at night while using uvBeast brand (uvbeast.com) LED flashlights. These produced a range of wavelengths with emission maxima that were near but not exactly at their listed value of 365 nm. Figure 1 shows examples of locally found fluorescent sodalite as viewed in (LEFT) visible light and (RIGHT) 370 nm UV light. Sodalite in seyenite, like the top rock in each image, were found along gravel roads in the area. Sodalite in seyenite like the left rock in each image were found along railroad tracks in the area.

However, other objects exhibited fluorescence when irradiated with UV light, complicating the search. We encountered minerals that glowed purple and fossils containing calcium carbonate that glowed yellow. Many non-mineral items glowed, especially human-produced objects that contained fluorescent dyes such as optical brighteners. Objects that appeared to be slag from smelting operations glowed purple. Algae on rocks produced a pink glow attributed to chlorophyll.⁴ Perhaps the most misleading were rocks and other objects such as the piece of concrete at the right of each image in Figure 1, which exhibited fluorescence that resembled that of sodalite. In reality, this glow appeared to be due to a type of lichen.

sodalite_rocks.png

 Figure 1. Samples of two rocks bearing fluorescent sodalite and a piece of concrete bearing fluorescent lichen (LEFT) under visible light and (RIGHT) irradiated with 370 nm UV light.

Fluorescence Spectroscopy

Fluorescence spectroscopy was used to help determine which samples contained sodalite. Figure 2 shows fluorescence of sodalite when excited at 368 nm by a UV LED flashlight and a 406 nm laser pointer. In both cases, a broad emission was observed that contained about five peaks with a maximum of about 630 nm. These spectra resemble spectra found online.² Spectroscopy also shows that lichen fluorescence was not the same as sodalite fluorescence. Figure 2 shows the spectrum of lichen excited by a 406 nm laser pointer, which appears to be comprised of two broad overlapping peaks. These spectra were acquired using an Ocean Insights OCEANHDX spectrometer and a fiber optic reflectance accessory. Fluorescence spectra from some brighter-glowing sodalite samples were also acquired using a Vernier Fluorescence/UV-VIS Spectrophotometer. The spectrophotometer was turned on its side, detector side up, and the sample was placed in a cuvette slid sideways into the system. The light source was then shined onto the sample though the open end of the cuvette. In either measurement approach, care needed to be taken to have the samples in just the right positions to optimize their fluorescence spectra.

sodalite_fluorescence_graph.jpg

Figure 2. Emission spectra showing the fluorescence of sodalite excited at 368 nm and 406 nm and the fluorescence of lichen excited at 406 nm.

Chemical structures

Sodalite is an aluminosilicate mineral with the empirical formula Na₄Si₃Al₃O₁₂Cl, (or sometimes twice those values, consistent with the number of atoms in the cubic unit cell of the compound).^5-7 The arrangement of atoms within structures can be represented by a variety of models.^8-11 Figure 3 shows a LEGO model that illustrates superimposed layers of atoms in the sodalite unit cell. Here sodium is blue, silicon is yellow, aluminum is gray, oxygen is red, and chlorine is black. The orange beads represent the edges of the unit cell. To establish where to place the bricks representing atoms on the baseplate, a face-on image of the sodalite unit cell with atoms was retrieved from the mindat.org site for sodalite.² The image was moved into PowerPoint, and colored dots were drawn over the atoms in the image and grouped together. The unit cell image was removed, and an image of a blank LEGO baseplate was placed behind the colored dots. The array of dots was then scaled to best match the pegs on the baseplate to show where to place the bricks. To find out which bricks really belonged to which layer of the unit cell model, the unit cell image was rotated on the mindat.org site for views of the unit cell from different angles.² Rotating the unit cell image online permitted a better visualization of the ring-like structures formed by the silicon, aluminum, and oxygen atoms. The fluorescence of sodalite is related to its chemical composition. The yellow glow has been attributed to negatively charged disulfide ions replacing the chloride ions in the structure. The sulfur is also thought to expand the unit cell of the structure.²

layer_eclipsed.jpg

Figure 3. LEGO model illustrating superimposed layers of atoms in the sodalite unit cell, where sodium is blue, silicon is yellow, aluminum is gray, oxygen is red, and chlorine is black. The orange beads represent the edges of the unit cell.

The Supporting Information contains a PDF file showing how 2 peg x 2 peg LEGO bricks can be placed on a flat LEGO baseplate to model the sixteen-layer sequence in the unit cell of sodalite.

Classroom Connections

The search for fluorescent sodalite produced a number of predictions that required testing. Fluorescent sodalite was predicted to be found among igneous rocks brought into Central Illinois, either from continental glaciers or other sources. Searches using UV flashlights revealed that sodalite-bearing rocks are here in Central Illinois, although their sources are unknown. The rocks themselves might provide clues to their origins. For example, the stones found in Central Illinois are rather angular and not rounded like gravel associated with glaciers. Two different local rock sources were predicted to both contain sodalite, and their fluorescence spectra were consistent with known sodalite spectra shown online. The yellow glow of the lichen produced a false positive result in a simple UV flashlight fluorescence test that did not hold up under more extensive spectral analysis. The sodalite fluorescence was predicted to be different from that of lichen, and this seems to be the case for at least one lichen sample. These tests could be used in discussions about the scientific method.

The molecular weight, empirical formula, and percent composition of sodalite and other minerals can be found on webmineral.com, making it a good source of information for crafting chemistry problems.⁷ The fluorescence of sodalite is impacted by the concentrations of sulfur present, which is an example of how chemical impurities can impact the physical properties of compounds. The strategy of utilizing images of unit cells and LEGO baseplates to build LEGO model layer sequences worked well for a complex sodalite unit cell. This approach is anticipated to be successful for a number of other unit cells, especially those cells containing faces that meet at right angles since they resemble a square arrays of LEGO brick pegs. We look forward to exploring this approach more in the future.

Safety

Do not shine UV light or laser pointers into anyone’s eyes. Anyone exploring in the dark for rocks risks injury, e.g., from falls or being run over. Trespassing on private land is also a bad idea. Small children could possibly swallow LEGO bricks and small rock samples.

Acknowledgements

This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium.

References

1, GEOLOGYSCIENCE. Yooperlite. https://geologyscience.com/gemstone/yooperlite/ (accessed August, 2024).

2.  NIGHTSEA. Sodalite Fluorescence – a Viral Sensation. https://nightsea.com/nature/minerals/sodalite-fluorescence/ (accessed August, 2024).

3.  McClarren, C. Yooperlites, aka Glowdalites: what are they and where can I find them? https://www.michiganrockhounds.com/articles/glowdalites (accessed August, 2024).

4.  Campbell, D. J. “Algae Connections to Chemistry Classrooms.” ChemEd Xchange. https://www.chemedx.org/blog/algae-connections-chemistry-classrooms (accessed August, 2024).

5.  Hudson Institute of Minerology. Mindat.org: Sodalite. https://www.mindat.org/min-3701.html (accessed August, 2024).

6.   Mottana, A.; Crespi, R.; Liborio, G. Simon and Schuster’s Guide to Rocks and Minerals. Simon and Schuster: New York, 1978.

7.  Webmineral.com: Sodalite Mineral Data. https://webmineral.com/data/Sodalite.shtml (accessed August, 2024).

8.  Campbell, D. “Modeling Unit Cells and Layer Sequences of Solar Cell Materials using Dimpled Packaging.” Green Chemistry Teaching and Learning Community. https://gctlc.org/modeling-unit-cells-and-layer-sequences-solar-cell-materials-using-dimpled-packaging (accessed August, 2024).

9.  Campbell, D. “Tissue Paper Banners Connected to Chemistry.” ChemEd Xchange. https://www.chemedx.org/blog/tissue-paper-banners-connected-chemistry (accessed August, 2024).

10.   Campbell, D. J.; Walls, K.; Steres, C. “Paper Snowflakes to Model Flat Symmetrical Molecules.” ChemEd Xchange. April 6, 2022. https://www.chemedx.org/blog/paper-snowflakes-model-flat-symmetrical-molecules (accessed August, 2024).

11.   Robinson, K. F.; Nguyen, P. N.; Applegren, N.; Campbell, D. J. “Illustrating Close-Packed and Graphite Structures with Paper Snowflake Cutouts.” The Chemical Educator, 2007, 12,163-166.

Supporting Information

Attached is a PDF file showing how 2 peg x 2 peg LEGO bricks can be placed on a flat LEGO baseplate to model the layer sequence in the unit cell of sodalite.