Mineral of the Month – Fluorescent Syenite “Yooperlite*” November 2019

Syenite is a coarse-grained intrusive igneous rock formed when molten lava cooled slowly under the Earth’s surface. It has a general composition like granite.

Ultraviolet Light (UV) is light categorized between the wavelengths of 100 nm and 400 nm. These wavelengths are shorter than what humans can see.

Fluorescence is the emission of light by a substance that has absorbed light in the ultraviolet range of the spectrum and emits light in the visible spectrum. The emitted light has a longer wavelength, and therefore can be visibly seen, than the exciting invisible UV wavelength. Fluorescence happens on the atomic level. When a substance fluoresces, the electrons in the atoms reacting to the UV light get excited by the wavelength and use the energy to jump away from the nucleus to a higher electron orbit. Nuclei do not like empty electron shells, so they pull electrons back down to fill the empty slot. This movement back down releases energy, which is displayed by light in the visible spectrum.

“Yooperlites” contain a variety of sodalite with an impurity that reacts to the specific UV wavelength of 365nm. This wavelength was developed to detect cat urine, as well as for use in manufacturing processes. Fluorescent syenite has minimal fluorescence with a 395nm wavelength.

In most cases, syenite is not fluorescent. In most cases, sodalite is also not fluorescent. The suspected source of this variety of syenite that has a fluorescent form of sodalite is believed to come from central Ontario. The rock was transported to the Michigan area 8,000 to 10,000 years ago by glaciers and/or by icebergs on Lake Algonquin, a larger predecessor lake to Lake Superior. The east coast of Lake Algonquin was in the area of the fluorescent syenite deposit. Thus, once all the fluorescent syenite specimens are picked up from the Lake Superior beaches, the supply will be gone since it cannot be replenished – at least until the next glacier scrapes across the landscape. So PLEASE, if you go out to find these amazing rocks, just take a few and leave the rest for others to find.

Some people have attempted to find fluorescent syenite during daytime hours. This is almost impossible since the syenite looks very much like granite. Without the 365nm wavelength, and the darkness needed to see its effect, there is no way to identify the fluorescent component during the day.

Both granite and syenite are similar in that they are intrusive igneous rocks that formed when molten magma cooled slowly under the earth’s surface. During cooling, minerals of the same type combine to form mineral grains. The slower the cooling, the larger the mineral grains. The difference between granite and syenite is primarily due to the quantity of quartz. Granite has as one of its components more than 20 percent quartz; syenite has five percent or less. In the photos below, notice the granite on the left is “whiter” due to the quartz. Syenite is on the right with little to no white quartz.

Here are two more photos of syenite.

The search is underway to find other deposits of fluorescent syenite. One of the museum’s customers had several samples of syenite he collected on the north shore of Lake Superior. He brought some into the museum and it was not at all fluorescent. Once these glowing rocks have all been picked up from the beach (and we are almost there already), there will be people searching for a new deposit source.

*The term “Yooperlite” is a trade named by Erik Rintamaki who discovered these fluorescent rocks on the Lake Superior beach in 2017.

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