The Yooperlite story

(Taken from the May 2018 issue of Mineral News) - by Raymond Laughlin and Shawn Carlson



A New Kind of Fluorescent Sodalite

Back in January 2018, it was brought to the attention of one of us (RL) that a gentleman named Erik Rintamaki was selling pebbles and cobbles of fluorescent syenite that he had collected along the Lake Superior shore in Luce and Chippewa counties of Michigan's Upper Peninsula, marketing them under the unofficial name "yooperlites". For those that may be unfamiliar with our geography, denizens of Michigan's Upper Peninsula are colloquially known as "Yoopers", a corruption of the abbreviation "U.P." for Upper Peninsula - ergo yooperlites. Apparently, the fluorescent material in these syenites had been visually identified as sodalite by California geologist and fluorescent mineral collector Gabe Reyna.

Erik's Story

Salting in some details of the discovery provided by Mr. Rintamaki: "I have a little story that goes with my discovery. About 15 years ago I taught my brother-in-law Jason Klein how to find agates on the shores of Lake Superior north of Newberry, and he took to agate picking like no one I've ever seen. He became a rock hound just like me. Well he stumbled across some literature from the Fluorescent Mineral Society about UV lights. And we always dreamed about hitting our beaches at night and seeing the beach turn into a world of fluorescing stones and minerals. So fast forward 15 years and I happened to be on eBay and saw an affordable longwave UV flashlight. So on a whim I ordered one. So in June of 2017 I left for the beach at 4:00 am. I got to the beach just in time to use my cheap 3 LED longwave UF flashlight and found 2 very small Yooperlites about the size of a dime. I went 4 more times and only found have a dozen stones. Then I upgraded to a 100 LED UV torch and that's when the fun began in the fall of 2017. My largest find so far was over 5 pounds. Now I have upgraded even further to Convoy S2+ UV and a shortwave lamp. So I hope to open even more secrets of our Lake Superior beaches."

To our knowledge, sodalite has not been reported from Michigan before; no mention of it is made in the Mineralogy of Michigan (Heinrich, 1976), Mineralogy of Michigan by E. Wm. Heinrich (Robinson, 2004), or in Mineralogy of Michigan Update (Robinson and Carlson, 2013). In order to study this occurence further, our research team purchased several kilograms of sodalite-bearing syenite from Mr. Rintamaki for analysis. Upon receipt, we observed that the clasts do indeed resemble syenite at first glance and most likely are; however, we note here that we have not conducted any whole-rock analyses to classify these rocks chemically (TAS) nor have we performed any thin section point-counts to classify them via modal mineralogy, as our primary interest is with the fluorescent sodalite andnot with formal igneous rock-type classification. Expressed differently, all or most of these rock are probably some type of syenite but we have not performed the necessary tests to confirm this.

Mineralogy

The mineral reported as sodalite does indeed display a strong hackmanite-like fluorescent in response to longwave UV illumination. Examined under incandescent illumination and binocular stereozoom microscope, the mineral is generally a very pale gray but hints of yellow and teal are observed. And it is translucent, although this is probably due to heavy fracturing, as small unfractured domains appear transparent. Examined via petrographic microscope the material possesses a refractive index between 1.480 and 1.490 (consistent with sodalite) and appears isotropic in crossed polars.

Preliminary SEM/EDS analyses conducted at Michigan Technological University showed only Na, Al, Si, Cl and O in the mineral's spectrum. Sulphur was sought but not noted with EDS (below detection limits) but was detected by follow-up EMP/WDS. Microscopic studies were conducted using a Kyowa SDZ-P binocular stereozoom microscope and a Kyowa ME-POL2 petrographic and ore microscope equipped with a Supper spindle stage. EDS spectra were obtained at Michigan Technological University on a JEOL 6400 tungsten-source SEM equipped with a 4pi Analysis (Inc.) ultrathin window EDS detector and operated at 20 kV. WDS analyses were provided by the Saskatchewan Research Council on a Cameca SX-100 LaB6 microanalyzer operated at 20kV, using standards from Astimex Standards (Ltd.).

We therefore concur with Rintamaki and Reyna that this fluorescent mineral is indeed sodalite, and that this is the first verified sodalite documented from the state of Michigan. Our congratulations and thanks to the discoverers for bringing this find to our attention.

Although no mention of syenite is made in Susan Robinson's artistic study Is this an Agate? An Illustrated Guide to Lake Superior's Beach Stones Michigan (Robinson, 2001) it has been our experience that syenite clasts are not that rare in Michigan, and can be found along many Lake Superior beaches as well as in gravel pits within Uper Michigan's interior. Perhaps this clast lithology has been traditionally underestimated because of it being misidentified as granite and thus dismissed by collectors as not very intersting.

Although there are some small syenite bodies in Upper Michigan's Marquette County, to the west of this study area, we believe that it is unlikely that Rintamaki's sodalite-bearing syenite clasts are locally-sourced. It is more probably that the bedrock source of this material is located in Canada, specifically the Coldwell Alkaline Complex in Ontario, and that fragments of this complex were transported to their approximate present-day location by continental glaciation, further slightly moved, concentrated, and somewhat wave-polished by modern Lake Superior. Indeed, the presence of minor orange-red natrolite (alteration of nepheline) in these clasts argues strongly for a Coldwell Complex origin (Mitchell and Platt, 1982).

Approximately 187 valid mineral species were known to occur in Michigan in 1976 when Heinrich published the first edition of our state's geographic mineralogy. Today, there are about 383 minerals known in Michigan, more than double. Yet despite this obvious advancement, the fact that discoverlies like this fluorescent sodalite can still be made, a mineral not previously confirmed in Michigan, but now known to occur in kilogram quantites along Lake Superior beaches - suggests that the mineralogy of our state is significantly under-studies, and that hundreds of minerals new to Michigan (plus possible brand new species for the world) await discovery here. We warmly encourage future collectors and researchers to go out and discover them.

Acknowledgements
We thank Erik Rintamaki for making us aware of this discovery, and Gabe Reyna for mineral identification. Vancouver Petrographics Ltd. are acknowledged for preparing polished thin sections, and Steven Creighton of the Saskatchewan Research Council is credited for providing microprobe analyses. Finally, Vicky Underhill, Audrey E. Smith, Mark Smyk (Ontario Geological Survey) and Shannon Zurevinski (Lakehead University) are thanked for their helpful reviews and comments.