April Presentation: "The Geology of Rare Earth Element Minerals" by Dr. Leigh Broadhurst
Synopsis by Andy Thompson, MSDC Secretary
Dr. Leigh Broadhurst is a geologist trained in planetary geochemistry and physical chemistry. She is also a mineral collector familiar to the clubs in the DC area. She described her evening’s presentation as being “about the rare earth elements (REEs), their mineralogy, (and) some of the mining and geology background of the rare earth elements.”
She explained that the term “rare” was mistakenly applied because originally the REEs were thought to be found only in Sweden. Today, REEs can be found throughout the world. With technological developments in recent years, there has been an increasing high demand which strains the supply chain. Also, there are production issues including environmental challenges and difficulties separating the elements from one another and from impurities. Leigh added she was not going to spend a lot of time showing things that that can be readily picked up on the internet.
With that brief overview, she showed where to find the 17 REEs in the periodic table, including the 15 elements ranging from Lanthanum (La) to Lutetium (Lu) in dark blue and Scandium (Sc) and Yttrium (Y) in light blue in a nearby column. Those last two are included in the REEs because they also “fill the 4-f electron shell” and have similar properties to the 15 Lanthanides. Promethium (Pm), she said, is man-made in a super-collider, is radioactive and has an extremely short half-life. “So it is not found in geology.”
Readers, the purpose of this program report is to encourage everyone to view the full-length video of Leigh’s presentation. You can find it and the MSDC YouTube channel by clicking on the link HERE.
How REEs are Essential for Today’s World
Leigh then began sharing her understanding of the 17 REEs and their implications for our world. Using the “Key Applications” slide below, she pointed out the profound changes the REEs are making in today’s world. She called attention to the major problem of China controlling 90% of the world’s production of the REEs. These elements are of critical importance because they are used as catalysts to enable so many of our essential technologies as shown below. “And so the overwhelming consensus for all the Western countries and in fact even for China is that this has got to change.”
Referring to the first of the above seven applications (used in computer drives, MRI medical equipment, etc.), she explained: “Any time you are going to generate electricity you’re going to need a magnet. And, anytime you want to generate a tremendous amount of electricity in a very small location such as in an automobile you are going to need very powerful magnets.”
Readers will want to listen to Leigh’s explanation for how REEs are heavily used across all seven applications, especially in national defense, and also used in:
· phosphors which help create bright colors in electronic displays,
· catalysts which are central to refining petroleum,
· ceramics and capacitors for electronics, and
· the defense industry with many used in satellites and communications.
She noted that REEs applications with “catalysts is probably the number one use of these rare earth elements... and defense is the primary reason why this has become important lately.” Given China’s worldwide dominance in producing REEs, all nations “are on the lookout for reopening mines, finding new mines, finding new resources, and finding new ways to mine these elements and separate them from each other.”
The Geology of REEs
Shield rocks are “one of the first places we go to find these” rare earth elements.
As shown in the above slide, shield rocks are found throughout the world and in many cases “these old rocks have been exposed by repeated glaciations.” Leigh focused on the Canadian Northwest Territories Great Slave Lake area near the town of Yellowknife. Initially its importance was as an early gold mining site in the three-billion-year-old rocks. But then the miners also discovered its rare earth minerals.
The slide shown below shows Slave Lake in the center bottom just north of Alberta. She emphasized the lake is the size of Belgium. The area is largely uninhabited and resources such as fuel, food and equipment are in very short supply. Readers will want to view Leigh’s YouTube video in which she shows beautiful photos of the area’s glacially striated rocks and illustrates some of its gold mining history.
This area has now become a very important REE site known as the Nachalacho mine, Canada’s premier location for mining REEs. The mining operation was structured as a consortium of the Canadian government and the local Native American peoples. Its location is shown below in dark brown at the edge of the Great Slave Lake and within the area outlined in yellow.
One of the important minerals found in this ancient metamorphic, igneous rock formation at the Nachalacho mine is eudialyte. While not a rare earth itself, it contains cerium (Ce) which is one of the REEs.
Leigh also pointed out that eudialyte also contains Zircon (Zr) as shown in the chemical formula below:
“Zircon is found with the rare earth elements again and again, . . . where you find Zircon mineralization is a great place to start looking for rare earths.”
The type of rock found in the Nachalacho mine is nepheline syenite which dates from 2.2 billion years ago with an alteration which occurred between 1.9 and 1.8 billion years ago. Nepheline specimens from this mine are shown below, often featuring blue or green colors. As its formula shows, (Na,K) SiO4, does not contain REEs.
But there are many hydrothermal minerals containing REEs associated with this nepheline syenite rock and the nine primary ones are listed below, with bastnäsite being the primary ore and all of them containing cerium (Ce).
In summary, the unique importance of the Nachalacho mine is that it is the richest mine for REEs in the world, replete with the more valuable heavy rare earth minerals, as described in the following slide below.
How Did this REE Mine Form?
Leigh then described the geologic conditions which supported the deposit of the REEs billions of years ago. She stressed that those conditions were unique to the early formation of our planet, with its intense heat, radioactivity and other factors. Those factors resulted in the lithosphere subducting at a “Low-Angle” illustrated below as b).
In contrast, today’s tectonic plate subductions, illustrated below, as the “Normal-Angle Subduction,” a), shows the lithosphere taking a steep angle decline.
“So the conditions for these rare earth elements to be in these ancient Archaean crusts, in these ancient shield rocks, they simply don’t exist today.” To learn more about this important contrast between the Archaean crust subductions and those commonly seen today (e.g., the world’s best known subduction zone on the extensive West coast of South America), watch the YouTube video of Leigh’s presentation.
Leigh’s geological explanation of the Archaean plate subductions was simply an introduction to how the REEs were deposited. There was much more to come.
Where are the REEs and What is their Mineralogy?
Leigh began her discussion of mineralogy by describing a century-long puzzle that once miffed geologists. How could the very well-known sedimentary mineral, calcium carbonate, become an igneous mineral containing iron and magnesium?
To find the answer to this and the following questions readers will be delighted to view Leigh’s YouTube video of her presentation to MSDC.
There you will discover answers to:
· the above calciocarbonatite sedimentary mineral mystery,
· where the shield plates are found across the globe,
· the significance of California’s Mountain Pass REE mine,
· the association of REEs with some rift zones but not with most volcanoes
· how carbonatite chemistry supports the slow formation of the heavy REEs with their highly valued plus three electron valence, and
· how the “Boring Billion” time-frame provided most of the world’s REE deposits.
Viewers will also learn what was happening with the Earth’s plates, shown , during the time of the Proterozoic supercontinent Columbia period when all the REE deposits were gathered in the same place.
Recognizing that many in the MSDC audience were mineral collectors, Leigh showed a variety of beautiful minerals, such as the fluorapatite and others pictured below. She explained their connection to rare earth minerals. Hint: it often has to do with many minerals having chemical components that allow substitutions, including REEs.
By watching Leigh's YouTube video, viewers familiar with the zircons, tourmalines, and aquamarines shown below from the world-famous Minas Gerais state, Brazil, will learn that the region also has a late Cretaceous (80 to 90 million years ago) carbonatite ring and ancient rift zones. That site contributes 90% of the world’s rare earth niobium. The very familiar minerals shown below are neighbors of the REE deposits but not chemically associated with the REEs.
Viewers will also learn about the characteristic ruby red color associated with many of the REE minerals, such as those shown below.
Finally, viewers will be interested in Leigh’s concluding observations of the Ukraine nation’s potential as a source for REEs, as illustrated below. Given its Archean shield and many Proterozoic carbonatite localities, Ukraine has recently captured the world’s attention.
The extensive question and answer session which followed Leigh’s presentation was replete with additional interesting information. The topics ranged from the potential for REEs in the deep sea basaltic mid-Atlantic ridge (which is not carbonatite based), to the recent discovery of REEs in a Wyoming deep coal mine deposit. Questions also arose about the REEs found in the heavy sands of Australia’s Perth region and China’s near-total monopoly of REE production and the associated implications for the environment and international trade.
MSDC President Dan Teich then thanked Leigh for her extraordinary presentation which was seconded by extensive applause by viewers in the Zoom community.
Readers: you can find Leigh's presentation on the MSDC YouTube channel by clicking HERE.