They have been found in what is called the Rhame bed.
The North Dakota Geological Survey has identified a second interval of rocks that contain elevated concentration of critical minerals.
Those critical minerals are defined as essential to the economic or national security of the United States.
The Geological Survey has been looking at the recovery of critical minerals from lignite coal beds in North Dakota.
Right now, the US has little or no domestic production of those minerals – and has to import them from other countries, such as China.
"Recently, China and Russia, to some degree, have flexed their muscles, saying, 'Well maybe we won't be supplying those — we'll keep them for our own domestic production," said geologist Levi Moxness.
Moxness said the federal government has been working on alleviating that strategic vulnerability, and find domestic sources here for these minerals.
"We just haven't produced them for decades," Moxness said.
Moxness said the Survey and Department of Mineral Resources have been looking at lignite specifically, because of its ability to "catch" the minerals in the carbon. He said the Survey has been working with the Institute of Energy Studies at UND. Moxness said the challenge will be to make any removal of those minerals to be economically feasible.
"Maybe get into a small-scale mining operation, close to the surface, to see how deep does it need to be, how thick the bed needs to be, how much over-burden would you have to remove," Moxness said. "Those are still some questions that aren't answered quite yet. But there are a lot of 'very smart' groups working on it,"
Moxness said other coal producing states are looking at the availability of those rare earth minerals as well.
"North Dakota, so far, has some of the highest numbers in the country," Moxness said. "If the US Department of Energy is serious about one day producing rare earths from coal, we think North Dakota lignite has a pretty good chance of it being economic to do so."