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Rare Earth Element Critical Minerals Studied in a Hydrothermal Diamond Anvil Cell

— Dr. Bryan Maciag

The world is changing fast. Advanced electronics such as smartphones and tablets are a staple of everyday life. Critical to these devices are the rare earth elements (REE). Unfortunately, the supply of REE around the world is limited, thus research into how REE mineral deposits form is needed to help guide us to new sources of these metals. One aspect of REE geochemistry that is not fully understood is how REE are transported in the hydrothermal fluids that form these deposits. How easily these elements can be transported depends upon the composition of the fluid and what ligands (negatively charged molecules) the REE elements bond to in the fluid, which is called complexation.

Using the newly installed Horiba LabRAM HR Evolution Raman spectrometer, we are investigating the REE complexation in hydrothermal fluids at various temperatures and pressures. Raman spectrometry provides molecular information by measuring the vibrational state, which is unique to each bond. By measuring the complexation in solutions with a controlled composition and temperature we will develop a lexicon of possible REE complexes and which ones are dominant.

Raman Spectra
Figure 1a: Raman spectra of O-H stretching band.
vibrating OH bonds in water
Figure 1b: A schematic sketch of a water molecule (red – oxygen; white – hydrogen) showing the stretching vibrational mode.

Firstly, we are looking at how REE bonds to Cl. We are exploring this variable by deconstructing the O-H stretching band, in solutions with over a range of Cl concentrations at 25ºC (Figure 1). This research aims to develop a method measuring the abundance of REE-Cl species present at higher temperatures. So far the results at 25ºC are consistent with thermodynamic predictions, although our experiments indicate that REE-Cl species might be present if the Cl concentration is much greater than the REE concentration.

Our next step is to explore the complexation of Yb at temperatures up to 500ºC. Several recent studies indicate that the SO42- may be important to transporting REE, yet several variables such as pH are yet to be explored. We plan to explore these sulfate fluids using a hydrothermal diamond anvil cell (H-DAC; Figure 2). The H-DAC squeezes a small volume of fluid two special cut diamonds to generate high pressures while heating to 600ºC. The transparency of diamonds allows the Raman laser to interact with the sample, letting us probe the molecular composition of fluids at the temperatures and pressures that they would experience flowing through the Earth's crust.

vibrating OH bonds in water
Figure 1b: A schematic sketch of a water molecule (red – oxygen; white – hydrogen) showing the stretching vibrational mode.

This project is supported by DOE grant DE-SC0022269 and NSF-MRI EAR-2039674.

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