Capillary Raman Cell Experiments: The rare earth elements are in hot water (and feeling salty)!

— Dr. Sarah Smith-Schmitz

Somewhere in the Earth’s crust a hot fluid is seeping through tiny cracks and fissures in the rock. The fluid is water and it carries with it a cargo of dissolved ions like chloride, sulfate, or carbonate. It might also carry dissolved metal ions useful to humans such copper, gold, or, in the case that we are considering, rare earth elements (REE). Fluids like this play important roles in forming ore deposits where the REE are present in high enough amounts to be mined. We want to understand how the REE interact with other dissolved ions and the water itself in order to better understand the conditions that allow water to mobilize, transport, or deposit REE.

In the Raman Microscopy Lab at the New Mexico Bureau of Geology, we are conducting a Raman spectroscopic investigation into the complexation of the REE dysprosium (Dy) using a new capillary cell heating stage built at New Mexico Tech (Fig. 1). Complexation is the formation of a chemical compound or cluster with a metal ion in the center surrounded by bound molecules and/or ions (e.g. chloride, sulfate, or carbonate) called ligands. Raman spectroscopy uses the interaction of light with the chemical bonds in a molecule to derive information regarding the chemical composition and structure of the molecule. We are currently investigating what complexes the REE dysprosium (Dy) forms in a chloride (Cl-) bearing fluids at varying pH, Dy/Cl ratios and temperatures of 20-300 °C.

Raman spectra for pure water, as well as solutions made with DyCl₃ and NaCl have been collected at temperatures up to 300 °C (Fig. 2). Our current results show that, as temperature increases, Dy complexes with more and more Cl. The ongoing experiments and analysis will allow us to gain more insight into how Dy complexation with Cl- changes not only with temperature, but with the other factors considered above. These insights with help us better understand the potential roles of Cl- bearing fluids in REE deposit formation.

This research is supported by the U.S. Department of Energy, DOE, Grant – DE-SC0022269 and by the National Science Foundation under Grant NSF-MRI EAR-2117061.

Presentations

1. Smith-Schmitz, Sarah. E., Gysi, Alexander P., Hurtig, Nicole, Migdisov, Artaches A. (2023) Hydrothermal solubility experiments and Raman spectroscopy for the determination of Dy hydroxyl complex stabilities as a function of pH and temperature. Goldshmidt, Lyon. Abstract 20293.
2. Smith-Schmitz, Sarah. E., Gysi, Alexander P., Hurtig, Nicole, Migdisov, Artaches A. (2023) Determination of Dy complexation in hydrothermal fluids as a function of pH and temperature using Dy(OH)3 solubility experiments and Raman spectroscopy. ACS Fall Meeting, San Francisco. Abstract 3927842.