September 29, 2022

Understanding the Structure of a Complex Mineral

New measurements of natrophosphate point to an old solution

Illustration of a woman examining a pipe with a magnifying glass, thinking of two chemical structures

Characterization of the natrophosphate mineral revealed that the local chemical environment of ions within the crystal structure did not vary across a wide range of synthesis conditions.

(Illustration by Nathan Johnson | Pacific Northwest National Laboratory; Image adapted with permission from the Royal Society of Chemistry)

What do arctic mines and tank waste at the Hanford Site have in common? Both are places where natrophosphate, a mineral with a complicated crystal structure, occurs. Originally isolated in the late 1800s, natrophosphate’s crystal structure is still being explored by researchers.

Natrophosphate is a hydrated salt made of sodium, fluoride, and phosphate ions. Currently, there are two schools of thought about the structure of natrophosphate. One says that the relative amount and local chemical structure of the ions can change. This results in an altered crystal structure and composition of the material. Potential alterations include a different ratio between the fluoride and phosphate or the formation of hydrogen phosphate. The other hypothesis is that the natrophosphate structure stays the same.

Understanding the structure and composition of natrophosphate is important because nuclear waste processing operations are guided by models that predict solubility. Operators must select conditions that keep all the different components soluble.

A team of researchers led by Pacific Northwest National Laboratory (PNNL) chemical engineer Trent Graham performed new experimental measurements to look for changes in the crystal structure and composition of natrophosphate. By integrating the results from different spectroscopies, the team found no evidence of changes in the structure in their data. The data, published in Inorganic Chemistry Frontiers, were consistent with classical salt and mineral crystal behavior.

“Natrophosphate is a fascinating mineral,” said Graham. “The complexity of the crystal structure offers an opportunity to understand the strengths of different experimental techniques and how to combine them.”

Comparing crystal environments

The researchers created natrophosphate by varying the ratio of sodium fluoride to sodium phosphate in the starting solution. Compositional measurements by electron microscope showed that no matter what the ratio of fluoride to phosphate was in the initial solution, the fluoride to phosphate ratio in the final mineral remained the same. That helped answer one question about natrophosphate.

Graham and the team then deployed the wide variety of material characterization techniques available at the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility. They extensively employed nuclear magnetic resonance spectroscopy (NMR) to probe the natrophosphate structure. NMR uses magnetic fields to study the structure and environment of different elements.

They used NMR to study specific phosphorus, fluorine, sodium, and hydrogen nuclei. Studying these nuclei led to direct measurement of the phosphate, fluoride, and sodium ions, and the water molecules, that compose the structure of natrophosphate. By using NMR and other complementary techniques, including X-ray diffraction as well as Raman and infrared spectroscopies, the researchers found that the mineral structure remained constant across the different starting solution compositions.

“One way we test the interpretation of our solid-state NMR data of certain nuclei, such as sodium-23, is to perform experiments at different magnetic fields,” said Graham. “If you can simulate the shape of the line from one experiment with parameters extracted from an experiment at another magnetic field, you can feel confident in your analysis. The great thing about a place like PNNL is having access to a fleet of NMR spectrometers across a wide range of magnetic fields. This makes an integrated approach possible.”

Graham’s focus on tank waste relevant minerals comes from the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center. IDREAM scientists are also exploring whether salt hydrates like natrophosphate could be used as model analogs for the assembly of ions in concentrated solutions. Multi-atom ion aggregates are important in many environmental and industrial processes, including radioactive waste processing.

This work was funded by IDREAM. PNNL authors are Graham, Emily Nienhuis, Jose Marcial, John Loring, Kevin Rosso, and Carolyn Pearce. Jacob Reynolds of Washington River Protection Solutions collaborated on the paper. The article was featured on the inside front cover of the journal.